# WMW - 2nd frame - 1st TIG welded



## abarthx19 (Jan 13, 2008)

Hi,

After the best part of two years chipping away at it I've finally completed my second frame, and thought I might share it with you. Hopefully it'll be of interest to some of you, especially those into full suspension design. Apologies in advance for all the text, but there are some pics in there too!

It's another steel full suspension downhill bike but this time TIG welded. Having enjoyed 7 days back to back riding of my first frame (yellow bike below) in the French Alps a couple of years ago I was keen to make a second frame and try to develop my fabrication skills, address a few issues from the mk1, and try out a few ideas.









Lessons learnt from the mk1, and future aims included:

Simplify the construction - reduce the reliance on bolts.
Lighten - reduce size of linkages, and use of bolts.
Longer front triangle, and shorter swing arm / chainstay length.
Slacker head angle and reduced suspension travel.
Review cable routing - decent routing is a black art.
Keep singlespeed setup, basic suspension layout and mud clearance.
TIG welded instead of fillet brazing - I don't like the responsibility of handling acetylene tanks.

Design / geometry ideas (to test):

160mm travel front and back.
65 deg head angle, with rear travel path matching front as closely as possible - effectively also 65 deg (with 7mm deviation from parallel path at mid point due to arc).
17" chainstay length at 30% sag to lesson effect of reward axel path.









Construction:

Paragon 44mm head tube. 
Swing arm, seat tube and seat tube brace is all straight gauge chromoly. 
Reynolds 853 top tube and down tube. 
Chromoly sheet / aluminium plate sandwich construction for dropouts and disc brake mount.
BMX mid bb shell for main pivot.
7075 Aluminium for rocker and linkages, pivoting on skateboard bearings.









Its early days but my initial impression (so far based on a limited number of runs) is the new bike rides quite similar to the mk1 - which isn't a bad thing as I had so much fun riding that. Its weighing in at 40.9 lbs compared to the 46lbs of the mk1 so about 2kg lighter which is a move in the right direction, and appreciated on the push back up hill! The swing arm is a little wider so can rub a bit on the calfs, but I tend to ride with a bit of a bow leg stance anyway on dh stuff so not too much of a problem for me.









One thing I noticed when I was giving the frame a once over to clean and check for cracks etc, is that when you load up the frame laterally by putting weight on the pedals and pushing sideways you can see quite a bit of flex. Its probably a result of only being able to fit in one swing arm cross brace (I've designed myself in to a corner on this frame as I can't add a secondary cross brace - something to address next time). Which is a bit of a shame, but I didn't really notice it whilst riding (wasn't looking for it either to be fair) and could have been hidden by the muddy/slippery conditions on the day causing the rear to step out, especially on the wet roots! Maybe a hard packed/rocky track would expose this a bit more? I don't think a bit of flex is a totally bad thing, and its certainly nice to have a bit of 'twang' on a hardtail.









I want to give it few more sessions to dial in the shock, and then if it looks ok I'll strip it down and get it powder coated, as having to keep rust at bay with WD40 is a bit of a pain and my attempt at a rattle can paint job on the mk1 was a bit rubbish in terms of durability.









I hope my little project has been of interest. And thanks to the forum for the advice and feedback along the way, especially regarding TIG technique.

Cheers,

Mark


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## TacoMan (Apr 18, 2007)

How does it ride over sharp ruts? It looks like it should soak them up pretty good with the angled wheel path. Vertical wheel paths are good at big drop offs, but not when hitting deep ruts at speed.

I don't think another brace would add a lot of swingarm stiffness. Does the front attachment link float or does it add some stiffness? I think you need to make the connection between the pivot and the wheel as torsionally stiff as possible by going to large cross section tubes. Or make the front link super rigid and increase both tube sizes. Maybe even stress-skin the whole arm like the old Cannondales and Mountain Cycles. Also anything you an do to spread the two link points farther away from each other.


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## Feldybikes (Feb 17, 2004)

Homemade DH bikes are rad. I'm usually more interested in crazy FS bikes at the NAHMBS than super carved up lugs.

I used to be into the "push sideways on the pedals to check flex" test, but I'm not sure it translates into anything meaningful for the ride.

You might consider throwing on some cable guides _before_ you get the thing powder coated.


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## sasquatch rides a SS (Dec 27, 2010)

Wow, that really is awesome looking. I'd love to ride it!


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## ferday (Jan 15, 2004)

looks a lot like my balfa bb7, especially pivot location....i'll bet i have a good idea how it rides 

awesome work man, thanks for sharing and i agree with feldybikes, there's something pretty special about a DIY FS bike.


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## Mr.Magura (Aug 11, 2010)

You could gain a fair bit of stiffness in the rear, by running a wider rear hub. The stays in the swing arm, are almost parallel, which does little good for the weight to stiffness ratio in general. 
At least it would be an easy fix.

Magura


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## abarthx19 (Jan 13, 2008)

Hi,

Thanks for the comments.

TacoMan: 
The trails I’ve so far ridden haven’t aren’t all that rocky, they’re more mud and roots.
The original frame was pretty good at covering rough/rock ground and seemed to feel better at speed or rather I maybe appreciated it more, so I’d expect this frame to be similar. The rear end of the old frame certainly used run freer when off the rear brake. Under braking the suspension used to be noticeably less compliant/sluggish – maybe a high single pivot thing?

The rocker arms moving the damper unit are connected by a brace/rod to help reduce any unwanted twisting loads being transferred to the damper. The linkages connecting the swing arm to the rocker are effectively floating, and maybe subject to some twisting. I did look at trying to run a brace/rod between these linkages but its really tight for space in there and would most probably clash with the piggy back – though I might have a another look at this option.

Ideally I’d like to do away with the linkages and simplify the design to just the main pivot as it was a bit tricky to get everything to align to ensure they work/swing correctly. Initially this was my intention but the software programme (Linkage) I used was predicting a massive falling rate with this geometry, hence the use of them. I’m tempted to have a go a making a more ‘rough n ready’ frame (i.e. all cheap/thick wall chromoly) for the next attempt to see/feel how bad it really is – except it probably won’t be that nice to ride but that it’d be more good building experience. 

Fledybikes: 
Cable stops would finish the frame off better, but I don’t own a gas set up to braze the guides on. Plus I don’t want to put any more heat into this frame… its take more than enough already to get to this stage! So I'm going to make do with electrical tape or zip ties.

Mr Magura: The swing arms should be parallel on this design (in reality they’ve pulled in a little from welding). They were designed this way on both the mk1 and mk2 to work with the 110mm spacing of the BMX hub to allow the transfer sprockets to be inboard of the swing arm whilst (just) clearing the cranks. I think next time I would try for a regular 135mm hub and position the transfer sprockets out board of the swing arm which would also triangulate it in plan.

Cheers,

Mark


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## TacoMan (Apr 18, 2007)

Did not notice it before, but yeah braking is not going to be very good with the wheel path. I would make a floating brake mount and run the tie rod up to the frame horizontally, that should take care of it. That's what I run on all my FS bikes regardless of wheel path, and braking is excelent.


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## bmxplorer (Nov 11, 2007)

Nice Full Suspension Frame!

You can attach cable guides by tig welding them to your frame as alternate method to brazing them.

Thanks for sharing your craftsmanship, that is really clean how you integrated the main pivot and jackshaft.


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## customfab (Jun 8, 2008)

bmxplorer said:


> Nice Full Suspension Frame!
> 
> You can attach cable guides by tig welding them to your frame as alternate method to brazing them.
> 
> ...


Might want to chase the pigeons out of your shop before you fire up the welder next time.


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## thehammertownhead (Jan 23, 2011)

Nice work!!!

Cool to see a FS build here....


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## jay_ntwr (Feb 15, 2008)

Great project and thanks for sharing so much of the detail.


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## Hack On Wheels (Apr 29, 2006)

This is awesome, I was looking at your first bike a couple of weeks ago. Very nice.

Would you mind explaining how you did your bearing selection for the main pivot and linkages? And what are you using for the eccentric BB? I've been chipping away at a BMW inspired design but I'm finding it to be a bit of a design challenge.

One more thing - what did you do for a jig? Making sure the pivot locations are close enough in position and in plane to not cause binding or premature wear seems like quite a task, but I don't have any hands-on experience with full suspension frames.

And again, awesome work!


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## Yogii (Jun 5, 2008)

Show us pics of your Abarth X19. Bro had a racing Abarth 850. Always broke down...


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## abarthx19 (Jan 13, 2008)

Hi Hack On Wheels.

The main pivot is effectively a BMX Mid bottom bracket. The chromoly shell was sourced from the bike manufacturer Solid BMX, and the Mid bearings were for a 19mm axle.

The eccentric bottom bracket is a Bushnell ebb and chromoly shell. Nova Cycle Supplies Inc, sell them as a set or separately.

The bearing to the linkages are sealed skateboard bearings, as they are pretty easy to get, and I'm sure I had seen them used on other bikes before.

In terms of construction and jigs I broke the process in to sections/sub assemblies using a mixture of mdf/ply and extruded aluminium sections to create / bodge together jigs for each task. This was the only way I could get my head around the problem. My method for the swing arm sure threw up a few challenges - especially trying to get the main pivot, linkage pivot and rear axle in the right positions when the two sides of the swing arm were slightly different post welding - answer: make an alternative dropout to one side to accommodate the difference.

Below are some pics of the various jigs/subassemblies used (roughly in order of process):

Mdf swing arm jig (one side at a time):







Dropout jig (metal and plastic shims featured quite heavily throughout the different stages to get the correct setting out):







Checking symmetry of swing arm sides - making sure pivot points and axle line through, mismatch identified (see shim under dropout where there should be no gap between tube and mdf - solution was to make an alternative dropout to accommodate the difference):







Swing arm jig (dummy axles at pivot and dropouts set perpendicular to extruded aluminium section. Aluminium flat bar to either side of swing arm sets out axle spacing, help ensure they are parallel):







Rear brake mount jig:







Seat post / main pivot / EBB jig (the seat post is the backbone of this design. The jig is to ensure main pivot and ebb are parallel and true to one another as possible as clearance between cranks and swing arm is very tight):







Main jig for front frame (extruded aluminium sections, heat sinks stay bolted in to head tube / pivot / ebb shell if frame is removed after tacking):







Shock mount jig (to position shock mount fixing bolt in correct position relative to main pivot and linkage pivot):







Geometry jig (head tube had pull in during welding effectively steepening the head angle. This jig allowed the linkage between the swing arm and rocker to be adjusted slightly to attain the desired head angle).







I hope the pics / description are of help to you. It's probably not the best system, and there's a lot of room for improvement but it got me where I wanted to be. The jig for the main frame was a bit of a pain to set up correctly, and I'm sure the swing arm construction could be simplified or made easier.

Yogii - sorry I don't have an Abarth X19. I wish I did though! Really like the look of that car years ago hence the name.


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## mickuk (Jul 6, 2007)

Fantastic homebuild! Some serious work went into that frame.

How did you find tig vs brazing with regard to distortion of such a complicated structure? 

Did you have prior tig / welding experience? I'm sort of in the same situation regarding gas bottles at home - brazed a few frames but frustrated having to drag everything back and forth to use gas bottles after hours at work. I spent this afternoon filing brass fillets and have a couple of hollows that need 10 mins touching up with the torch - so job stops until monday.


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## abarthx19 (Jan 13, 2008)

Hi mickuk,

I found I had more localised distortion with the brazed frame. The head tube ended up a bit banana shaped, and the bb shell was slightly less than circular when I had finished. When I brazed that one I was outside during a really cold christmas break, and was struggling with heat control partly due to the cold and partly due to being out of practice. I found the swingarm to be the hardest part to weld due to the tight angles, number of joints and not much space between tubes. I was quite suprised with how much difference there was between the two sides of the swingarm when I first put them next to one another.

Following a redundancy I managed to get on a vocational retraining scheme and learnt the basics of tig welding - mainly fillet and butt joints on sheet metal. That was just over a year ago, then its just been a case of practising loads on mild steel tubes and working up to thin wall chromoly, getting advice off the experienced forum members and then practice some more. Still got loads to learn but thats a big part of the fun.


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