Help with crank!

Hiya, I just cannot seem to keep my left crank tight. It comes loose after I hop on it like 5 times, or if i just ride for like 2 minutes. I have tightend the nut and bolt really really tight, I even used a hammer to help tighten it by hitting the ratchet. Does anyone have any suggestions on what I might do? I appreciate your help.ohhh, and by the way it is a united 20’ in trials with alex dx 32 rim.
Thx, Derek:D

you’ve probably stripped the threads by tightening it so much. you can try using loc-tite on the threads. you can get it at your local hardware store. But if that doesn’t work you will probably need to get a new hub.

It is possible your crank or your hub is damaged. Riding with a loose crank can damage the taper on the crank and make it such that the crank will never be able to stay on.

Here are two threads about installing cranks:

If you continue to have problems with the crank then either the crank or the hub may be damaged and need to be replaced.

Do you have steel or aluminum cranks? Do you have the hub or some other hub?
Aluminum is softer than steel so you’re more likely to permanently damage an aluminum crank by riding it when it’s loose. Steel cranks are harder and that means that a loose steel crank can do more damage to the taper on the hub.

I have steel cranks, and the hub i have was an hub, it says,… CrMo on it.

The UDC hub has internal threads.
Check that the threads are all still good and that nothing has stripped. Then check the tapers on the hub. Are they still smooth? Have they been dented, rounded, or deformed? Then check the tapers on the cranks. Have the tapers on the crank been dented, rounded, or deformed?

If all looks good then put it all back together using Loctite on the threads and a little bit of grease on the tapers. Then use a torque wrench to tighten the bolts. If you don’t have a torque wrench then have a bike shop tighten the bolts or find someone who does have a torque wrench. If you have a Harbor Freight tool store nearby you can get a torque wrench for cheap ($10 on sale now). You need one that can measure up to 40 foot-pounds.

If the crank still comes loose then you’ll need to replace the cranks. Hopefully the hub itself is all OK.

stripped crank securing thread

one of my cranks worked loose on my super cheap uni that i’m learning on. I thightened it. I went to check the tightness of the other crank and the thread stripped instantly. they are square section tappered cranks and shaft.

on inspection it is the male thread of the shaft that has stripped (it was only holding by a couple of threads) and the nut is unaffected. I think the shaft is made out of fairly cheesy steel.

i’v thought of a few possible solutions to hold the crank on. I’v worked with cars all my life and have access to most tools, various nut and bolts and some machines and welding if i call in favours and do a bit of travelling so im fairly well equiped to repair or replace things.

the solutions:
iA) replace the shaft. are uni shafts special or will one off of a bike do the job? are replacement shafts available somewhere on the interweb if they are special to unis? how do the shaft and hub interface?

iB) call in a favour and get a friend with a lathe to modify a standard bike shaft

ii) grind and file the shaft so that the crank sits closer to the wheel allowing the nut to grip onto fresh threads that have not been stripped.

iii) glue the crank to the shaft

iv) call in a favour and weld the crank to the shaft

v) cut the wrecked threaded part off, drill and tap the shaft and add a bolt to hold the crank instead of a nut (make it an iny instead of an outy!)

vi)cut the crank radially, cross drill the shaft, add a nut and bolt and make it into a clamp fit

any thoughts, experiences or more ideas? personally im leaning towards ii) or v) as being fairly sturdy solutions.

If the taper on the hub is knackered and you need a new hub then welding the crank on will give you a bit more life out of it. I’ve done this quite a few times on older unis.

the tapper is fine. the crank was held on tight and has never been loose.

iA, the shaft and hub are sold as one for all unis, as they’re usually welded or sometimes pressed trogether. you could just get a new hub, you’ll have to check your bearing spacing but something like this will do the trick. You’ll need to build the wheel tho, and may well need new spokes if you can’t get a hub with the same flange diametre as your current one.

iB, the actual axle is pretty much the same, indeed many unis hubs use stock bike axles, you need to weld the spoke flanges on. Again you’d prob need new spokes, and learn how to build a wheel/pay for it to be built.

ii, good idea if youve got enough clearance on the inside, those tapers have to be quite accurate though, if the angle is much different from that inside the crank the cranks will sit on a single point and always work loose. You could do it on a mill if you’ve got one handy.

iii, I’m not sure how the glue will hold up, but worth a try if you don’t want to go overboard with it, ive heard of people glueing pedals in using metalweld stuff, so if you use a proper high grade adhesive with good surface prep (abrasion, alchol washing etc.) it mgiht well do.

iv. probably better than the glue, but one that will limit the life of your uni. if you ever wnat to change a bearing, crank length or badly strip a pedal hole you’re done for. (same applies to glueing I guess, unless you use something that has a solvent specced for it)

v. more recent hubs are made like this, it seems by far the best idea to me as long as you’ve got some way of holding the hub perfectly vertical in a press drill without dismantling the wheel.

vi. is there enough material to accomodate a bolt hole through the crank? most taper cranks are pretty skinny in this area. I don’t know how the clamping forces would work out, couldn’t say either way

In short, either buy a new hub and learn how to build wheels, or go from outy to inny. Hope that helps.

yeh i was thinking this was the most practical untill i looked up my drill press and found that it can only fit wheels of 14" diameter in the table. im gonna see if the shaft is a press fit so i can remove it for drilling, otherwise i guess it’s out with the pistol drill and spirit level!

I found if you aply lock tight it acts as a lubricant before it sets, then with a mallet drift the crank on by smacking it realy hard then keep tightning the crank bolt and smacking it again untill you can not get it tighter leave it. when the locktight is set your crank should be as solid and shouldnt loosen.

The pro’s answer

If you strip threads in an expensive part, say an engine block, or uni axle, is the helicoil.

You can buy at kit at any auto parts store that will have the correct size bit and tap to install a special tread insert.
Somewhat cheaper, if there is enough metal, would be to just drill out the axle threads, and tap new threads for a larger bolt.
Be sure to clamp the axle well, and use a drill press, or hire someone to do this. A hand drill is unlikely to make a straight enough hole. :sunglasses:

All this talk to answer a question as simple as, Square Taper sucks. I had a UDC hub and it did the same thing. I had one crank that was never tight. I finally bit the bullet and moved to a flanged hub. They are not that expensive and the aggrevation this crank is causing is worth every penny.

His hub has a male rather than female thread, there is no hole to retap, hence his disucssion of grinding away the end of the axle to drill a hole for such a thread.

Maybe other udc hubs are not female thread

I have a udc 36 " square hub, and it is female threads for the crank bolts.
It is difficult to tell if the smaller udc hubs use a bolt or not, from this picture.
A new hub, shipping, labor to lace it up, is 50 ish $. That or the drill and heli coil trick will be the only lasting option if the threads are really gone.
Welding may be a good idea. Fast and cheap. If not overdone, the weld can perhaps be ground down and the crank changed again later.

Indeed all the modern UDC hubs are female threaded, but he specifically stated that his was a ‘super cheap uni’ (so not UDC) and male threaded.

Maybe, maybe not

He makes mention further down in the thread,of having a UDC hub, marked cro molly. So what is it Uni_ok ? I kinds hope for you that Kingston is right, and you have stripped a bolt on the end of the axle. That design would be especially thoughtful for a novice uni commonly sold to someone who tightens things by banging on the rachet with a hammer, (hint-torque wrench is needed ).
Should this be the case, examine this “bolt” carefully. Perhaps you are so fortunate to have stripped a "stud ". Were this the case, the stub may be unscrewed with large vice grips and replaced for 2 $. :sunglasses:

Did i? erm…no:p that was someone linking to show a possible replacement.

yip the male threaded bit has the last two threads stripped off.

it was made of cheese and only 2 threads were gripping. for a standard thread to have full strength it requires 3 FULL threads (not the tappered run in or out) engaged. I much prefer to do things by “feel” than with a torque wrench, especialy with tappers and gaskets. With this one i didnt get anywhere near “feel” because it stripped as soon as i touched it: my hand was as close to the driving part of the ratchet as possible and i was not turning hard. I was so surprised when it stripped i initially thought it was a left hand thread and i had slackened it since many things on cranks and pedals are left handed threads. (i think i mean left hand thread, the backwards ones where lefty loosey, righty tighty is reversed).

affraid not.

i’ll fix it tomorrow night and let y’all know!

I’ve never owned a splined (not flanged - hubs without flanges are very difficult to build into wheels) hubbed unicycle. I don’t do trials, and I roll over/off stuff rather than leaping off it on a muni, but either way that isn’t the problem here. A properly installed square taper crank that is checked for tightness occasionally, mainly for the first few rides after installation, will be fine for a long time. The choice of crank lengths is better, and it’s a standard that isn’t going to disappear any time soon - compare availability of cranks for square taper hubs with that for the 1st gen splined KH hubs.

Having said that, the internal/female threaded axles as on the UDC hubs (and all current bike square taper BBs) are a vast improvement over the external/male threaded hubs that used to be common.

I’ve had the same problem with the left side crank on my hub. I just went out and bought the gluestick threadlocker loctite and I’m waiting for it to set. I really hope this fixes my problem because I’m tired of having to ride around with an allen wrench in order to keep the darn thing tight.