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Discussion Starter · #1 ·
So, I'm helping someone who is small by designing a frame and fit for them. She is pushing the front center dimention to the very minimum. My question is, what is the smallest FC you folks have been able to do for an offroad bike that hasn't produce an issue with shoe/wheel interferance?

This is for 26" wheels. I'm going to measure her shoes later this weekend. She does have small feet. I can put more info into the CAD then.

Right now I'm looking at 595mm.
 

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Bike Dork
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If she's really that short 24" wheels are the way to go using a 26" sus fork. One I built using an Action-Tec "fork" and blades designed for a 24" wheel. I've built three 24" wheel frames and they worked great.
 

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pvd said:
Sorry. 24" wheels arn't going to work in this area. It's gotta be 26".

Shoe size - 37 euro
Crank length - 170mm
You likely already know this, but be sure to include crank length and "Q" factor, and pedal width in your model. A bike with toe overlap in profile view can be fine when the fork is turned.
 

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depends how "out of the box" you think you can go with angles & rake & still have it ride well. IMO < a 21" - 20.5" Eff. TT x 74*/70* angles x ~ 45-50mm rake gets kinda wierd/tight. i have built 10", 11" & 12" C-to-C S-tube bikes. tiny bikes are by far the hardest...........i trashed 3 pairs of SS's on this one.........the stem is because she has bad neck issues. it took allot to get her comfortable but it was well worth it. she loves this bike!
 

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Discussion Starter · #6 ·
Interesting...

It's a colder wet morning here and I decided to pass the morning drinking coffee, listening to NPR and modeling this FC problem in solidworks.

I still need to make some acurate measurements of the persons' shoe and cleat position but it's already obvious that the position of the wheel at the end of travel is where problems will occur. Cool.
 

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pvd said:
Sorry. 24" wheels arn't going to work in this area. It's gotta be 26".
The bikes I built were all for rocky rooty North Idaho and I knew a good number of folks doing 20" wheels too from my Bike Friday days. I don't know where you folks are riding but the SF bay area never struck me as needing 26", but that's you're choice. You might want to check out the Arrow Racing tires before dropping the 24" wheel size totally.
 

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pvd said:
Interesting...

It's a colder wet morning here and I decided to pass the morning drinking coffee, listening to NPR and modeling this FC problem in solidworks.

I still need to make some acurate measurements of the persons' shoe and cleat position but it's already obvious that the position of the wheel at the end of travel is where problems will occur. Cool.
And how likely will the fork be fully compressed and the wheel turned at the same time?
 

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Discussion Starter · #9 ·
shiggy said:
And how likely will the fork be fully compressed and the wheel turned at the same time?
It's about safety at that point. I always want the bike to work properly in the extreme cases since that is when the rider needs all the help they can get to survive the ride. I may let the toe just graize the tire, but I'm not going to let it slam. I'm just trying to squeeze every millimeter out of the 26" wheels. The design is really coming along. If I can trust gitting the front center down to 580 or 575 I'll be really happy. I'm pretty sure that 585mm is going to work but I have to know for sure.
 

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Rough calcs

...say that assuming a 71/73 set of angles, 38mm rake, 335mm wheel radius, and 25mm BB drop, toe overlap goes to about zero at 572mm front center. That's making some assumptions about stance width and cleat position, though. I would imagine 580mm would be no problem at all unless the clear position is really weird.

-Walt
 

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pvd said:
Crank length - 170mm
Pete, if she's really short, would 165 or 160mm cranks be an option? Or is she mostly leg? Perhaps she's just never been exposed to the correct length cranks...

The shortest adult I know riding still runs 170 mm cranks and she looks like she's running hurdles when she rides...but that is what she has always used and likes it just fine.

B
 

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Discussion Starter · #12 ·
MDEnvEngr said:
Pete, if she's really short, would 165 or 160mm cranks be an option?
She had been on 170mm cranks so that's what I was initially using. I got a chance last night to do some more measurements on her and I'm definated recomending 165mm cranks. (745mm crotch height, 400mm femur) That is now the length I am designing the frame around.
 
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