Minimum acceptable ground clearance below crankarm?

[Jim Bronson]

Is there any rule of thumb on this?  Put the 650b wheels on my custom Riv with 32.8mm Nifty Swifty tires and there is less than 3 inches of clearance from the crankarm to the floor.

I think it's ridable...if I never pedal through corners.  I'm just afraid it's going to bite me sometime when I'm tired and not thinking about it.

I suppose bigger tires and shorter crankarms would help some.  There is a huge amount of clearance available underneath the R559 brakes I installed.  And I'm running 180s now so could go down to 175s or even 170s.  Might be able to get another 15mm of clearance or so with 170s and 40mm tires.

But anyway :)

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Keep the metal side up and the rubber side down!

[Toshi Takeuchi]

My bottom bracket height is 260 mm minus the 170 mm leaves 90 mm
clearance from the ground. This is about 3.5 inches, and I never
worry about pedal strikes (it's never happened), but my pedal isn't
very wide, and this is an important part to consider. I have 42mm
wide tires on my conversion...

I would run 38mm tires minimum on my conversions if I could (one of my
conversions doesn't have the chainstay clearance). With Xpress,
B-line, Pari-Moto, Lierres, there are great options. The 42mm Hetre is
superb and the 40mm SOMA (is it out yet?) one sounds like it would be
great too.

I have struck my pedal off the ground a couple of times on another
bike, but I just bounced off--it was more or less a non-event for me.
Has it caused others to crash?

Good luck,
Toshi

[Michael]

Would the 44-584 Marathon HS420's that RBW sells work?

This is a guess, but a call to RBW could confirm.

Would they make your bike sit higher off the ground than on the Swiftys?

Marathons appear to have a tall profile. Maybe because of the Green Guard layer on top if the casing.

My Swiftys looked a rounder profile, if I remember right. The Marathons seem more oval and tall to my eye.

And if you are going puncture protection, you might as well go all the way and get Marathons.

[Jim Cloud]

I clipped a pedal on a 73 Pogliaghi Italcorse bicycle years ago while heeled over in a curve.  I went down very quickly and the tubular tires on the bike were pulled right off the rims.  I think it all depends on the force dynamics, how fast you're going and how much pedal contact with the road occurs.  I certainly wouldn't recommend it as an exercise!

Jim

[Jim Bronson]

I had short reach brakes before with 700x32mm tires and they barely fit under the brake bridge.  Don't know what that converts to in 650b but they need to be a bit smaller than that to clear the fenders that will be bought.  I'm not sure a 44mm tire would fit.  But I do have the R559's installed and there is a LOT of clearance currently.  There is certainly room to go somewhat bigger.

The reason I bought the Nifty Swifys (not from Rivendell) was because they were cheap and I needed a 650b tire in order to ride the bike.  I don't think they will be my ultimate tire but for now it was a good way just to try it out for not too much.

I rode the bike around the neighborhood a bit today and aside from the concern about the pedal strikes it felt great.  Handling was excellent and seems unaffected by the wheel size change, if anything in fact it seems better. Maybe because the bottom bracket is even lower.  I pedaled around some gentle corners and didn't suffer a pedal strike even though I was prepared for one. 

The front wheel has a Shimano generator hub in it and I was surprised how smooth it was compared to the 700c SON28 that I have.  There was no perceptible sensations that a dynohub was installed.  

On the downside, the shifting is not even in the neighborhood of acceptable with my Campy/Shimano mixed drivetrain.  Of course, I didn't even adjust the limit screws so maybe that's to be expected.  Also the R559s need further adjustment, they're functioning like modulators rather than brakes.

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[Steve Palincsar]

On 11/13/2013 11:54 PM, Jim Bronson wrote:
> I had short reach brakes before with 700x32mm tires and they barely
> fit under the brake bridge. Don't know what that converts to in 650b

subtract the difference in radius (622 - 584 /2) of the smaller rim and
add half the additional width (42-32 / 2) of the new tire

[Jim Bronson]

OK, so 19+5=24.  I majored in liberal arts.  What is the significance of the answer?  A 650bx42 tire is 24mm smaller than a 700Cx32 tire?  (assuming sizes printed on the sidewall are more or less correct).

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[Steve Palincsar]

On 11/14/2013 10:21 AM, Jim Bronson wrote:

OK, so 19+5=24.  I majored in liberal arts.  What is the significance of the answer?  A 650bx42 tire is 24mm smaller than a 700Cx32 tire?  (assuming sizes printed on the sidewall are more or less correct).

19mm is the additional clearance you get because of the smaller rim.   As you can see, I'm a liberal arts major too: you lose 5mm, so you have to subtract rather than add it because the tire is larger, so you have a net gain of 14mm clearance.  I should have said "add back", that's what I meant.

On Thu, Nov 14, 2013 at 7:31 AM, Steve Palincsar <pali...@his.com> wrote:

On 11/13/2013 11:54 PM, Jim Bronson wrote:

I had short reach brakes before with 700x32mm tires and they barely fit under the brake bridge.  Don't know what that converts to in 650b

subtract the difference in radius (622 - 584 /2) of the smaller rim and add half the additional width (42-32 / 2) of the new tire

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[Jim Bronson]

OK, so I should be able to run Hetres with fenders then.

[Bill Lindsay]

Jim

Yes, you should be able to do that.  Steve got the rim part right, but the tire part has a small mistake.  You don't half the tire width for this calculation.  

Your existing 700x32 wheel, which barely clears the brake bridge is (622/2)+32 = 343mm in radius and barely clears the brake bridge

Your 650bx42 will be (584/2)+42 = 334mm in radius.  You will have 9mm of additional clearance under the brake bridge, with Hetres.  

That same 9mm is what you use for estimating your change in BB height.  Your bike will be 9mm lower to the ground when you go from 700x32 to 650x42.  Additional tire deflection when you get on the bike might be worth considering, as well, but that 9mm is a good start.  

You use the full tire width when estimating wheel radius, not half the tire width, as Steve indicated above.  You would take half the difference in tire width when you look at the sideways clearances, between the chainstays and the fork blades.  A 42mm tire will be 5mm closer to the chainstays and the fork blades, so you need to check there for potential clearance issues.  

[Jim Bronson]

The wildcard I didn't mention was the 700x32 tires barely cleared the old brakes, Tektro RX40 short reach.  I installed the Tektro R559 and the 650B wheels the other night and rode it yesterday with Nifty Swifty 650Bx32.8.

There may actually be better clearance under the R559 just given that they have a way more optimal shape for big tires and fenders.  I don't really know how you calculate for that, but anyway I am starting to feel better about this conversion.  I'm planning on acquiring the 42mm tires and shortening my cranks to 170.  That should put me right about where I started with the 700c wheels as far as clearance goes.

[Bill Lindsay]

Do what feels right to you regarding crank arm length.  I am one of those people who would never change crankarm length.  I think running shorter cranks to solve pedal strike issues makes as much sense as squeezing your feet into smaller shoes to solve toe clip overlap.  

Many other people who are much more experienced than me believe that crank arm length doesn't matter much and that 5mm or even 10mm changes are hard to detect and if detected are easy to grow accustomed to.  Again, do whatever feels right to you.  

[Jim Bronson]

I already decided that I don't like the current TA 180s on my Riv.  I was going to try to live with them (see Kneesavers thread) but I think I would prefer to reclaim some ground clearance now that I've seen how low it is.  

I do like the 175s on my Paul Taylor so my thinking is maybe it's another 5mm in the right direction?  And I reclaim a full centimeter of ground clearance.

Or I could just get 175s, I'm looking at some Sugino XD600 46-36-26 and the vendor I'm looking at has both 170 and 175.

[IanA]

If you still have the 700 x 32 wheels/tires on hand, just measure the radius.  An owner of the Hetre 42mm told me that he measured the radius of his wheel at from the middle of the tire at the skewer to the edge is about 33.5 cm.  My true 33mm x 700 wheels/tires on the same measurement basis give 350 mm.    In my case, that would be a physical drop of 15mm.  Your nominally 32mm might be a smaller radius, but you're probably looking at a difference of at least 10mm.  Were you pedal striking before?  If not, the 10mm might be still within a safe margin and you can hold off switching cranksets until you have tried the bike in its new iteration as 650b.

Ian A Canada.

On Thursday, November 14, 2013 8:21:12 AM UTC-7, Jim Bronson wrote:

OK, so 19+5=24.  I majored in liberal arts.  What is the significance of the answer?  A 650bx42 tire is 24mm smaller than a 700Cx32 tire?  (assuming sizes printed on the sidewall are more or less correct).

On Thu, Nov 14, 2013 at 7:31 AM, Steve Palincsar <pali...@his.com> wrote:

On 11/13/2013 11:54 PM, Jim Bronson wrote:

I had short reach brakes before with 700x32mm tires and they barely fit under the brake bridge.  Don't know what that converts to in 650b

subtract the difference in radius (622 - 584 /2) of the smaller rim and add half the additional width (42-32 / 2) of the new tire

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[Bruce Herbitter]

Not sure I understand your question. You usually talk in terms of BB height. You mean from pedal to floor with crank arm at 6:00 position? My 650B conversion of an early Road has 2 7/8" under the pedal measured this way. I can't recall a pedal strike in over 5,000 miles, but there may have been one or two. I don't corner like a racer, however.  The bike has had Pari Motos, Maxy Fasty and Cypres tires

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[Jim Bronson]

Thanks Bruce.  That's close to where my bike is also, even with the 180 cranks.  I don't pedal gonzo around corners either.  I'll have more clearance with taller tires and shorter cranks also.  Gaining confidence in this conversion.