donglow

I have found some good information on the bike forums, but one I have never seen and decided to undertake is shorting the chainstay on an aluminum/carbon bike to make it more responsive to acceleration and climbing. I decided to give back to the forum and share my experience.

First let me share that I am a former bmx parent and big time tinker of bicycle frames. My daughter was national ranked and I tricked her bikes out a lot. We also spent a lot of time training and researching and one thing I found is a shorter chainstay aid in acceleration, and acceleration is 90% of bmx success. We had (still do if anyone wants to buy it) a timed starting gate with a 175’ first straight and I have loads of data to support the theory shorter chainstays improve acceleration.

We had a felt Z womens bike with 650c wheels for my junior road racing girl. I really liked the bike except I felt the chainstay were too long. Felt says they are for more comfort, but for kid who spent three years on a 12” bmx chainstay, the handling was way too slow. I looked for a long time trying to find a frame geometry I like and at a reasonable price with close to the same quality as what she had, and could not. The seatstays of this bike were carbon fiber and the chainstays scandinavian aluminum. After much reading, plus a few years of playing with carbon fiber, I decided to shorten the whole rear triangle and use carbon fiber to hold it in place. Here are some pictures of the process. She has raced and road it now for about a year with no problems. I used West Systems Aluminum Etch on the bare metal, 80 grit sand paper to prepare the aluminum, painted areas and between epoxy layers, acetone for the paint and between layers of epoxy (carbon fiber), a thin fiberglass matt first layer to keep cathodic corrosion from happening between the aluminum and carbon, Carbon Fiber Plain Weave 3K 5.7 oz cloth, and System Three General Purpose Epoxy. For reading I referred a lot to How I Made a Carbon Fiber/Epoxy Composite Bike in my Garage, by Damon Rinard and Carbon Fiber Q & A. Without these I don’t think I would have felt like I knew enough to do this project.

It came out very functional, but some things I would have done different are: 1. use carbon fiber tape rather than fighting weave cloth trying to unravel, 2. Not mask off the ends. Masking made a layer of epoxy that I had to cut and to cut it I had to go through the paint and into the aluminum a little. Just taking the epoxy to the painted surface and wiping excess would have worked better. 3. A black powder color additive in the final surface coat of epoxy. Clear coat with wrapping of the seatstay when it comes into contact with the seat tube, makes ever imperfection of the underlying fabric (which isn’t straight) stick out when light hits it. The light goes through the clear epoxy and accents the carbon fiber.

All in all it did just what I wanted, although the frame probably doesn’t have much resell value because of cosmetic imperfections of the job. But she should be on the frame a for at least 4 more years, and it did not cost much to do. More road races and triathlons (the sport she shows the most promise in) are coming.

CliffordK

Interesting project.

I'm not sure I would have done all that work to take out an inch or so. I've been having a tough time with welding aluminum, but an option would have been to just get it welded (and perhaps sleeved).

5 year lifespan for a "junior"? That seems to be quite a while for someone who is growing. I would think that she would be ready to jump to a short 700c bike by now which would open a lot of possibilities.

donglow

Thanks for the input. I grew up in Eugene and went to college at U of O. I’m not going to comment on the worthiness of taking 3 CM out of the chainstay length. That can be debated forever, but because I came to conclusion it was worthwhile is the reason I choose to do it, and I can say I believe handling changes I was seeking have happened.

The chainstays change in diameter, oblong shape, and width from the bottom bracket to the dropouts. Fabricating sleeves for the chainstay would have been prohibitly expensive and time consuming. You can see from the bottom left picture the carbon fiber molded to make all three of these shape changes happened. Plus scandinavian aluminum is considered one of the hardest to weld. Also it is butted and extremely thin in the middle of the chainstay. If welding had been possible, then for it to have strength it would have needed to be heat treated which means I would have been looking at preparing and repainting the whole frame.

Then the seatstays also had to be dealt with. To keep the triangle so the dropouts are in the same horizontal plane, the seatstays must be shorted too and I decided to keep the triangle angle at the dropout the same and shorted it where it intersected with the seat tube, thus moving it down the seat tube about 2cm. I spent about $100 and 8 hobby hours by doing it in the method I described.

You can see by looking at the right bottom picture of the bike on the trainer, the seat post is turned around to shorted the effective top tube. If you look closely you can also see the cranks are kids bmx cranks considerable shorter than the 165mm cranks which come on the smaller of adult and many junior bikes. This is because at age 10 she is still 5” shorter than the smallest Felt says can fit this frame and she can grow 5” above that before she too large for the frame. As she grows, I plan to change the crank length, which means the seat height does not have to change much, change turn the seatpost back to where it was meant to be, put the original stem (longer) and bars (wider) back on. This is why I estimate she will have approximately four more years on the frame.

Although the 700c bikes do add a lot more possibilities, they also have the drawbacks of not accelerating as well as the 650c, toe overlap problems on small frames, and with 650c losing popularity among triathletes, after being very popular for years, harder to find as good of high performance used wheelsets as the 650c. There are currently some steals in the used market for performance 650c wheelsets.

duanedr 

Well, I don't know enough about carbon fiber construction and I can see some things that worry me but, i'll give the benefit of the doubt and assume you did the right things to mitigate the worries I have (which may be valid or not!).

I will say, it's amazing what a dad will do to help a kid feel good on a bike. This is for my 7yo. He loves MTBing.

unterhausen

cool bike, are those 24" wheels?

duanedr 

Thanks! Yes. it's for our 7yo. Hydraulic brakes, 140mm discs, super light rims and spokes with red/silver/blue alloy nipples and XT hubs. I have some XT and XTR stuff that will go on it.

Lots of pictures on my flickr site of the framebuilding part. It's currently hanging in the garage waiting to go to the painter. He won't be tall enough to ride it for another few months anyway. It's purposefully light with a 3/4in top tube and 1in butted top tube as a downtube. I used the slimmest parts of slim seatstays to build the wishbone rearend etc. He only weighs about 55lbs and won't likely add much in the next few months so, I figure why not. The one place I didn't skimp on material to save weight is the fork.

There are a few places i'll keep an eye on for cracks and if we develop some, i'll build another. There are some things I don't like about this build. I'm sure the next one will be perfect!

https://www.flickr.com/photos/543195...7647295218844/

donglow

The bike has had no problems. Been riden and raced many times. The problem is the girl grew 8 inches in two years and now is too big. Even changing the seat post and stem only lasted 10 months. My 5 year prediction was about 2 1/2 years too long!

Brian25

My take away on this is that you are an amazing parent, to be so into working/ upgrading your girls bike for her.
The reason that they make the chainstays long on the 650 wheeled bikes is as a matter of chain line angle. (probably does not matter since only one chainring.) Shimano does not recommend chainstays shorter than 40 cm, I believe. BMX is a little different than road riding, I would have tried to talk you out of the shortening. If she were into velodrome racing, specifically the kilometer, where you push the bike up to speed from a dead start like BMX, that would be important to have a bike with short chainstays.

duanedr 

Ha, I realized after reading my (lengthy!) response that I didn't technically answer your question. They are 24 x 1-3/8.

After a few rides, I could see the steep geometry (mistake) and light narrow rims/tires were causing the handling to be very twitchy so, I re-laced the hubs to wider "normal" 24" rims and used wider tires which slowed things down a bit. He's still riding it but the front center is too short at this point. I'll start on a new one in November when I finish my current project (650b commuter). I may bump him up to 26" wheels.