That’s in lb.ft isn’t it? I’ve seen 40 lb.ft recommended elsewhere.
That (ideal) translates to about 56 - 70 Nm if I didn’t mess up the
conversion.
But I’ve been meaning to ask: is this related to hub strength? Could a
Suzue, being a reasonably strong hub, handle more nut torque than your
run-of-the-mill pisbakkenstaal hub before stripping?
While we’re on the subject, do foot-pounds convert directly to inch-pounds by multiplying by 12? My torque wrench reads inch-pounds, so I’ve been torqing to 480 inch-pounds. Just wanted to make sure. Thanks.
> While we’re on the subject, do foot-pounds convert directly to
> inch-pounds by multiplying by 12? My torque wrench reads inch-pounds,
> so I’ve been torqing to 480 inch-pounds. Just wanted to make sure.
> Thanks.
Yes. Torque is force (pounds, newtons, etc.) multiplied by the length
of the lever arm. One pound of force acting at a distance of one foot
is 1 ft-lb or 12 in-lb.
No. Just about any hub spindle will take bolt torques far greater than the recommended level.
The torque limit is to protect the crank hole from stretching too much. The bolt torque is directly related to the amount of pressure the bolt exerts on the crank, which is in turn directly related to the outward pressure that the tapers exert on the inside of the crank hole. Both the threads and the tapers are shallow wedges that multiply the force you exert with the torque wrench.
If the torque level is too low the crank isn’t firmly pressed on to the tapers and it will “squirm” under load and loosen. If the torque level is too high then it will micro-rupture the crank.
Fortunately the pitch of the threads and the angle of the spindle tapers are fairly standard, so the torque levels for various cranks tend to be a function of the quality of the crank. It’s a rough guide, but without the actual manufacturer’s recommended torque it’s better than nothing.
40 foot-lbs is about right for a good set of bicycle cranks. Sugino used to make a low-end set that could take only 25 ft-lbs, and I believe one of the Shimano Dura-Ace sets could take 50 ft-lbs. These are all fairly high quality, forged aluminum cranks. I have no idea what a cast aluminum or steel crank would require.
Thanks for your comments cyberbellum, your understanding in these areas is always highly valued.
I notice that the unicycle hub nuts have a pitch of 1.2mm & the hub bolts have a pitch of 1.0mm would it then be correct to reduce the nominal 40 foot-lbs (also quoted by various other people on this forum too) to 40/1.2 = 33.3 ?
At the moment I haven’t got the confidence to do my bolts up above 25 foot-lbs it just seems soooo tight.
On Wed, 16 Jun 2004 21:01:37 -0500, “cyberbellum” wrote:
>No. Just about any hub spindle will take bolt torques far greater than
>the recommended level.
>
>The torque limit is to protect the crank hole from stretching too much.
Oh, I seem to remember that in Sofa’s destructive experiment he
stripped the threads? To reconcile that with your remark, we have to
assume that the crank hole was already overstretched at that time and
would fail during riding.
Sofa used an old bent steel crank. Since the crank was already bent it was a throwaway, so no loss if the crank gets damaged. Since it was steel, it’s not going to stretch as much as an aluminum crank would.
Now I’m confused. I originally thought that the 40 lb.ft limit was based (at least partly) on Sofa’s experiment where he stripped the hub threads around 90 lb.ft. Then I learned that the cranks are the limiting factor, because they overstretch before the hub threads go. Now JC is making a distinction between steel and aluminium cranks, the latter being more vulnerable (if I understand that right). U-Turn, obviously someone with good technical insight, quotes 40 lb.ft but only states what hub he used, not the cranks which seem to be the limiting factor. So what hub/crank combo does the 40 lb.ft apply to? And what about other combinations? Are there official torque recommendations somewhere?
There may be some official numbers on some specific bicycle spindle/crank combinations, however, the uni world is the Wild Wild West. Magura brake installation instructions, in contrast, give specific torque numbers for each bolt.
A couple of years ago I was figuring out what to do on crank installation. Most bike shops do this “by feel”. Although I respect the mechanics in many ways, I didn’t feel comfortable with this for myself. I took the average figure of about 25 ft-lbs and raised it to 35 for unicycling and started working with that using a torque wrench for consistency. This is a generic setting for all combinations. After a while I raised it to 40 ft-lbs, which seemed to give the best performance over time. I have used that with pretty good success for all combinations of cranks and square-taper hubs (usually the Suzue or widened Suzue, but also the stock Coker and the uni.com, and the occasional generic low-end uni unmarked hub). This setting works pretty well although any setting requires constant attention for many reasons: riding style, environmental conditions, crank quality, and the like.
I rejected the Bridgman 20-ton arbor press approach for reasons discussed elsewhere, but also because it presses the crank on to a specific position, not to a specific resistance force, and because it is not possible for a) the typical rider and b) a built wheel.
People have been stripping axle threads (including one from EBay that I had to rebuild for a Unatic) and complaining about the Suzue external threads etc. I have never stripped the threads using this approach, so I think it’s reasonable to conclude that people that have stripped the threads on a better-quality hub have simply over-tightened the nut. Usually this is because they are doing big drops or lots of hopping that are damaging cranks that were not designed for such activity, and they are torquing harder hoping that will fix the problem.
Sofa did us all a favor by taking a damaged Suzue hub and trying to see what it would take to strip the threads off the undamaged side. It took him twice the torque, or about 80-90 ft-lbs. That just adds to the reasonableness of the number 40.
So if you are doing big vertical stuff, get a splined hub. If not, take care to torque your cranks on to a specific torque and inspect them frequently.
That said, my off-road uni is still a Suzue with Kookas put on at 40 ft-lbs. Although I’m not nearly as courageous and talented as lots of people, I do put a lot of stress on it and it does fine.
In summary, most of the cranks we work with have such low manufacturing standards that we are lucky that they fit on the spindles. I think we have all received cranks with mis-punched holes. This is not the high-tech track racing world that cyberbellum was dealing with. However, if you pick the better cranks, then 40 ft-lbs is an easy number to work with and remember and works pretty well if you inspect frequently. To go beyond that at this point is probably to go back into the noise of the manufacturing tolerances. Most crank installations are done by the customer him/herself, and if he/she used a torque wrench to do that at 40 ft-lbs, many of the crank and hub problems people pay to fix would go away.
I don’t remember if I was using my Kooka (alluminum) cranks in the experiment of steel ones, but seeing as the Kooka’s were the last cranks on that hub for riding, I kinda think those were the ones I used.
There may have been residual grease on the threads of the hub and the nut, but I did not specifically grease them for the experiment.
I have another ‘half hub’ (one taper was twisted) this is a ‘normal steel hub’ and I will redo the experiment on this one. Using grease.
If the purpose of the experiment is to see at what point the hub fails then use a steel crank. If the purpose of the experiment is to see if the crank will split and fail then us an aluminum crank. Since we’re interested in the hub here, a steel crank is the way to go.
I paticularly noticed the bit about lubrication:
… As an example, the maximum safe torque for the M8 grade 8.8 bolt was quoted as being 21Nm or 15lbft. If the threads are lubricated with a high efficiency lubricant such as “Xylan 1014”, the maximum safe torque comes down to 8.5Nm or 6.3 lbft! On the other hand, if the nut and bolt are bone dry when assembled, it would not be unreasonable to tighten an M6 grade 8.8 fastener to (say) 10lbft, an M8 fastener to 20lbft and an M10 fastener to 45lbft. It really does all depend on the condition of the mating surfaces…
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