Geometry and Fit Resources

From the perspective of the Education forum, there is no right or wrong. Here you can find a collection of resources on bike geometry to form your own opinion!

Feel free to suggest articles below and I will update the main topic.

Forum Threads:

Articles:

https://www.rodbikes.com/articles/700-compromises.html

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Thanks for putting all these resources in one place.

I’m designing a bike for a smaller rider now. He wants a similar geometry and feel to his 51cm Cannondale supersix, but with the ability to run up to a 38mm tire, (but more likely will use a 35mm). The problem I’m running in to is that the two carbon forks available that he prefers to use (Enve AR Disc or Columbus Futura Gravel) are both available in only a 47mm offset version and it’s creating a toe overlap issue and a somewhat higher trail number than desired (although 66mm is not bad).

Can anyone give me some insight into the best way to remedy this problem for a smaller rider who wants a lively handling bike without toe overlap and the ability to run a larger tire?

And also here is the same design (with .5 degree more slack HT) overlayed on top of the supersix geometry.

Adding this here:

It’s kinda long, some of it is wrong, and they could really learn a thing or two about data presentation, but the conclusion they come to is interesting. It puts some of the information sprayed by a particularly loud guy on the internet in context. I’m fairly certain the author is an actual engineer too.

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You can use a shorter stem and a longer front center to push the front wheel out too. I’d rather do that than make a really slack front end. Either way avoiding toe overlap is a good goal.

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I echo using a shorter stem. The benefit is it adds a bit more stability by increasing the wheelbase a little which on little bikes makes a big difference. The trick you’ll have is talking the customer down from the edge about using a particular fork when it paints you into a corner.

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Crazy solution: smaller wheels. Won’t work if they wanna race it if there are wheel size regulations though.

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if you go to 1.5 tapered, the no6 road fork has short axle to crown and clearance for 38’s

I agree with you, the fork offset availability is annoying. BIgger brands like Cervelo can even make a 55mm offset fork for their smaller sizes.

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We found the No6 fork option a few days ago and decided we’ll use it as it’s a much better fit for the frame. Also, I was emailing with No6 about availability and they let me know they offer discounts to frame builders off the MSRP prices on all their ready to paint forks, so that helped with the decision as well!

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Luis sent me this link because I didn’t know that 5mm here or there really makes a difference…

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A few years ago, I made a bunch of kids track bikes (48cm) for the local velodrome. They had several that used 650c wheels kids always wanted ‘adult wheels’. I did my best to balance it all out but regret not using the smaller wheels. We ended up using 70mm stems, 75* STA and more STA and rake than I would have liked for a track bike. They are still using them but I know they could have been better with smaller wheels.

I don’t know why folks have such an aversion to using an appropriately sized wheel. Anyway, it’s not just about geometry, it’s about all of the other choices we make as bike makers.

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I think scaling wheel size to the closest commercially available option is the go!

Out-of-the-norm shape riders are the best candidates for custom frames. Makes for an interesting design challenge.

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Interesting thread. Thanks for sharing everyone. I’ve searched far and wide and there is surprisingly little scientific research into bicycle geometry (lots of opinions and subjective testing though).

I found this article which applies to motorbikes but is nonetheless very interesting and some principles can be applied to bicycles. It basically states that HT angle is more important than trail for stability and handling.

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I don’t think that’s an accurate assessment of what’s out there. There’s plenty of researchers working hard to produce current and relevant work (1, 2, 3, 4, 5, among many others). Further, this just barely scratches the surface by summarizing the 111 references that went into making it. It seems to me that the math and physics can get as descriptive as you have the patience for. Whether or not they can practically be used to inform design is less certain, but there’s definitely no shortage of scientific research on two wheeled vehicles.

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Crank length! Custom frames are the perfect place to incorporate all the changes that crank length affects. While I haven’t written a Framebuilder’s guide to crank length… there’s tons of words and info in the Rider’s Guide to Crank Length linked below.

Depending on how deep the rabbit hole you want to go, changing crank length from a 170 to 155 can change the whole geometry of the bike (if you want it to). BB height, top tube length, handlebar drop, fore-aft… all sorts of rider-fit related goodies. Any feedback much appreciated.

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Lots of great links there! It will take a while to go through them all. I see there are a couple I have read. Are there some key ones that you would recommend?

To clarify what I was saying; while there are lots of articles on the physics of bicycles (primarily descriptive - How does a bike balance, How does a bike turn etc) but there are few that explore the effects of changes to geometry, especially with a rider.
For instance, trail can be the same across any number of HT angles but the handling is not the same.
ex 70HT 63rake 60trail vs 75HT 33rake 60trail .

Ideally, you’d need to test many bikes with many variations on geometry or just have fun and create an adjustable geometry bike like this one - https://www.youtube.com/watch?v=o3o1WkW3Dps

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Was schlepping through Reddit and found this…

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Rob English just published this on Escape Collective. Worth a read for both newbs + vets.

The whys of bike tech: Geometry and handling - Escape Collective

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I definitely haven’t read everything I linked to, I just wanted to put the existence of the research out there in case anyone was interested. I don’t have a favorite yet. It’s unrelated to the quality of his work, but my partner used to see James Sadauckas’ wife for physical therapy.

As I’m working my way through some of it though, I would like to see if this methodology can be used in design. In particular, I think it would be a cool exercise to try to tune the speed at which the Hopf bifurcation occurs by modifying the head angle or trail. It might be all for nothing though, as I suspect that the ideal values of these parameters (if they exist) are similar to what the industry has settled on. At least then I would have a good understanding of why other than “it works” or “we’ve always done it that way.”

I totally agree, and I would love for there to be more work that is relevant to a user or designer. But I imagine that the work out there is done to make studying a bicycle tractable. It seems to me that the parameter space would get really large really quickly if you tried to incorporate any nontrivial information about the rider. That’s not to discourage anyone from trying though!

Can someone 'splain to me what this website is…
Geometry Geeks

Coco I think this is a resource for people looking to compare one bike to another OR as a resource to figure out if a stock bike can fit you.

One way to use this might be to go to an independent fitter, and get the coordinates for your feet, butt, hands. Then you can place those in relation to the stack/reach for a few bikes. One brand/model might have you on a “58cm” whereas another might have you on a “54cm” and a third might not work at all.

For those of us making/buying custom bikes I think it might be useful to see trends or to get a bit of inspiration but I don’t see a lot more value than that.

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