# dually



## ktm520 (Apr 21, 2004)

I've been working on this design for way too long. It's going to be a 29er with 140mm of travel and clearance for a 3" tire, more popularly know as "plus". Getting ready to start cutting steel for this one really soon. Have everything but st/tt and they should be here tomorrow.

Specs:
dt - 1.5x.035 4130
tt - 31.8 1/.7/1 true temper
st - 35 1.6/.8 true temper
st brace - 1x.035 4130
pivot shell - 1.25x.12 4130
swingarm - 1x.035 and .75x.035 4130
shock mount fwd - .065 4130 sheet
swingarm pivot/mounts - 1" 4130
x-12 dropouts

geom:
fc - 720
cs - 415
hta - 68.5
trail - 95
sta (actual) - 63.4
bb drop - 31

kinematics:
leverage ratio - 2.66-2.89
anti-squat (32:42 @sag) - 94%

There is lateral clearance for a 75mm wide tire, but radially the tire would hit the st at 140 travel, based on the knard 773mm diameter. I highly doubt I will ever actually put 3" tires on this thing, but if I do I can run a smaller plus tire or short stroke the shock to limit the travel to 120mm. The compromises with the st angle and kinematics weren't worth it to me. I don't even have a fork that will accept a 29+ tire and probably won't for awhile. I'd love to have a Magnum but it's just not in the budget. I will actually be using a Tower Pro 120 initially.

Seeing as it was taking me forever to finish the design, I told myself I would not do any structural analysis/optimization on the swingarm and use the build/test approach. Such efforts can quickly turn into a black hole and Shirk had already demonstrated that a simple design can work. But . . . I just couldn't resist. The initial design was very similar to Shirks and I used that as a baseline for stiffness. I ran some quick models and made a few small changes that resulted in a rather significant improvement in stiffness. So, it was time well spent.

I would prefer a more flat leverage curve, but this very slight regression can easily be tuned for with spring volume. I initially started with 435 stays, but quickly decided to go short. This of course pushed the st fwd, which pushed the rear mount fwd, which pushed the front shock mount fwd and up, which pushed the leverage curve from flat to regressive. The seat tube is notched 12.5mm to clear the rear shock mount as is.

For the pivot, the first place I looked was the Santa Cruz heckler. The pivot kit comes with the axle (collet style) and hardware, bearings, bearing dust sheilds, and shock bolts, all for 38$.
https://shop.santacruzbicycles.com/parts-and-tools/by-model/heckler/propack-heckler-6-1.html

The pivot axle has 15x1.5 threads on it. This is not a common size. I ordered a tap off ebay, recieved it today and although the box was labeled correctly, I got a 18mm tap. Bummer.

More to come . . .


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## Walt (Jan 23, 2004)

So, 3x29" tires are ~390mm radius. You're going to leave ~5mm of clearance to the BB shell?

BTW all sorts of forks will clear 29+ tires, even older Fox stuff.

-Walt


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## ktm520 (Apr 21, 2004)

415 is the horizontal stay length, actual is 416 . . . just to clarify. The Knard I have is 386mm radius and would be ~10mm of clearance at st, which is closer. Would 5mm not be enough?

There is a very slim chance this frame will ever have a Knard on the rear of it. I would probably go with a shorter tire, more for seat tube clearance though when bottomed. I like 3" tires on my rigid ss, but really don't think I'd get into them on a bike like this.


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## Walt (Jan 23, 2004)

Maybe I'm not understanding, but BB shell is ~19mm radius. CS is 416mm, so you're left with ~397mm of space at the BB shell. Your Knards must run a little smaller than Surly thinks (they say ~390mm radius) but regardless, if you aren't going to actually run a 3" tire, it doesn't matter. Unless the seat tube is offset way away from the BB shell I'm not clear on how it can have any more clearance so it's even tighter there?

-Walt


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## ktm520 (Apr 21, 2004)

Well manufacturers ratings aren't exactly accurate. I'm sure there is some variance. I think we are on the same page. Chupa's and Fatty Trax are even shorter, that I've measured. Looks like the Fatty B is even shorter than the lot. 

What do you consider minimum clearance at the bb? I've had decent luck with 6mm . . . in my very limited experience.

You bring up a good point though that completely escaped my mind. Offsetting the st at the bb will give me better clearance at the rear shock mount and steepen the upper portion of the st. Might not be much, but every 1mm counts.


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## adarn (Aug 11, 2009)

Your chain stays are definitely for sure going to break if you leave them hanging off like that.


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## ktm520 (Apr 21, 2004)

adarn said:


> Your chain stays are definitely for sure going to break if you leave them hanging off like that.


Thanks for the feedback. Can you elaborate a little? Where exactly would you predict that they break? The loads I used in the analysis are fairly close to what it would see in a soft bottom out with a 1g lateral load and the margins are decent, but this was a simple beam element model that doesn't do the best job of predicting localized stress concentrations.


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## Walt (Jan 23, 2004)

Adarn, I think, is referring to the school of hard knocks/personal experience, rather than FEA or computer modeling.

I did a couple of bikes basically identical to this, with a little bit longer stays (though I don't recall by how much), about 10 years ago. Back then through axles weren't as much of a thing so I just used Instigator dropouts and 3/4x1.5x.049 rectangular 4130 for my swingarm, with some extra 3/4x.035" for lower braces. It rode nice and stiff and didn't fall apart. I'd be pretty sketched by 1"x.035, especially with all the joinery/stress risers in the dropout/brace area.

Also chainslap/chain clearance can be super annoying if you don't plan for it carefully. 

I mean, build it and see what happens. I'd beef it up there and just accept that it's going to be heavy as all get out. 

As an aside, what's the goal for this bike? I can't think of anything I'd take 3" tires on (at least if you make an exception for the old 26" Gazzalodis) that would require more than about 80-100mm travel. Knards or Chupas are way too flimsy for hucking or chunder, though I've heard good things about the Dirt Wizards. It seems like it's going to end up being really tall/oversuspended for the kind of riding you'd typically do on 29+ bikes.

-Walt


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## ktm520 (Apr 21, 2004)

I gotcha, and agree the weak link is the 1x.035 tube. I'll take the advice given here over my simplified analysis especially when it comes to stress. Relative stiffness models, on the other hand, are quite helpful. Didn't mean to be snarky by posting those analysis results.

I was mostly banking on the fact that Shirk has had good success with his swingarm and he used 1x.035.
http://forums.mtbr.com/frame-building/squishy-bike-frame-2-a-936800.html#post11687517

This is going to be an agressive all mountain bike and will replace my Yeti SB95. Which is why I don't think it will ever see 3" tires. Adding the lateral clearance was easy, so I figured why not. I agree completely with your opinion on the suitability of 3" tires even though everyone is coming out with 140-150 b+'s right now. Not my cup of tea.

I had been toying around with the idea making a more "race track" shaped tube by splitting a round tube length wise and sectioning in sheat. Maybe something like 1" wide and 1.5" tall. Would that be a terrible idea? I kind of tabled it because I was under the impression it was overkill. Bumping up to a 1.13 or 1.25 tube for that main member would much easier and get it closer to an oval section.

There is small amount of chain clearance with a 32/42 combo. I have never had the need yet to run a 42 gear so I think that should suffice. No?

I try to avoid that "build it and see" strategy especially if there are indicators up front that suggest don't do it. I don't have a ton of time to build in the first place, so I'd rather design it to my best abilities in the first place.


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## Walt (Jan 23, 2004)

Yeah, I would guess Shirk's bike will eventually run into problems too. You never know. Adam rides harder than most folks here so that comes into play as well. 

-Walt


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## ktm520 (Apr 21, 2004)

Cool.  I got some more homework to do. Glad I posted this up for feedback before I started on the swingarm. Taking a second look at the model results, there is definitely an area of higher stress right behind where the upper and lower tube meet in front of the drop. Just guessing, shirk probably hasn't put enough miles on it for any potential fatigue issues to show up . . . or, maybe it could be fine.

You think at a minimum, going with a 1x.049 tube would be a safe approach?


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## Walt (Jan 23, 2004)

Man, I just did a lot of nit-picking. Must be the sick 1 year old and no sleep.

Just to be clear: this bike is going to be awesome and I love this project. I only wish I could model stuff in CAD and do all this kind of analysis!

-Walt


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## adarn (Aug 11, 2009)

Walt said:


> Yeah, I would guess Shirk's bike will eventually run into problems too. You never know. Adam rides harder than most folks here so that comes into play as well.
> 
> -Walt


Oh You're making me blush.

Yeah unfortunately the school of hard knocks thing is true. 
KTM, Any tube cantilevering off of a stress riser where 100% of the force from the rear wheel is transfered is sooo sketchy dont do it.


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## ktm520 (Apr 21, 2004)

Walt said:


> Man, I just did a lot of nit-picking. Must be the sick 1 year old and no sleep.
> 
> Just to be clear: this bike is going to be awesome and I love this project. I only wish I could model stuff in CAD and do all this kind of analysis!
> 
> -Walt





adarn said:


> Yeah unfortunately the school of hard knocks thing is true.
> KTM, Any tube cantilevering off of a stress riser where 100% of the force from the rear wheel is transfered is sooo sketchy dont do it.


Walt, no worries man. I wouldn't expect any less. I remember those days with the little critters and feel your pain. Thanks for the encouragement.

Adarn, I'm with you man. Sometimes I'm so freaking stubborn it takes a smack in the head to clear my mind. Thanks for that.

I started on a redesign last night. Have a few ideas to play with but basically plan to run both the upper and lower tube back to the dropout. Transition down to the drop will be the trick. I don't think just increasing the wall or diameter of the upper tube would be the way to go without going really beefy/heavy.


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## shirk (Mar 24, 2004)

I've put some serious descending miles on my bike. I've not babied this bike here on the infamous North Shore and yet to have any issues with it. It's been g-d out, hucked, flat landed and bottomed out numerous times.

This weekend it will get raced at the Crankworx EWS (the bike was intended to be raced at it last year but it wasn't completed in time) so that should be it's ultimate test.

With that said I didn't do any calculations or modeling. I did some reading on what other guys have built and decided to try the 1x0.35. So far it's not broken or cracked.

Looking at the pictures I have less cantilevered off the end vs Adams.



















Adam is your's a smaller diameter than 1 inch? From the pic it looks like it might be thinner.


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## Eric Malcolm (Dec 18, 2011)

Gentlemen

Is there any reason why you need to keep that lower chainstay parallel and above the chain for its whole length, thereby leaving a loaded area to the drop-out?

Maybe look at the usual dual attachment to the drop-out and bending to clear everything. That is, S-bending along the length with a 90* rotated bend down towards the drop-out to finish off. Easy.....

Eric


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## ktm520 (Apr 21, 2004)

Eric, that's a great idea and would actually only take one bend in the vertical plan. The point of contact with the chain is closer to the first 1/4 of lower tube. After that, it could bend down and meet at the front of the dropout. Unfortunately, I don't have any bending equipment at this point, so my design is limited by tooling.


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## shirk (Mar 24, 2004)

I have no bendy stuff so straight tubes ftw.


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## ktm520 (Apr 21, 2004)

shirk said:


> I've put some serious descending miles on my bike. I've not babied this bike here on the infamous North Shore and yet to have any issues with it. It's been g-d out, hucked, flat landed and bottomed out numerous times.
> 
> This weekend it will get raced at the Crankworx EWS (the bike was intended to be raced at it last year but it wasn't completed in time) so that should be it's ultimate test.
> 
> ...


Thanks for the update Shirk. That picture makes it a little clearer as to what Adam is talking about and were he is coming from. It's really not so much a problem of stress riser, but how much the upper tube is cantilevered back to the drop from where the lower tube stop. Adam's drive side tube was cantilevered ~3 times as much as your design. That's a big difference.

I'm still moving away from this design and think I have a solution that will result in greater strength/stiffness and not add too much weight.


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## shirk (Mar 24, 2004)

I spent a bit of time learning CAD working on #2 and think i will make some plates that link the pivot to chainstays.


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## ktm520 (Apr 21, 2004)

Here's what I'm working towards. I don't like the boxy structure between the tubes and drop, but gives you an idea. Ran a quick FEM and the stress levels dropped considerably along with a bump in stiffness. Will have to redo the brake mount since the upper tube is higher now. This is basically a copy of the latest Heckler layout, sans the fancy drop forging.


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## Eric Malcolm (Dec 18, 2011)

Thought the limitation maybe the bending bit, but worth asking the question. Your latest iteration is going somewhere, so keep working in this direction.

Eric


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## shlammed (May 19, 2015)

with your latest design, you can still fit a chain?


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## ktm520 (Apr 21, 2004)

I think I've got a new design I like. Went through 5 different new swingarm designs, even with some curvy tubes, and played with the seat tube a little. Ran FEM's, hit on all the swingarms, hit the black hole trying to further optimize, and this one came out the best. The asymmetrical design does cause a slight asymmetry in the stiffness but I don't think you'd ever feel it on the trail. Not sure if the offset st will stay or not, but it does provide a little more room for the shock and steepens the st angle.

Ebay dude I bought the tap from refunded my money and let me keep the 18 tap. Got another one on the way and will have it friday. At mercy of my uncles schedule when it comes to lathe time, but hopefully I can get the pivot shell and swingarm eyelets turned early next week.

I'd say I'm getting excited . . . but I've been excited for 3 weeks now. It's been 6 months since I've done any building.


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## shirk (Mar 24, 2004)

That looks good. 

Are you going to tie the two sides together with a cross tube? I have one on the lower brace tubes. Kinda forget if I also have one on the upper tubes.


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## mickuk (Jul 6, 2007)

I did the bendy stuff on my E-stay frame 22mm diameter).

Bending was OK-ish, I just didn't think enough about the attachment to the dropout. It worked Ok but not very tidy - I've learnt a lot about designing joints thick-thin and since then.

The two front bends are all done in one plane (tube angles out from down tube, then runs parallel to longitudinal axis of bike, then angles outwards). The tube was then rotated (by an amount that I can't remember) before the final bend. So in one hit the final bend sends it down and also brings it back onto the longitudinal axis (so it runs parallel to the dropout face). Hopefully that makes some sense.....


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## ktm520 (Apr 21, 2004)

shirk,

I played with some cross bracing on the front end in some of the initial models and it didn't really help much, although it was just a single cross tube. An X brace would be ideal but I don't have the room. It mainly helps stiffen the front part of the structure laterally which will reduce side loads on the shock. It's seeing minimal lateral deflection at the shock mount, so I'm not too concerned. I'll put some more thought into it though. There is not room for a bridge between the tire and st. Big wheels, short stays.

mickuk,

Yes I recall that build. Amazing work man. In my design, a bent lower stay on the drive side actually was weaker and less stiff than the latest iteration. That member is the most heavily loaded in the structure. Under ultimate load, it tries to straighten out and this causes a stress concentration in the throat of the bend. It would probably work, but lower stiffness is a deal breaker.


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## ktm520 (Apr 21, 2004)

Is joining the seat tube brace to the butted section of the top tube a no no?


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## Eric Malcolm (Dec 18, 2011)

That join is not a hard working area of the frame, and most designs use a much smaller dia tube compared to the tubes the brace attaches to. That being said, if you are brazing, I would place a longitudinal half tube on the TT to act as a spreader if you wanted to. Be aware that if the wall thickness is under .6mm, the tube may 'heat wrinkle' on you. TIG, well, you are limited, so if worried, use straight gauge tube for the TT.

Eric


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## ktm520 (Apr 21, 2004)

Got all the pivot/swingarm parts machined last night. Now just 26 joints to fit . . .

















I redesigned the geometry last week. Going with 125 travel, 10mm lower bb, and same front end. I lost some tire clearance with the swingarm redesign, but a smaller + tire like a Fat B will fit. If I bow the left lower tube 10mm, a Chupa will fit. Screw Knard's.

With a + tire, I will have to limit the travel of the shock for 110 wheel travel. I just read Mike C's thoughts on + full suspension and it really put things into perspective.


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## Walt (Jan 23, 2004)

*Thoughts on plus stuff...*

Woot!

Use supetherm for the toptube. You can weld to the .7mm middle portion fine and the 1mm end is great at the head tube.

I just put mine together last week (with, as usual, a mishmash of inappropriate parts and no cable guides...not to mention no powdercoat). 425mm stays, 12.8" static bb height (yes, I like low) and 100mm travel front/rear.

I agree with MC (which is common, we are old friends from way back and both ride more than responsible adults should) - for ME, more than about 100mm travel and 29+ tires (Chupacabras) is I think going to be too much. Even at 100mm travel, the bike really makes XC type trails pretty boring/numb feeling (unless you're out at night, it's perfect then!) It shines on rough/fast/steep but most of the trails out my back door aren't super technical.

The other odd issue is that you want to go just hit jumps on this bike but... plus tires are horrible on jumps. I think I'll need to get a set of 2.5 Minions for riding flow trails or something, because man, big fat XC tires are just terrifying if you don't land everything perfectly. On the plus side, though 29x2.5s and 120mm front/rear will end up about perfect for the BB height. 








-Walt


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## ktm520 (Apr 21, 2004)

Very nice Walt! Mike's comment about making things too easy really hit home. And I agree with you on jumping + tires. I hit dj's on my + rigid every know and then and can be scary unless I air up.

I am using a true temper 1/.7/1 top tube. First thing I noticed about your stupid mobile 2, besides that clever yoke design, was the super short seat tube (upper segment). I like it. Droppers kind of change those design requirements.

How bad or not bad did that rear tire ride on the crest?


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## Walt (Jan 23, 2004)

Yeah, for me it's not even about making things too easy - it's also about just limiting speed on multi-use trails and still having fun. I have plenty of blind corners and other trail users to consider and if I have a bike that just isn't fun unless I'm going 20+ mph... that's not great. Honestly I've been riding rigid all summer because it lets me *feel* like I'm going fast even though I'm not and it's perfect for most of my backyard stuff.

I had to do the short upper segment seat tube for this frame to get enough clearance for everything (seatstay bridge/tire) and it does limit my choices on droppers. I was being very paranoid about clearance - the Chupacabra has like 12-15mm of clearance in all directions, it's silly.

Honestly I don't think I'll end up using a dropper on this bike anyway, though. The plus tires just aren't great on the terrain where I'd want (flow trails, DH) a dropper. If I do put some Minions on I'm probably on the chairlift so I'll just use a short rigid post.

The rear tire is fine on the crest, but I'll get a flow wheel or something along those lines when I have some time to organize the build a bit. I was in a hurry to ride! 

-Walt


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## shirk (Mar 24, 2004)

You guys and these zany + tires. 

ktm that pivot looks great. Very clean. 

Walt what pipe did you use for the yoke? Also what are you using for chainstay material? 3/8 * .035?


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## ktm520 (Apr 21, 2004)

shirk said:


> You guys and these zany + tires.
> 
> ktm that pivot looks great. Very clean.


I'm really happy with how this pivot setup is going to work. The Heckler hardware kit is cheap and the swingarm bosses are easy to make if you can barrow a lathe. Probably not as stiff as a 1.5" headset, but if it works for Heckler it should work just fine for this frame.

Ha, trust me I'm not a + fanatic. They are perfect for my rigid ss but beyond that their usefulness, for me, starts to diminish. I've already got a set of wheels/tires and the swingarm clearance wasn't any extra work, so why not? Chances are, I won't even take the time to make a shock spacer so I can mount them on the frame. Chances are even slimer that I pick up a + compatible fork anytime soon. My 6 yr old is getting close to out growing the frame I made him last year and his 4yr sister needs his current bike. Material for kids frames is cheap, it's all the other crap that hangs on it that's hard to do on a string. They are killing my bike budget.

The 140 was a curiosity thing for me. I've never ridden a 29er with anymore than 125 travel. I will build one eventually. It would be kind of crazy for me to overkill my first fs frame for something I really only need 5% of the time I ride. My perfect duo is a rigid 29+ ss and a 120'ish 29 with slightly aggressive geometry (soon to be replaced SB95). I have ridden everything from the local xc loop to lift access with that setup. And like Walt said, the rigid is a blast to ride on 98% of my backyard trails. I just have to resist the temptation to hit those few bigger gaps/drops. But, at the same time, the fs is still fun on the same trails just different.

Walt, what is the overall length of your st and the post? I don't think I'll go that short but I'll probably trim it down to where I only have 100mm inserted on a 400mm post. Which means I probably can't trim it down much, crap. There goes that.


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## Walt (Jan 23, 2004)

Seat tube is 350mm to the bend, then another 125mm. My saddle height is a super-goofy 820mm, though, so the Thomson post actually has to be out past the max insertion to get the saddle high enough. I think I'll probably just use a rigid post, though - this is really basically an XC bike, not a hucking machine. That's the disconcerting thing about FS and plus tires - you feel like you're on a DH rig until you get some air and then you almost crash, because the tire casings just can't handle that sort of thing.

Chainstays are 7/8x.035". The pipe section yoke came from:
ASTM A513 1020 Drawn Over Mandrel (DOM) Steel Round Tube | Buy Online | Cut to Size | No Minimum Order

-Walt


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## mullen119 (Aug 30, 2009)

Subscribed


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## ktm520 (Apr 21, 2004)

Quick update. Made some good progress over the last 4 days. Maybe I do better work on sudafed.

My uncle talked me into cutting the bores for the pivot bearings oversized to allow for shrinkage after welding. I took a guess and cut them .002" oversized. My thought was I'd be better of if they ended up oversized because using green loctite would be better/easier than trying to open up the bores. Well, the damn thing didn't shrink at all and it actually opened slightly where the weld is closest to the ends of the shell. Green loctite to the rescue. I think I could probably minimize the spreading if I do a better job at controlling my weld on those spots.

Other than that, it has gone pretty smoothly. Hope the swingarm continues the theme.

And . . . I found a deal on a Pike that I couldn't pass up last week, so I guess I have no excuses not to try 3" tires.


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## shirk (Mar 24, 2004)

Progress looks great KTM.

A question on your jig. What are you using for headtube inserts and your bb stand-off? I have acquired some 80/20 and planning the same style of jig. Need to make some drawings of the bits I'll need machined and get a friend to spin them up.


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## ktm520 (Apr 21, 2004)

I made some stepped bushings for the bb, but I don't have cones for the 44 ht yet. I just sandwich it between the angle brackets and line it up. Works well enough, I probably won't even take the time to make cones.

I welded the the front triangle last weekend, short of the seat brace which I forgot. It went back in the jug straighter than any frame I've welded but the pivot shell moved more than I would like. In the horizantal plane it is canted 1 degree and this equates to 1mm of lateral displacement at the rear shock mount between extended and compressed. Not a huge deal and this shouldn't kill the shock, but I need to figure out a better process for future frames. I was overly concerned about this when I first realized it, but removed the rear bolt on my SB95 and popped the shock up to find it's misaligned by slightly more than 1mm. I've put a lot of hard miles on that bike and shock show no signs of accelerated internal wear.

I ran into a short set back. The Anvil 142 dummy I have is only for shimano/dt dropouts. He makes a separate dummy for the x-12's and I didn't realize that until I tried to mount them up to it. I kind of got a decent work around, but I'm going to get a x-12 dummy on the way. I could make one, but it's not worth my time right now.

I'm impressed with the Pike fork. Only fork I've ever ridden that was valved well out of the box. However, it's not even close to fitting a Chupa and I'm not even sure a Fatty B would fit. I could of sworn I saw guys running plus tires in the Pike. Oh well, + compatibility wasn't really the reason I bought it and you couldn't pay me to ride a Fox fork. Sooo, plus tires got dropped from the requirements for this frame.


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## seankanary (Feb 19, 2013)

Might be silly, but I found that using 3/4 drive sockets for my headtube cones for 44mm headtubes works pretty well off the shelf.

Duralast/3/4 in. drive 1 7/16 in. socket 75-962 at AutoZone.com


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## DSaul (Dec 13, 2012)

Shirk,

1 7/8" trailer hitch balls work pretty well for 44mm head tubes.








I could make you a set of these if you need them.


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## ktm520 (Apr 21, 2004)

Ready for some swingarm tubes. This is going to be interesting . . . and frustrating I predict.


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## shirk (Mar 24, 2004)

Rear triangle??? 

With a 3 month old on my hands I don't have time yet to work on my own project so I am going to pester you for updates.


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## ktm520 (Apr 21, 2004)

Hey shirk, congratulations buddy!

Made decent progress over the last few days, but the holiday weekend turned out to be bust. I was hoping to get in some solid hours, but we had family stuff most of the weekend. Got a 3 day weekend coming up and my schedule is clear. I'm trying to get this frame done for a local race we put on dubbed "freecross" that is on the 20th. 11 days and counting.

Just got the main swingarm tubes fitted and tacked last night. Wasn't as bad as I expected but I'm taking extra time to get everything fitted as good as possible to minimize movement after welding. 6 more tubes and a brake mount left to go.

I've been thinking about the process alot and the plan is to fully weld in stages as I go. I'm thinking if I fit/tack the whole swingarm at once and weld everything last, it runs the risk of the rear shock pulling in/out and that will be very difficult to straighten. I guess this is where a lateral link between the sides of the rear shock mount would be beneficial.

In my quest for 3" tire clearance, I over looked a limitation of this design. Crank arm clearance at the lower tube limits the tire clearance without an s-bend on the that lower tube. I was still able to get 70mm +6mm on each side. It will probably still fit a Fat B, from what measurements I could find online.


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## D.F.L. (Jan 3, 2004)

I would strongly suggest designing in a provision for a scissor link up top to reduce the side-to-side flex. It will probably flex quite a bit. Don't ask how I know.

To avoid making those links, you could order something from Foes or another company that has their own.


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## ktm520 (Apr 21, 2004)

D.F.L. said:


> I would strongly suggest designing in a provision for a scissor link up top to reduce the side-to-side flex. It will probably flex quite a bit. Don't ask how I know.
> 
> To avoid making those links, you could order something from Foes or another company that has their own.


Can you explain a little more what you mean by a scissor link? Not following.

Wheel and crank test fit tonight. Right half of swingarm shrunk a little so I'll have to take advantage of the Syntace eccentric inserts.


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## Walt (Jan 23, 2004)

Steve means a little rocker to actuate the shock that allows the seat tube (or in some cases toptube) to act as a second bracing point for the rear end for side loads. 

I personally have done bikes like this using headsets and BB bearings for the main pivot and didn't find lateral flex to be a big deal. But that was a long long time ago. I bet it'll ride great, and it looks fantastic!

-Walt


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## ktm520 (Apr 21, 2004)

Thanks Walt!

A rocker driven shock link won't work with this design. It's a true single pivot.


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## Walt (Jan 23, 2004)

Yep, I got that. I think maybe Steve meant in the future on other bikes?

As an old rider of Super8/Bullit/etc type single pivots back in the day, I've never had a problem with this sort of setup as long as the pivot is built right. As I said, I'm guessing it'll rock.

-Walt


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## D.F.L. (Jan 3, 2004)

Here, look at the top pic: http://forums.mtbr.com/foes/fly-problem-swing-link-635917.html

(OK, horrible title for something I'm suggesting you adopt, but it's the first pic I could find. (; )

The link doesn't have to actuate the shock at all, it just controls lateral movement from flex. I recommend it because it's probably not too late to incorporate it into your design.

You're doing great. As satisfying as it is to build your own hardtail, making a from-scratch FS bike is another level of cool.


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## Walt (Jan 23, 2004)

Oh, neat. Regardless, I doubt it's needed here, though. Beefy main pivot bearings = rides just fine, at least for the frames I've done (and all the old Santa Cruz's I've ridden - I had a Super 8 with v-brake mounts and no disc tab back in the day!)

-Walt


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## ktm520 (Apr 21, 2004)

D.F.L,

Ahhh, have never seen that before. That is a cool mechanism, however this current design is probably short on room for that. I'm committed to this design at this point. We'll see how it goes. I'll put that in my pocket for the next revision. It wouldn't be all that hard to make. Shirk, take note.


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## D.F.L. (Jan 3, 2004)

They can also be placed beneath, from the st to the swingarm, but the length of each link may need to longer, all the way up to each being approx. 1/2 the shock stroke, if you know what I mean.

WAY back in the day, Arrow had a 24"-wheeled (because smaller wheels were the future!) FS bike that was a single pivot. It used some sort of teflon pads on each side of the seat tube to rub the swingarm and limit lateral flex. From before cavemen invented the linkage.


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## shirk (Mar 24, 2004)

There is no more lateral flex in my single pivot without link than there was in my Xprezo Super D that did have a link.

When the writer from Pinkbike that saw my bike at Crankwerx did the classic step on the side of the crank to look for lateral flex he was very surprised at how stiff it was. 44mm pivot shell with 1.125 pivot axle is a burly pivot.


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## D.F.L. (Jan 3, 2004)

OK, I take it all back.


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## Walt (Jan 23, 2004)

Ah, yes, the push-on-the-driveside-crank test. Man, fatbikes are really flexy!



Steve, your idea is good. I had never seen that. We all learned something. No need to take anything back!

-Walt


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## ktm520 (Apr 21, 2004)

Walt said:


> Steve, your idea is good. I had never seen that. We all learned something. No need to take anything back!
> 
> -Walt


I'll second that. Always good to have options.

12hrs fri, 15hrs sat, 8hrs sund, and I got it built up late afternoon yesterday and snuck in a quick ride. I'm completely sold on short stays now. The shock/spring tune was a little off compared to how I had it setup for SB95, but a little more comp damping and more spring mid/end and it will be good to go. This is partially due to the slightly regressive leverage curve, but more so to the change in rear weight bias due to the short rear end. It's been a long time since I've ridden a monopivot and it did not disappoint. I like how active they are. This is my main complaint with the switch link on the SB. Rear linkages are no different than forks in that any reduction in friction between the axle and frame results in an increase in sensitivity.

I did not take it easy at all, probably rode it harder than I should have for a maiden voyage, and I honestly can say that it's any more or less stiff than the SB. Let a buddy jump on it who's got about 20lb on me, and he was impressed. He had also never ridden a fs 29 with super short stays. In the model, I didn't really care for how the slammed tt looked, but as built I like the looks and the extra room when throwing it around on the trail.

In my haste to get it built up, I didn't take any detail photo's of the raw frame. Got a few minor details to finish up, mainly cable guides and a cap for the fwd brake post. Man, those brake post are a pita and I've got a Post Punk, but they look so clean. The fit up on the rear post was terrible but at that point I said f it and just welded the damn thing. Surprisingly, it didn't move at all.

Here's a crappy pic


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## thedudeman (Nov 10, 2006)

Looks fantastic! I think I'm most blown away by how fast you work! 

You and shirk have inspired me to go for it on a dually- super cool- thanks for sharing


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## ktm520 (Apr 21, 2004)

This was a boner move, and what happens when you are rushing to throw together a fresh frame and race the sunset for a ride. I installed the angular contact pivot bearings . . . backwards on both sides. The driveside inner race was sitting in the frame when I tore it down to weld on cable guides. It must have happened when I pulled out the pivot axle because there is no way it would have functioned as well as it did all last week like that.

I discovered this sat night at 10pm while prepping the frame for a race on sunday morning. Insult to injury, the punch slipped when I was driving out the bearings and about 6 of the balls bounced across the floor. Luckily I found them all and was able to salvage the bearing.

































It took a few revalves to get the shock dialed, but the more I ride, the more I like it. I tend to push the limits a little farther when racing, must be adrenaline, and it came out of some bad situations well composed. Stiffness it not at all an issue. The only thing I'm not happy with is the cable routing on the swingarm. Should have put more thought into that when designing it. I'm hoping to get an internal dropper soon, so I'll leave zipties on the tt for now.

I'll take some more detailed photo's a post them up soon.


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## Walt (Jan 23, 2004)

Fantastic work!

Man, I love this forum. So much awesome stuff getting built, so little drama.

Except when Feldybikes questions my infinite wisdom, of course. I hate that guy.

-Walt


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## cable_actuated (Jun 7, 2012)

Awesome work. 

All these successful full suspension builds have gotten the gears turning my head. Might have to go full-Enduro myself.


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## ktm520 (Apr 21, 2004)

I've been riding this thing for the last 3yrs without a single issue, other than poor geometry choices, but at the time I didn't know any better. Have ridden it on everything from xc to double black. I just finished a new dually, and I'll post that shortly, but this one is being handed down to my wife. Maybe I'll get it powdered for her . . .


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