SF Cyclotouring

Ride reports and other ramblings from a San Francisco cyclist.

9/08/2010

Notes on 650B Conversions and Handling



I've now done two 650B conversions: first my RB-1 and now my Trek 400. Learning the handling changes brought about by switching to the smaller wheel size has been an interesting experience.

The RB-1 has a 73.5-degree head angle and a fork with 45mm of offset. With a typical 700Cx25mm tire, that bike has about 54mm of trail and 15mm of flop. When I 650B'd the bike I used Nifty Swifty tires, which measure about 34mm wide -- the trail and flop values changed to 51mm and 14mm, respectively. In its stock form, the RB-1 rides as if "on rails". The bike is very easy to ride no-handed at any speed. Swoopy, fast, descending curves make the bike feel like it wants to drift out to the guard rail -- which has been rather unnerving at times. It's very difficult to correct the bike's line during these sorts of turns. In contrast, with 650B wheels, the bike's straight-line handling feels much less "on the rails". It's still easy to ride no-handed, but the front end feels much livelier and less solid. During the same fast downhill curves, the bike now feels like it wants to "oversteer" -- that is, instead of fighting the bike's unwillingness to turn, you now have to consciously keep the bike from turning too much. It only takes a ride or two to adjust to this difference, but it is noticeable the first time out.

The Trek 400 has a 73-degree head angle and a fork with 45mm of offset. Using a 700Cx28mm tire results in 58mm of trail and 16mm of flop. Now that the bike has a 650Bx35mm Panaracer Col de la Vie tire, the trail and flop values change to 54mm and 15mm, respectively. WIth the 700C wheels, the Trek has fairly staid handling. It is less obnoxious during fast cornering than the RB-1, but gives a similar, though less intense, feeling of wanting to drift toward the fog line. Straight-line riding is uneventful, and the bike is fairly easy to ride no-handed. After the switch to 650B wheels, the steering feels much lighter -- similar to how a low-trail bike feels with no front load. The bike still tracks fairly straight when ridden no-handed. I haven't yet had the opportunity to test it on a fast, curvy descent -- but based on the riding I have done, I think this bike will feel fairly neutral vs. the 650B RB-1's tendency to oversteer.

What I find really interesting here is this: The stock RB-1 has trail/flop values of about 54/15mm. The 650B'd Trek 400 has trail/flop values of about 54/15mm -- the same numbers -- yet the two bikes handle really differently! I didn't expect that. Clearly there's more going on....

7 Comments:

Blogger Steve Chan said...

The Trek is a bigger frame isn't it? My guess is that the shorter top tube on the RB-1 results in proportionally more weight on the front wheel than on the Trek, which would change the handling.

11:05 AM  
Blogger Jim G said...

Hi Steve, yes, the Trek is a bigger frame, but its TT is only 1cm longer than the RB-1. I'm also using a shorter stem on the Trek than on the RB-1 -- the RB-1 actually has more overall reach. Hmmm.

11:11 AM  
Anonymous alex wetmore said...

I'm guessing that one or both of the bikes have different head tube angles than spec'd. You really should measure them.

Digital angle finders have gotten pretty cheap (under $40 at most woodworking stores) and make measuring these angles pretty easy.

11:37 AM  
Anonymous James Black said...

Agreed - a small difference in head angle makes a large difference in handling. And if the actual head angles don't match your assumptions, then the trail figures aren't equal either....

12:52 PM  
Blogger portland_allan said...

Wheelbase?

12:59 PM  
Anonymous Anonymous said...

A lot of what afeects how a bike "feels" WRT handling has to do with wheel weight. Usually heavier wheels slow down the steering just like more trail does. If your bike is too "nervous" sometimes this makes a difference. Although the wheels that make your bike handle better downhill might not be the same ones that you want to go UPHILL on !

Pneumatic trail is a red herring. It's the differences in wheel mass and inertia that affect handling to greater degree, IMHO. Although they all work together so it's hard to isolate the different effects.

9:59 AM  
Anonymous PedallingPete said...

Many thanks for taking the time to analyse and share your experiences! I have a RB-1 and like you say it's very nice to ride. So I'll probably leave it as it is and use another frame for my conversion.

7:28 AM  

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