Friday, February 20, 2009

Bolting a Bicycle Wheel

In general this applies to Single Speed or Fixed Gear bicycles with a horizontal slot dropout but could also apply to Tandems or to any bicycle where you want to insure a secure wheel. How to insure your rear wheel doesn't slip (with subsequent chain throw).

Although this is common sense, most of the hub manufacturers don't have it figured out (using small diamter serrated locknuts means you don't have it figured out). Not even the expensive "Boutique" hubs. (yet another re-inforcement of my opinion that most of these expensive hubs are yes beautifully machined but poorly engineered).

The proper solution is simple: proper clamping surface area is required. Just like snowshoes are used to prevent "post-holing", so should large enough lock nut and track nut washer diameters be used to prevent permanent deformation of your frame .

Serrated nuts and small-clamp-area are fine for derailleured bikes where digging into the frame doesn't matter. But it doesn't work that well for single speed and track bikes

Serious indentions shown in pic

Fine adjustment of chain tension becomes difficult if not impossible depending on severity of indentations and the nuts will tend to slip "downhill" into the deformity and preload will be lost and wheel will slip and maybe the chain too.
[BTW, an easy chain adjustment method is described by Sheldon Brown: Basically walk the wheel back in the slots by tightening and loosening one nut or the other and pushing sideways on the tire. http://www.sheldonbrown.com/fixed.html#wheel . ]

Another solution is to have track fork slots be made of high strength steel. Something with high yield strength, or maybe even case hardened. But If you don't have that already, it would entail a frame or dropout replacement which is of course absurd.

One could also use “tug nuts” but they are huge hassle.

How much clamping surface area is needed to prevent frame deformation and to properly hold a wheel in the frame? That is, how big of a diameter should the "lock nut" and "track nut" be? It depends on:

-Strength of frame track slot or horizontal dropout. The contact stress should not exceed the yield strength. If we assume an inexpensive stamped steel dropout, the yield might be as low as about 35,000 psi.

-Amount of Clamping Force. A 10mm bolt class 8.8 can easily be tensioned to 7000-8000 lbf.
[compare to quick release force..]Most likely this much will not be used. Assume a torque of 20 ft-lbf (which is reasonable, 6 inch wrench, 40 lbf). This will generate approx 3000 to 4000 lbs of clamping force (calculated using torque vs clamp force equation...) . To be conservative assume 5000 lbf.

bc defg
2ID mmOD mmClamped Area mm*mmClamped Area in*inForce lbfContact Stress psi
3111640.10.062500080,387
4111754.00.084500059,732
5111869.60.108500046,350
6111986.90.135500037,131
71120105.80.164500030,484
81121126.40.196500025,519
91122148.60.230500021,703

[ fyi, from ms excel, the complicated equation above is finding the clamped area : D3=C3^2*(0.25*PI()-0.5*ASIN(B3/C3))-0.5*B3*C3*(1-B3^2/C3^2)^0.5 ]

Only OD is changed in the table above. At 16 mm outer diameter, which are most lock nuts, the contact stress would be way too high. Thus why many have issues with chain adjustment or throwing chains. Higher quality hubs usually run with 18mm luck nuts, which is still not adequate. I would want at least 20mm. But even larger diameter would be smart if you want to play it safe on not deforming your frame or if clamping on aluminum

Far left is overkill at 24mm diameter. Far right is 18mm.

A related aside, how much holding force is really needed to keep a wheel from slipping forward in the frame? Being conservative, at the most 300-400 lbs could be put into a crank (if you are really strong). This would translate into 800 to 1000 lbs of chain force. (well if much more the chain is going to break).

Assume someone only clamps with 3000 lbf , assume friction of 0.15 of steel on steel (which is about as low as it should ever be), equals only 450 lbs of friction force capacity (compared to 800-1000 lbf in chain !). Oops, not enough !? Nope, because there are actually four faces that the clamping action is on. (so this is the reason for multi-plate clutches...).

Too be conservative, of course only two of those, on the drive side, should be considered, giving 900 lbf of capacity before the wheel would slip. But also this could be more than doubled with more wrench torque (which you are going to do if you if able to generate 1000lbf chain force). So if your wheel slips, its either not clamped with enough force, or too little clamp area, plastic flow occurs (frame indentations). When things go plastic, force isn't maintained.

What can a person do to get a large diameter locknut? (I made the one pictured below, on a metal lathe). Do the best you can. I did find a locknut in my spart nuts bin, that was a little over 20 mm OD. I think it came from a Surly hub. Yah smart people in Mn.

Yeah this works. 24mm dia and some rust (magic friction increaser).


0 comments: