Ciufalon

Can anyone tell me if converting a bike to 650b wheels changes the standover height? I am curious if it lowers the height allowing for a slightly larger frame or larger tires. All things being equal (other than wheel size), how much does it change? I haven't found any information on change in standover height on sites addressing such conversions. Thanks for any information you can share.

will015

I've read converting from 700-23 to 650-38 you'll drop the BB 8-10mm. The top tube in turn would follow.

Generally the reason to convert to 650b is to allow a larger tire to fit in a frame not built for them. If you already have decent tire clearance in your frame a 650b conversion doesn't make much sense.

noglider 

Bead seat diameter of 700c is 622mm. BSD of 650b is 584mm. Radii are 311mm and 292, respectively. The difference in radii is 19mm. The 650b tire will probably be taller because that's how they tend to be, so the difference should be less than 19mm. So how could the drop be 8-10mm? Am I missing something?

redxj

Your math is flawed. You are using BSD only and not taking in account tire size. A 700x23 tire is roughly 676mm in diameter; 650b x 38 tire is 668mm; 27" x 1 1/4" is 702mm. (using the trail calculator https://yojimg.net/bike/web_tools/trailcalc.php)

The change in the BB height is going to depend on what tire size you started and ended with of course. Going from 27" to 650b would be more than 700cX23 to 650b.

Zaphod Beeblebrox 

**edit** Redxj beat me to it.

Tom, From 700c to 650B the brake contact point on the rim will drop 19mm. The overall standover drop depends on what width tires you're starting with and what width you're moving to, and the width of the rims.

For an approximation of Diameter you can add the tire width to the Bead Seat Diameter. So a 700x23 would be (622+23+23) = 668mm and a 650 x 38 would be 584+38+38 = 660mm. approx. 8 mm difference in diameter.... call it 8-10 if you take rim width variation in consideration.

And this is all diameter so in terms of dropping standover height thats 4-5mm total difference.

noglider 

Thanks, guys. What an embarrassing mistake.

Ciufalon

Thanks everyone for answering that with such complete information. That math was all new to e and it gives me a far better understanding of how to calculate things. I have a set of 650b wheels/tires and have just kind of been keeping my eyes out for something to put them on. I see bikes and frames I like now and then but have not been sure if they would work or not. I appreciate the expertise here in C&V.

southpawboston

Just keep in mind that not only will top tube height be lowered, but so will the BB height. If your bike already has a relatively low BB, lowering it further may cause increased risk of pedal strike around sharp turns.

David Newton

By converting a 27" wheel Bridgestone 300 to 650A (my preferred size wheel) I was able to turn a "slightly too large for me" frame into my best bike.
Brake reach is always an issue, but there are some good choices in long-reach brakes out there.

noglider 

And if you're going whole hog, you can put drum or disk brakes on.

surreal

i'm in the process of converting an old bike, designed around 700c wheels and rather tight clearances, to 650a. the allure was fat tires plus room for fenders. the 700x23c i'd been rocking are only a few extra mm in diameter, so geo and standover'll be roughly the same. if you're thinking 650b, and had 700x23c on the frame, you'll want true 42mm tires; the 650b market tends to be generous with sizing (ie, 650x42b tires might only truly be 39mm wide, depending on the rim you use.

as d.newton pointed out, a conversion such as this can help drop the toptube height, which mayhelp with fitting a big frame to a smaller rider. This only works if the frame fit wider tires to begin with. of course, the worry with bb height enters in here, and you'll want to avoid pedalstrike. if you're keeping the overall diameter the same, neither parameter shall be effected.

hth
=rob

ps im using drums to sidestep brake reach

NormanF

650B conversions are done for two reasons: to make a road bike into an all weather machine by allowing it to be fitted with fenders and to provide a cushier ride with the use of medium width tires. An additional reason is to reduce standover height to allow a bike to be fitted to a smaller rider - this is a consideration with a larger frame. It works quite well and the late Sheldon Brown has an article dealing with the nuts and bolts of 650B conversion.

surreal

agreed, but it'll only have the latter effect if the overall diameter of the wheel w/tire is reduced. And, then you need to make sure your BB isn't too low, depending on what your needs are in terms of clearance and lean-angles.

IMO, the best frames for 650b/a conversions are tight-clearance frames with skinny tires and short-reach brakes as original equipment. These will accommodate the smaller wheels/fatter tires without effing with the geo. IMO, if the frame is too big, it will be cheaper/easier/"betterer" to buy/trade a frame that fits.

-rob

rothenfield1

I have a particular potential dilemma that may just fit this discussion. I bought an Italian race geometry frame that I would like to convert, if possible, to something more like an all-rounder/rando/mo’ practical/what-ever-u-want-to-call-it type bike. I would like to get the equivalent of a 28c tire on it. If I went 650b wheelset, would the brake reach be my only issue?

jtgotsjets

Brake reach is one problem. Your next biggest problem is the change in geometry/bottom bracket height. A change from 700c x 23 to 650b x 38 (what everyone's been talking about) is a lot less drastic, geometry-wise, than the change to 650b x 28 you're suggesting..

rothenfield1

I'm reluctant to waste the few brain cells I have left to try and compute the math laid out above; but, by your comment, I'm understanding that the reduction in rim diameter from 700 to 650b is too drastic to put a 28c in a tight race rear triangle. That wasn't the answer to the problem I was hoping for, but I believe you.

rothenfield1

Yes, I've looked into it and see what your saying. That's too big a wheel diameter drop. On-to Plan B: Sell frame immediately!

jtgotsjets

Are you sure your frame can't take 700x28s? It's not that much bigger—and if you're riding in good weather (no fenders or mud to worry about), you'd be surprised at the tiny clearances that still work.

rothenfield1

You are correct, I haven't even received the frame yet. It may work...and if it does, I have some ideas about domesticating an Italian race frame. Thanks for talking me down Jtgotsjets.

southpawboston

I would agree with this. The frame I am converting has the ideal seat tube and top tube lengths for me, but the bike was designed around 27 inch wheels and 27 x 1-1/4" tires. The bottom bracket is also unusually high, making the standover height unusually high. Conversion solves this problem, and will allow me to go from 27 x 1-1/4" tires (32mm) to 650 x 42B tires (42mm) with room for fenders (barely). After conversion, the bike should be about 13mm lower.

I wouldn't necessarily agree with this. I agree that going from 700 x 23C to 650 x 42B will not lower the bike significantly (theoretically the height will remain the same) and therefore keep the geometry the same, but the problem is that most bikes designed for 700 x 23C tires will not have clearance to fit fat 650B tires. If I tried to convert my Trek 560 to 650B, the fattest 650B tire I could fit would be around 30mm, and that would limit me to I think one model of tire. And, the geometry would still change as the bike would be 12 mm lower.

Ciufalon

All of these responses actually addressed my question very well. I was wondering if I could convert a 57cm bike to 650b and bring it down to 56cm height and it sounds like if I find the right bike designed for 27"x1 1/4 wheels/tires it would work really well and I could put together a good randonneuring or porteur style bike that would be fun for other things as well. Thanks everyone!

ColonelJLloyd 

I can't recall seeing too many bikes converted from ISO 630 to ISO 584.

Edit: . . . without frame modification.

southpawboston

Just wait and you shall see...

nlerner

My St. Etienne was built for 27" wheels w/, fortunately, short-reach brakes. I'm running 75mm-reach brakes w/ 650B wheels. Fitting fenders was a bit of a big-spacer challenge.

Neal

David Newton

This is 27" to 26 x 1 3/8", very little reach left in those Tektro 661's
I suppose a Bridgestone 300 had longish forks and YMMV.