Thread: BB drop across wheel sizes

1. BB drop across wheel sizes

Im planning on taking UBIs steel brazing frame building class down the road, and I have been trying to plan out the bike I would like to build.

It will be a 650b AMHT. Only catch is, I only own 29ers at this point. I understand completely how different BB drops feel on a 29er, but what I would like to know is how that compares to a 650b or 26er.

Theoretically, to convert across wheel sizes you would need to compare the radius of each wheel size and add or subtract that number from the original frames BB drop.

Is this an accurate way of thinking about it?
Also, the diameter of our 3 wheel sizes seem to be a touch off... I measured a 700c rim I had laying around, and instead of measuring 622(24.48), it measured just under 25 inches.

So, if this is an accurate way to compare frames across wheel sizes, what are the true diameters of the wheels? I could measure my 700c and 26er, but I do not have a 650b to measure...

Thanks for all your help!
Sheepo

2. Bottom bracket height +BB drop = Wheel radius

So yes, you can convert by looking at the difference between wheel sizes.

622 is the bead seat diameter, but again assuming the same tire size you can figure out wheel radii by adjusting for changes in ISO size.

3. Figure out what bb height you want, figure out what size tires/wheels and amount of suspension sag you will run, and set BB drop accordingly.

-Walt

4. Originally Posted by dr.welby
622 is the bead seat diameter.
Ahh Thanks! It all makes sense now!

5. BB drop across wheel sizes

Originally Posted by Sheepo5669
Ahh Thanks! It all makes sense now!
And 650B BSD is 584mm

6. So for the same BB height between 29 and 650b, a 650b bike would have 19mm less BB drop than the 29er.

7. Obviously tires and fork a-c will be factored into the equation as well.

Thanks for all the help gentlemen!

8. BB drop across wheel sizes

Originally Posted by Sheepo5669
So for the same BB height between 29 and 650b, a 650b bike would have 19mm less BB drop than the 29er.
Given the same width tires, yes.

9. BB drop across wheel sizes

Originally Posted by Sheepo5669
Obviously tires and fork a-c will be factored into the equation as well.

Thanks for all the help gentlemen!
The fork A-C and sag are a major factor in the whole frame geometry.

10. Originally Posted by shiggy
The fork A-C and sag are a major factor in the whole frame geometry.
Yes, I believe I have read that somewhere.

11. Originally Posted by Sheepo5669
Obviously tires and fork a-c will be factored into the equation as well.

Thanks for all the help gentlemen!
You can get the BB down as low as you can stand, but with as long as forks are these days it's challenging to get the bars low enough in relation to the BB. Upping the BB a touch from as low as practical, especially on a bike with long travel 29er fork, can make the bars/seat/bb relationship easier to balance.

12. I'm going to quickly disagree - if you can't get the bars low enough, you have a wheel size/fork travel/head tube length/etc issue. Changing the BB height to try to raise the rider relative to the bars is putting the cart before the horse - 5-10mm of BB height will make a huge difference for pedal strikes but is pretty negligible (and not even noticeable to many people) in terms of bar height.

I mean, feel free to raise the BB to get the bar position you want, I guess, but to me putting the rider's COG where it needs to be (generally as low as possible) trumps most other stuff.

-Walt

13. I'm going to quickly....agree...with your disagree-al. I wasn't implying setting the saddle to bar drop using BB drop, just that it might be worth giving up a "low" bb for a more "standard" height if you're fighting a long fork. And that's also why I used 650b wheels instead of 29in on my second XC frame.

14. BB drop across wheel sizes

Thanks for the comments guys.
I'm 6 feet tall, so getting the bars low enough has never been an issue for me. Even with a 140mm travel fork on a 29er with a low BB.

I hate pedal strikes, but I love carving corners low and in control. On my frame, it will be built for descending fast and shredding the corners. Don't care so much for pedal strikes on techy climbs.

But, I have no clue how it will ride any better than my Honzo... This bike shreds.

15. One thing the folks at UBI recommend is to copy the geometry of a bike that fits and rides well rather than experimenting on your first one. There is plenty to learn in two weeks without also throwing in frame design. Nice to know that after all the work you are going to put into it that you'll have something that you are going to enjoy riding.

16. The short stays on the Honzo help to prevent pedal strikes, even with its low BB.

17. Originally Posted by eMcK
The short stays on the Honzo help to prevent pedal strikes, even with its low BB.
You've got me thinking about that one. I'm not trying to argue your point but I'd like to hear an explanation of the relationship between stay length and pedal strikes. I can't figure out how they relate.

18. Anybody...Bueller?

19. Re: BB drop across wheel sizes

I'm curious as well.

I suppose you could make an argument based on a comparison to 4wd vehicles not getting high centered with shorter wheelbases but I'm not sure how applicable that is bikes.

20. BB drop across wheel sizes

Originally Posted by graviT
I'm curious as well.

I suppose you could make an argument based on a comparison to 4wd vehicles not getting high centered with shorter wheelbases but I'm not sure how applicable that is bikes.
Not very. Many of my pedal strikes have little to do with where the wheels are but how big the rocks/roots/brush/backslope is at the edges of the trail.

"Short"CSs or wheelbase (+/-20mm) also do little to gain clearance when going up ledges.

21. I guess in theory a shorter wheelbase will keep the BB away from trail obstacles better if both wheels are rolling over the obstacle (ie you're not riding in the low spot between some rocks) - so yes, the 4x4 high-center analogy would work. My cargo bike nails pedals on everything (even just frost/root heaves in pavement sometimes) because the wheels are so far apart, but that's on pavement.

It breaks down some here, though, since you're often riding through the lowest line (ie the obstacles are to the sides where the pedals are) and we're talking about pretty small increments - even a very short chainstay bike compared to a long one is going to be no more than a couple inches shorter total wheelbase. So IMO you should decide on BB height basically independent of the desired chainstay length.

-Walt

22. I've ridden through a lot of rock gardens in my time, and much prefer the ride of shorter chainstays, as they seem to keep pedal strikes in check, all else being equal (which of course it rarely is, suspension, suspension travel, wheelsize, bb height, riding postion, gear section, crank arm length et cetera).

Maybe it has something to do with the short stays making it easier to loft the front wheel?

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