# Finally building that 36er



## TLKD (Mar 29, 2010)

Hi all,

after putting the 36er project described here https://forums.mtbr.com/frame-building/36er-questions-design-ideas-742095.html on the backburner for ... a WHILE, I took a few hours to get back at it and threw a couple of lines at the project since I can't get this 36er idea out of my head. Redesigned quite a bit of stuff, what stayed s what I really wanted.

Here's the 2D geo drawing :









3D sideview:









ISO









ISO








That upper render is with an old Race Face Forged alu crankset that I happen to have drawn about 10 years ago, that I have here and that I'm finally gonna use for this build. I'll have the thing repowdercoated to match the color theme I'll pick.

Custom Lefty details : 
Paragon Tapered Steer + Supertherm 1-3/4" Tube for both the leg and the "bridge", a custom spun part with a tapered section as the housing for the axle. Axle is Aluminum, has a male taper to fit the female, screwed in the back side with a special washer and a bolt. I'll probably add some length to the tapered section too. This is a first draft and I judged this is simple enough to make happen and would work.









According to the calculations I have done this leg out of Supertherm 1-3/4" 1-7-1 will be plenty plentystrong, in comparaison of using two 1-1/8" x .058'' Cromo legs. The steer tube can handle it too for sure. We'll see how it turns out. 36H Lefty Hub is ordered already. Cad says 2.8 pounds for the whole fork complete with the axle, washer and bolt.

Since the rear wheel is slammed pretty much all the way in, I had to use plates in order to clear everything. I also ditched the custom swigner dropouts I designed for 36er V1.0. I used the 12mm x 142mm round Paragon Droupouts for my 27 and a halfer build and loved them so much I decided to use them for this build too. I am no singlespeed rider yet so no need for swigners :









This frame had to have "something cool & unique" since I ditched the twin top tubes... so I decided to reproduce a segmented fork build for the seatstays and weld pre-drilled Paragon bullets at the end of each tubes and run my derailer / brake cable in there. Cables would run along the bottom of the top tube and then enter the stays :









Here's one last shot at it :










Like all my builds, this is gonna take some time to complete. I'm supposed to pick up the old jig that I build with a friend quite a few years ago, we'll see. Next step is to send an email to Walt and secure some tires, and probably get some more .045'' rod and a Curved top tube I guess... !

Thanks for following !

Thierry


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## todwil (Feb 1, 2007)

Thierry Nice rendering can't wait to see it in metal very interested to see the fork work!

Todd


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## Walt (Jan 23, 2004)

You keep teasing us with this stuff! Build this thing, it's going to be awesome!

I'm a little dubious about the 1mm wall single leg fork but since I've never done anything vaguely like that I'll just say, as with any weird project - be careful when you start riding it. My own findings have been that anything thinner than 1.2mm wall (for 28.6mm blades) will fail pretty quickly. That's a totally different animal in many ways (2 blades, not one, much smaller diameter, etc) though.

-Walt


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## Meriwether (Jul 26, 2007)

Psyched to see this thread and get some inspiration for my own 36er build. Subscribed!

I just bugged Walt myself but see that I have lots more research to do here in the forum before starting any cutting.

Good luck and i await further updates!


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## RCP FAB (Jun 15, 2011)

Looks great, but I don't think I would use that fork blade.


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## jcaino (May 26, 2007)

Really like the idea of the segmented design with the cables entering. You might as well route them through the top tube to get there - that'd look super clean. I think it would probably look great with a matching segmented fork, but the lefty is pretty cool, too.


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## Eric Malcolm (Dec 18, 2011)

Cool.

Had a look at the fork blade numbers, looks fine to me. I like to check things like that, and from a simple .7mm wall only calculation compared to a twin blade .058" sample, it matches well, you're using 1-0.7-1mm, so it will be stronger. Sometimes what you see may appear scary, but do the numbers, and reassure yourself.

Eric


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## TLKD (Mar 29, 2010)

*"Calculations"*

Eric, as far as I'm concerned, I am no engineer but I am fortunate enough to work with some "awesome" ones who are willing to answer some of my questions and share some of their knowledge... and then once they're done explaining, they immediatly add : "By the way you're doing all of this at your own risk and I have never told you anything, understand " So I start laughingMAO like No effing problem man  For myself, I can only assume, "recommend", build and test before I can be 100% sure that what I've calculated is good enough ... Pretty much like all the builders out here I guess, most of the designs are based on built and tested stuff. Would a 1-3/8'' Supertherm down tube do for a 300 pounder ? Might do... In doubt ? go 1-1/2, eh...

So as a very very useful guideline for some design improvements on all the stuff I build, I have put up this spreadsheet that allows me to COMPARE tubes to another, like how stonger and heavier is this one compared to that one, kind of thing. This thing automatically calculates weight, moment of inertia, section modulous and the ratio between the weight of the said tube and its moment of inertia and section modulous of every all the commercially available cromo tube sizes. I have hidden most of the lines so we can only have what's useful for this Lefty leg talk here.

Before some of you guys start assuming things off of this very partial chart, I am gonna warn you that, like myself, you're doing it at your own risk too ! Hahahaha !

Here she goes :










What we're after here is the *Section Modulous* column. When calculating a "beam" or deflexion of a beam, this is one of the variable in the equation (that refers to the strength of the "shape" of your section) you're gonna fit in there.

If we assume that a traditionnal 36er fork is made out of two 1.125'' x .058'' cromoly tubes then our "rough" section modulous for the blades of this fork design would be .0492 x 2 = .0984

This said, it means that the section modulous of the single blade we're gonna use *MUST be at least .0984 or greater *to achieve the "same strength" as twins 1.125" x .058''. This is if we're staying with the same material, in this case : cromoly. If we go with an alloy that's 35% stronger, then a guy could choose a tube that's 35% less section modulous and still achieve the same strength.

I wish I had the exact yield and ultimate strength numbers for the Supertherm alloy. I've red on the internet 135 000 to 150 000 psi in yield and 185 to 220 000 psi for the Ultimate strength. With numbers like those you always have to go with the lowest possible strength to "assume anything". If anyone out here was sure about the exact numbers, I'd like to have them.

According to this, if guy was to hand pick a tube to make this fork blade, he'd have 1 logical choice and one "less logical choice" !
- *The "logical choice" *: 1-3/4" x .049'' Cromoly, which is the tube that achieves the .0984 section modulous with the lowest weight. 
- *The "less logical choice"* : 1-3/4" Supertherm 1/0.7/1. What bugs me with this and I only realized it lastnight is the possible constraint concentration where the butt is gonna be.
- *Another good choice* : 2'' x .035'' Cromoly (not listed) but I didn't want to put a 2'' tube there, 1-3/4" is big enough.

I'm still debating which one to go with... or build 2 forks. Might as well do that.

Some other cool stuff about this spreadsheet is the Section Modulous to weight ratio ! We can clearly see that the lower the number, the more strength you get per pound ! Again here, outside diamter plays a big role in the final number.

Another problem would be buckling or the tube tearing off but I turely think it'll be ok, especially with the 1-3/4'' x .049'' cromo blade.

Cheers.

Thierry


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## Eric Malcolm (Dec 18, 2011)

Hey thanks for that. 

I'm with you all the way and have similar expertize help available to me. I hope your chart will be of help to others, as often, this is a grey area. And, unfortunately, I have read all to regularly, opinions that are un-qualified that put down a fine idea, making innovations seem like they are in the realm of fantasy, without fact.

Keep up the good work, this is a good build, and thanks for sharing so far.

I have a fork that I made from 29" blade material supplied by Nova for a 26" build. It is so hard riding and strong that I replaced it with a Smaller Dia version that is much better. If I can get a hold of a lefty hub ( extremely scarce around here ) I have told this fork that will lose a leg !!! and do something along the lines that you are doing. If I recall correctly, it has 1.25" dia at the steerer and tappers down to .875" at the drop-out and is 1.2mm wall at the top down to 0.8mm at the small end. I have already done the calculations.

Anyway, back to 36"......

Eric


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## MrCookie (Apr 24, 2005)

Just thinking out loud...
What is the risk of toe overlap? Bumping tips of shoes on front wheel makes for awkward feelings of falling.
With such a unique design, give yourself some margin of safety... better to overbuild, and back off with the next revision than to have an unexpected catastrophic fail. We have all seen what happens when a fork lets go of it's steerer.


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## TLKD (Mar 29, 2010)

Meriwether said:


> Psyched to see this thread and get some inspiration for my own 36er build. Subscribed!
> 
> I just bugged Walt myself but see that I have lots more research to do here in the forum before starting any cutting.
> 
> Good luck and i await further updates!


Hi there Whit, that's some cool ski gear hauling stuff you're making there. Loving the use of a streamline tube for the elevated chainstay bridges in this particular case.

Speaking for myself here but the only tricky things I ran into when "designing" a 36er are the following : 
-Making sure you clear your toes (slack HTA like (69 deg) may be required, long TTL (like 24.5) and 4'' offset on the fork kind of thing.
-Making sure the length of the downtube is long enough, cauz in order to clear your toes you'll have to add a bit to the top tube lenght, which throws your downtube further forward of course. For the above design I need 747mm... so it's flat out maxed on using a 750mm long DT.
-Making sure you clear the rear tire with the seat tube, lenghtening the chainstay makes this easier. Making it straight and weld it to the DT could be a simpler option than rolling a tube to follow the contour or the wheel.
-I'm guessing the geometry is a little different that anything else in the ways that your BB drop is like 4 to 5'' (depending on where you want it), the top tube must be long enough for toe overlap, Head tube angle slacker for toe overlap again and the fork needs to have a lot of offset to reduce trail and handle good (also helps with toe overlap!), mine has 4''. Head tube would have to be as short as possible to avoid having too high of a handlebar but there ain't much you can do here. A lot o stuff has been done already

Besides that, just cut & weld like the usual bike you're doing and you're done... Sound so easy said that way, eh !



MrCookie said:


> Just thinking out loud...
> What is the risk of toe overlap? Bumping tips of shoes on front wheel makes for awkward feelings of falling.
> With such a unique design, give yourself some margin of safety... better to overbuild, and back off with the next revision than to have an unexpected catastrophic fail. We have all seen what happens when a fork lets go of it's steerer.


Toe overlap is taken care of... unless a guy with size 14 shoes or above jumps on it, then it might be a problem !

I sure don't want that fork to snap !


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## Meriwether (Jul 26, 2007)

thanks, appreciate that info! 
I was trying to figure out what trail figure to get and what offset that'd mean. 
The guy i'm building this for is thinking about a truss fork for this thing. Could be fun, something I've never done before. 
The Lefty fork will be cool to see. Have you considered a double-crown lefty? Then you could get a lower front end not having to use a tapered steerer and an external cup headset? I'm not versed in the math for this but seems like you could also use a lighter tube for the leg? (I'm going to be studying and digesting your chart above for a bit, thanks for providing that.)

If it would just SNOW in the Sierras and get out of this drought I'd be all set on the new fatbike. Stupid weather.


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## Eric Malcolm (Dec 18, 2011)

Thierry

Have you thought of using the fork crown area for mounting the Handlebar? Using the 'head' of a H/bar stem directly would be WOW on this bike if it suits your ride position.

Eric


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## TLKD (Mar 29, 2010)

Meriwether said:


> thanks, appreciate that info!
> I was trying to figure out what trail figure to get and what offset that'd mean.
> The guy i'm building this for is thinking about a truss fork for this thing. Could be fun, something I've never done before.
> The Lefty fork will be cool to see. Have you considered a double-crown lefty? Then you could get a lower front end not having to use a tapered steerer and an external cup headset? I'm not versed in the math for this but seems like you could also use a lighter tube for the leg? (I'm going to be studying and digesting your chart above for a bit, thanks for providing that.)
> ...


Double crown lefty, yes I considered it in the past as per images below:




























Then questionned myself on the need for the double crown... Sure saves about 1-1/4" of handlebar height (in the "bar up" position, even more on the "down position") versus a single crown. When a guy starts to question himself on the reasons why a fork needs a double crown, I guess he ends up saying to him that the suspension components that composes the fork needs to be containd in this space in order for the fork design to work. Not a problem here since it is a non suspended fork. The torque applied to the steer tube, in the case of a standard 2 leg fork versus a lefty has to be roughly the same, although I would totally beleive that the lefty fork design applies some side load to the headtube a little more than a standard fork would (nother good reason to go Supertherm 1-1/2" downtube !). That said, we've seen the standard 36'' 2 leg rigid fork steer tubes handle the task of holding the fron wheel quite well in the past... and those steer tubes are the standard 1-1/8". With the availability of the extra HD tapered steer tubes, there is no more reasons for me to go double crown with 2 pretty complicated CNC parts to make / subcontract.

In my researches, I ended up on those pictures :





































So single crown unsuspended lefty forks have been done before... just not on a 36er, yet ! Just a madder of figuring out the tubes to choose. I guess there is no way anobody would question the possibility of making it happen safely if I was to use a 3" OD x .188" wall 4130 tube as the leg. Where is the "sweet spot" between bulletproof strenght and weight is where the "maths" are useful (if we can call those numbers "maths").



Eric Malcolm said:


> Thierry
> 
> Have you thought of using the fork crown area for mounting the Handlebar? Using the 'head' of a H/bar stem directly would be WOW on this bike if it suits your ride position.
> 
> Eric


No need for this ! Drawing says 40-3/8'' handlebar height from ground if I were to use an inset headset, 35mm stem with a flat bar ! That's only 3'' higher than my current 27.5er is at ! I can deal with this nooo problem, so no need for a complicated handlebar setup like I first drew on 36er V1.0 !

Cheers. Thierry


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## MrCookie (Apr 24, 2005)

Someone save me an hour, and model a double crown lefty springer fork... I have a feeling that'll be next...


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## TLKD (Mar 29, 2010)

*Burner...*

Mr Cookie, don't give me any more ideas, I got already enough to deal with by now... !!!!


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## Eric Malcolm (Dec 18, 2011)

Hey, I know what you mean, complicated H/Bar that is. You've spent 200hours doing the planning, engineering details, played all the options etc, and some jerk makes a suggestion. You're doing real well here. Its your build and I guess there are lots of ideas floating in others minds, so it kind of explodes with creative thoughts because so few have done this kind of build. I'm impressed, and you can pull this one off. 

Have lots of FUN...

Eric


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## todwil (Feb 1, 2007)

Thierry any progress worth sharing yet??


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## ktm520 (Apr 21, 2004)

Love the design! A simple comparison of cross section will probably keep you from busting your grill, but I'd be looking at torsional stiffness and combined bending and shear stress. What are using for your loads? Fairly easy stuff to calculate but the analysis is only as good as the loads/boundary conditions.


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## BungedUP (Aug 18, 2003)

Forgive me if I'm wrong, but doesn't comparison of just the section modulus of the tubes themselves, ignore the effect of triangulation that comes from using a pair of fork blades attached to the steerer? I don't know how one goes about calculating that, but it seems like there is something more to comparing the 2 structures than just looking at the section modulus of the constituent tubes. I'd be interested to know how one does indeed do this comparison - I'd love to have this explained to me, in layman terms.


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## TLKD (Mar 29, 2010)

todwil said:


> Thierry any progress worth sharing yet??


Hey there Todd !

not much done on that thing ! Been suuuuuper busy making and putting together those fully suspended ski trikes called Axis Snow-Trikes here the last couple of weeks. You guys can check our FB page at Axis Snow-Trikes or our Website at axissnowtrikesdotcom. We got a few interesting pics on our FB page.

We're currently touring British Columbia with those things these days and so far it's been a absolute blast riding them. They perform like a DH bike going down, they basically are a full squish mountain bike and the trail isn't just a single track, it's EVERYWHERE you wanna go. The 3 ski fully independant suspension design allows for so much control to happen it's crazy. Steep and choppy no prob. Here's a pic of yesterday's ride in Revelstoke, BC









So, that in my plate right now, nothing done on the 36er yet, besides receiving the tires. The box containing them was probably 15" x 15" 12" !!! Those things are just HUGE ! This 36er is the next in line after a one ski snowmobile build I gotta finish here. Can't wait to get at her !

Cheers.

Thierry


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## todwil (Feb 1, 2007)

todwil said:


> Thierry any progress worth sharing yet??


 Ok its been four months^^^^^


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## MrCookie (Apr 24, 2005)

Mostly you focus on the weakest axis. 

In the case of a two bladed fork, fore and aft will typically will be weaker (assuming straight round blades and ignoring the fork crown).

Also, the direction that the wheel rotates... I am pretty certain that most blade breaks happen when the front wheel runs into something inanimate. So when comparing 2 forks using the same material for blades, a fork with 2 blades will be able to withstand 2 times the load vs a fork with a single blade.

I am sure that is pretty obvious.

What gets tricky with a single blade fork is the moments created by forces that are not in line with the axis of the tube (intuitively they will appear as rotational forces anchored on a fixed/solid joint).

Ultimately, there is more involved than is reasonable to go through in an online posting, but in laymans terms you evaluate the forces in each of the 3 axes and sum the forces in each. Then you look at the material properties (strength of the metal) and the properties of the tube (based on the shape and wall thickness) to see if the tube can withstand the forces (with a margin of safety to compensate for flaws in material, effects of joining the tubes, calculation errors, etc.).

The basics are taught in second year engineering curriculum's, but there are so many factors and nuances that it would be prudent to have a person (typically a mechanical engineer) with experience review the calculations.

I hope I shed a little light on the subject without making an ass of myself. When it comes to taking a fork with one leg onto the trail, there are a lot of bumps and crashes that are hard to conceive, so you have to think of a worst case, and then add a safety factor.


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## chernichovsky (Mar 11, 2011)

Sweet project - hope its coming along well!


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## verytricky (Mar 22, 2014)

Sounds like a great project. I've also noticed a few 36ers coming onto the interweb. Got a bit confused with these two though: the Dirtysixer we know and love, and the Wheeler dirtysiXXer




__ https://www.facebook.com/testcentertoscana/posts/777287228961165



https://de-de.facebook.com/WHEELER....7520000.1413161508./10153215109849466/?type=1

Bit close to home??

Love the concept TLKD et al. Keep them coming please.


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## todwil (Feb 1, 2007)

Cool bike but I think they need to get creative and come up with their own name.......and Payaso is already taken!!!


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## Tall Boy Bram (Jan 12, 2006)

Don't forget to mention my new 36-er. Finally after 5 years it's ready to rumble!

TBB


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## todwil (Feb 1, 2007)

Tall Boy Bram said:


> View attachment 931830
> Don't forget to mention my new 36-er. Finally after 5 years it's ready to rumble!
> 
> TBB


 Hey Bram we need you to start your own build thread with LOTS of pics!......nice bike!

Todd


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