Homemade Trials Frame (pics)
 

Hey guys. I'm a 4th year at Cal Poly San Luis Obispo. I took singletrack vehicle design last quarter, where we design and build a bike. I wanted a trials bike so, I worked off of current designs and built this.

The parts are all taken off my dirt jumper mountain bike:

http://i114.photobucket.com/albums/n...IMG_0455-1.jpg

Got the tubes from Aircraft Spruce. I HIGHLY RECOMMEND them. I ordered these tubes online around 2pm on a Wednesday, and had them by 10:30am Thursday (the next morning). They will even cut your pieces to the nearest foot, which is really cool.

I did some rough FEA/stress hand calculations and 1.5" x .049" gave a pretty good safety factor, so I went with that for the main triangle (down tube, top tube, seat tube). I ballparked 1" x .049" for the rear triangle (chainstays, seatstays). I bought the headtube and bottom bracket from Nova Cycles to make sure a headseat would pound in and my existing bottom bracket would thread in.

http://i114.photobucket.com/albums/n...d/IMG_4091.jpg

I then started cutting and taping the mitering templates onto the tubes at the proper lengths. This took much longer than expected (about 6 hours). In order to make sure the cuts on each tube were in line with each other (around the axis of the tube) I made center lines on the tubes. The poor man's way to do this is to drag a straight edge across two similar tubes lying next to each other on the floor, just make sure they don't rotate at all while making the mark. I got the mitering templates from here.

http://i114.photobucket.com/albums/n...d/IMG_4185.jpg

http://i114.photobucket.com/albums/n...d/IMG_4186.jpg

http://i114.photobucket.com/albums/n...d/IMG_4189.jpg


The first day in the shop, I cut all of the miters on the main triangle. The tube notcher in our shop is a little sketchy. It is basically just a big belt sander with interchangeable rollers to make the right diameter notch. It works well for a cut or 2, but blowing out belts was way too common, so I cut the rough miters with a hack saw, and then cleaned them up with a hand file and/or a pneumatic angle grinder. It took almost 8 hours to finish all the miters.

http://i114.photobucket.com/albums/n...d/PB063822.jpg

http://i114.photobucket.com/albums/n...d/PB063824.jpg

http://i114.photobucket.com/albums/n...d/PB063825.jpg

Head tube joint mocked up.

http://i114.photobucket.com/albums/n...d/PB063826.jpg

You can see the gap between the top of the down tube and the front of the seat tube in this picture. That junction was hard to cut because there are 2 miters superimposed on each other for both the bottom bracket and the seat tube.

http://i114.photobucket.com/albums/n...d/PB063828.jpg

I had a long weekend the next week for Thanksgiving, so I brought my stuff home to work on it in my garage and have my Dad help with the construction. One of the more challenging parts of the construction was keeping everything aligned. I didn't have access to a jig, so we kind of just did our best with what we had.

We did the initial assembly flat on the bench.

http://i114.photobucket.com/albums/n...d/IMG_1814.jpg

http://i114.photobucket.com/albums/n...d/IMG_1816.jpg

Once we got the main triangle tacked together, we slid the fork in the head tube to get the frame sitting at the right angle (the rear axle is horizontally back from the bottom bracket, so this was easy). Then we put a 1x2 tube across the down tube and clamped it to the bench to try and keep the frame from moving too much.

http://i114.photobucket.com/albums/n...d/IMG_1818.jpg

I didn't do any design work on the rear triangle, I basically just wanted to build something that would fit. This is what I came up with, and I think they turned out pretty good. One thing I would have liked to experiment with is making shorter chain stays to bring the rear wheel closer to the seat tube.

http://i114.photobucket.com/albums/n...d/IMG_1822.jpg

I did the same thing with the seat stays, just sort of working my way from the rear axle up to the seat tube.

http://i114.photobucket.com/albums/n...d/IMG_1830.jpg

At this point, the frame was completely tacked together, so we slid (I wish, more like wedged!) the rear wheel in the dropouts.

http://i114.photobucket.com/albums/n...d/IMG_1832.jpg

Once I was sure everything was the right size, I welded it up using my Dad's MIG welder. I had to pulse my way around every joint because the material was pretty thin, so it took a while, but it turned out pretty good.

http://i114.photobucket.com/albums/n...d/IMG_1836.jpg

http://i114.photobucket.com/albums/n...d/IMG_1839.jpg

Close up of the chain stay/seat stay/dropout junction.

http://i114.photobucket.com/albums/n...d/IMG_1840.jpg

Rear triangle

http://i114.photobucket.com/albums/n...d/IMG_1841.jpg

Beginning assembly

http://i114.photobucket.com/albums/n...d/IMG_1847.jpg

Most of the way put together

http://i114.photobucket.com/albums/n...d/IMG_1850.jpg

http://i114.photobucket.com/albums/n...d/IMG_1851.jpg

Personal touch (N for Neil)

http://i114.photobucket.com/albums/n...d/IMG_1852.jpg

Thanks for the help pops

http://i114.photobucket.com/albums/n...d/IMG_1854.jpg

If you have any questions, feel free to throw them out, I love to talk about this build.

Here's a shot of what it looked like completely assembled

http://i114.photobucket.com/albums/n...1129101138.jpg

That full on kicks ass.

dave

That's a lot of welding for not having a bender. I like that you didn't let that stop you. How does it ride?

Yeah that rocks!

Nice! Great project!

It was a lot of welding for sure, but that really didn't take too long. Deciding where and what angle the cut the tubes at took forever though. I didn't have very much extra material either, so it was pretty crucial that I get it right the first time (or at least miss long).

It rides pretty good for the most part. My detail design called for chain stays that were almost an inch shorter than what I could build, so it's kinda hard to get the front wheel off the ground (especially for a trials bike). Other than that, I'm really happy with it. It's held up to quite a bit of abuse so far, and it's kind of turned into my city/urban mess-around bike. It's pretty light so it's fun to just mash around downtown jumping curbs and stairs and stuff.

Thanks for all the comments guys!

I think u did great!!! COngrats! :) I envy you :) Havent been able to built cr@p ever hahaha :)

Good work :)

Awesome build, great pics! I live out by Morro Bay. Does Cal Poly offer that class to old guys (like me?) I have a stable of bikes. I don't really need another one, and I doubt I could build anything as good as what I have, but the idea of riding something I built myself is really appealing.

I'm not sure what the policy on taking just a few units of classes is. With this particular ME class there was a lot of math the first half of the quarter. We learned all of the kinematics behind a bicycle and analyzed the effects of geometry changes on the handling. It's probably not the greatest class to take unless you've had some previous engineering experience. We also have a welding class that's pretty cool listed as IME 142. I know that Cuesta does a metal shop/welding class too, so maybe someone there could hook you up with a frame jig or something.

I can't tell if you ever finished welding the rear disc tab.

I left it the way it is in the pictures. All the clearances are so tight around the calipers and disk that there must have been some kind of interference issue, I don't really remember.

Getting the tab on there in the right position was one of the harder tasks. It's not trivial, and there's not a lot of room for adjustment. If I did it again, I'd either slot the holes in the mounting tab or buy ones that had slots.

Nice project! And the fun thing about trial bikes is, they really test your welding skills :) Happy riding!

UPDATE:

Surprisingly, this bike has held up perfectly for a few months of riding. I've been doing some pretty heavy (for me) trials stuff, with drops of up to ~4 ft. urban riding (just mashing through traffic and curbs, looking for something to potentially jump). The frame is actually really really stiff, which I wasn't really expecting. Nice surprise though. I haven't felt any flexing don't see any fatigue stressed areas, so I think it will probably last a while. For whoever's interested in frame materials:

Rider Weight: 155 lbs
Frame Material: 4130 Aircraft Cro-Moly from Aircraft Spruce (very very highly recommend dealing with these guys)
1.5 x 0.049" Tubing
90 ksi Yield Strength
Safety Factor (worst case loading): 3.0

Glad to hear that the bike held up well. How heavy is the frame?

I couldn't agree more. I visit their Corona, CA store to pick up tubing, and they are really great to work with. I wish more businesses were like them.

I don't have a scale, but I just rigged up a balance using a 2x6 resting on a piece of spare tubing like a teeter-totter. Put the frame on one end and 23 oz of water in a light cup on the other end and found the balance. Turns out the frame is about 4.5 lbs without any components on it at all.

Sweet frame bro! Thinking of making a 4130 trials bike myself this year, with that fork? Thought it looked good on ebay but wasn't sure it would be strong given its weight. How is it holding up?

Looks good, I'm curious to see how mig'd joints hold up!

I'm the guy who made the comments about the amount of welding. Good to hear you have been riding it and having some fun. 049 is really stout, so you shouldn't have a problem there. The rear drops seem very thin to me, but then maybe you did some serious math brain stuff on them.

The fork is holding up just fine. No problems to speak of, really. All the welds, tubing, joints, etc. have all held up!