700c to 650b Conversions

[Jim L]

A general question...

What are your opinions on the max BB drop on a 700c to 650b conversion?

Is 75mm pushing it?

Assume 38mm tires in both sizes.

TIA

Jim

[Steve Chan]

   Can you go to shorter and/or lower q cranks, as well as pedals that are narrower?

   Many people seem to have relatively high range of crank lengths that work for them, and narrowing the distance between the outside edges of their pedals should buy a few mm as well.

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[olof...@gmail.com]

I am riding a bike with 80 mm drop and 38 mm Pari Motos (650B) with no less than 177,5 mm cranks. Still no problem, though I have the habit of turning sharp with my inner feet high.

 

Olof Stroh

Uppsala Sweden

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[Nick Payne]

I rode for a couple of years on a Litespeed with 76mm BB drop converted
to 650b - bike as converted here:
http://www.users.on.net/~njpayne/bikestuff/litespeed_650b.jpg. I used
32mm tyres (Grand Bois Cypres) as that was all that would fit between
the chainstays. I did get pedal strike in corners once or twice until I
learned not to pedal that bike through corners.

I was using fairly narrow 170mm cranks. Both crank length and width at
the pedal will also affect the amount of clearance if you corner while
pedalling.

Nick

[Mark Bulgier]

When I was a racer, a team mate showed me a trick: cut a little piece of pop can and put it on the outside of your pedal pointing down.  Back when pedals had dust caps, we'd loosen the cap a little then tighten it with the edge of the piece of pop can held in place by the cap.

Now ride around, someplace safe, like an empty parking lot when the business is closed. We called it criterium practice.  Pedaling through corners increasing your lean until you hear the thin pop can piece dragging.

Do that a lot until you can always predict when it's going to drag, then shorten the piece of metal.  Repeat as necessary, or until your nerve gives out.  Extra credit:  remove the piece of pop can completely and calibrate your lean so that you can make the pedal dustcap (or cage, whatever hits first) just skim the ground, on demand.  I did that a couple times, but probably not as much as I should have for optimal muscle-memory.  It is just too nerve-wracking.  You have to be going pretty fast to lean that much, so the stakes are high. 

Taking it to the ultimate (scraping the pedal on purpose) is not useful for the non-racer, or even most road racers really.  Only crit specialists benefit enough to make the time spent worthwhile.  But doing that with a fairly long "feeler" -- an inch maybe? -- might be worth it if only to give you a confidence boost.

Mark Bulgier
Seattle

[Tim Gavin]

I think 75 mm is pushing it, and 80 mm is danger zone.

That's based on my experience with an 80 mm drop frame (Rivendell Road Standard), designed for 700 x 25 and now riding 650 x 38.  I'm happier with 170 mm cranks, but it still is pretty easy to scrape a pedal.  I just don't pedal in turns.

My bike feels like a lowrider, but it rides great.

Tim

[David Banzer]

It depends on the terrain - road rides? I don't think it's a problem. I've converted my Riv Redwood to 650b (though it fits 47mm actual tires) with 77mm drop, using 170mm cranks and Vp-001 platform pedals. No problems at all with 42mm tires or the 47mm tires that it currently sports. I intend this to be my gravel/dirt bike - though those trails are pretty tame here - and don't consider the drop an issue, but the tires are certainly bigger than 38mm.
David
Chicago

On Monday, August 17, 2015 at 11:03:50 PM UTC-5, Jim L wrote:

[Jim Bronson]

I have 80mm drop on the bike I converted and have 38mm LoupLoup Pass tires.  Not only that, but I have 20mm pedal extenders and large VP-001 platform pedals.  So my pedals are pretty far out there.

While you do have to be more careful about pedaling around corners, IMO it works fine if you learn the limits and handle your bike accordingly.  I have scraped a few times but never had a serious pedal strike or a crash.  (Knock on wood!).

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[Benz Ouyang]

I can't agree.

A borrowed bike has 80mm of drop and was originally designed for Jack Browns that are 700x33.3C (it's not a Rivendell, seeing especially that it has disc brakes). The owner converted it to 650B and I'm now running it with Hetres, which are 650x42B. With 170mm cranks, I haven't scraped the Time RXS pedals yet, even though I've pushed that bike quite hard.

I have scraped the MKS Touring pedals on my Wilbury with 170mm cranks and Hetres. Often in fact. This bike was designed for 650B wheels and thus I assume the BB drop's less (between 64 and 67mm, if the other 650B bike geometries are indicative). So it should have more clearance and it is a Wilbury (meaning: maybe it shouldn't be leaned over so much). But this Wilbury and its MKS Touring pedals has seen the most spark action, other than my Brompton (but that's another story).

[cyclotourist]

That's funny, the only bike I've ever had clearance problems with was
a Rivendell Bleriot, which like your Wilbury, was a bike designed
around 650B tires.
It had 67mm drop, and it definitely could have been less. That was
with 175 cranks and Col de la Vie (36mm?) tires.
My current 650B bike has a 68 drop, but with 50mm tires, that's not a problem.

All that said, while you can do a conversion on a 75mm bike, it's not optimal.

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Cheers,
David

Member, Supreme Council of Cyberspace

"it isn't a contest. Just enjoy the ride." - Seth Vidal

[Jim L]

Thanks everyone for the great feedback!

Jim

[Brad]

In its first life bike with 650b tires lived in a world where 175 mm cranks were extraordinarily rare and expensive and people did not pedal through turns.

[mitch....@gmail.com]

I think a 700C 38mm tire on a 75mm BB drop bike is going to have a fairly high BB. I'd rather ride that same 75mm drop bike with 650B 38mm tires.

 

If archives are available, do a search on Doug Fattic for his take on this. He has said he likes a low BB and has built and enjoyed frames with BBs lower than 26cm. I think I remember him saying 25cm but I might have misremembered.

 

I have a big '75 Gitane with 26cm BB height, as designed, and no trouble. I rode that bike fixed and fast for years and don't remember ever scraping a pedal.

 

A higher BB (27.5cm) race bike I rode a lot of crits on has the outer edges of Record pedal bodies and dustcaps worn away from grazing the tarmac during turns.

 

Depends more on how you ride, whether you know and understand the limits, and whether the limits match your terrain.

 

--Mitch

 

 

 

 

[John Wilson]

Keep in mind, too, that when going from a narrow low profile 700c "racing" tire to a larger profile 650b such as the CDV, the effective BB height may not be much different. Such was the case with my Austro Daimler Vent Noir ll conversion.

John Wilson

Greensburg, PA  USA

[Brad]

These numbers for JimG's trail calculator on wheel circumference

700 x 23= 2123 mm
700 x 19- 2098 mm
584 x 38 - 2098 mm
584 x 42= 2123 mm

So ........ getting the tires to fit between the chainstays and the fork is a bigger issue than bottom bracket.

[Jim L]

Mitch,

Thanks for the reference to Doug Fattic! Being a numbers guy, it's nice to be able to see the correlation and comments regarding different designs.