Crank bolt came loose after about 6 hours, what did I do wrong?

Unicycles and Equipment
21

Well okay but that sorta throws out the whole measuring part. And I think that part is important.

22

The ISIS standard is:
“The standard instruction for determining the required spacer thickness is to push the crank on by hand and then select a spacer about 3 to 5mm less then the space from the bearing to the crank. With the correct spacer the crank will fit tight on the tapered ISIS spline and against the bearing when the attaching crank bolt is tightened.”
Some call for 2mm space but that seem low with the official ISIS spec calling for 3 to 5mm. So you should be ok with a thinner washer to provide a 3 or even 4mm preload.

2 Likes
23

UDC UK told me 6mm or 7mm for those cranks. I use 7mm on both sides. Why different spacers on left/right side?

1 Like
24

Just because the calipers measured different gaps on each side.

25

Thanks for that link. Haven’t read this information yet (didn’t know it existed).

26

Hey RB,
After you “tighten” the crank bolt, try this.
Tap the end of the crank or pedal a few times.

Is there any looseness?
If there is, then it means two things:

a.) That means the conical outside geometry of the spline + conical inside geometry of the crank is loose. Solution: buy some brass shims(.001-.010" thk are available from mcmaster-carr, or jam some aluminum foil) between the spline and crank. This will close the gap.

b.) The compression length wise of the screw against the crank + shim is loose. The screw bottoms out and can’t bring the two parts any closer. You are just “crushing” the bottoming out of the screw.
Solution=add a washer that will fit. Then screw has more threads to move and “crush” properly.

Good luck

1 Like
27

There isn’t any looseness after I tighten. The whole thing is super tight.

28

FYI for those that have helped - this is what I did today. And hopefully for anyone reading in the future that is searching for all-things-loose-cranks:

  • I took the cranks off and redid the whole thing
  • I used the 3-5 mm rule instead of 2 mm (honestly I think this is gonna fix it)
  • The above point led to smaller spacers (9 mm and 6.8 mm spacers)
  • I extended the arm of my allen wrench and was able to get the bolt tighter

I’ll probably follow up, but if I don’t - it’s because its working well.

Thanks again everyone.

2 Likes
29

In my experience, you don’t have to gorilla the crank bolt. Tight is right, but overtight is not. But indeed it is important to recheck the tightness of the bolts a few times after mounting cranks (and riding them some). More frequently in the beginning, but even after a year they might work loose somewhat. This is normal and no cause for concern

1 Like
30

Good to know. Thank you.

31

And cottered cranks as well, in case you encounter any antiques. :slight_smile:

Speaking as an old guy who as way more experience with square taper (and cottered), cranks are one of the parts on the uni that have the most torque applied to them. It’s not unusual for them to have some looseness during the first ride after re-install. In the past, when I rode mostly square taper (Coker), my personal rule was to give a little “push” with a wrench on the cranks and on the pedals. Keeps you from having to walk as much. Though in recent years, I haven’t done much of that due to splined 36" and splined Muni. The pedals stay tight because I basically never idle or go backwards on those.

1 Like
32

Heyyy I know of you! Thanks so much for the comment.

33

One side note: not all ISIS bolts are the exact same length (extreme example: the KH rolodisc bolts).
So it would be good to check/verify the length of the bolt whenever there is a suspicion of bottoming out :wink:

1 Like
34

Use the same on both sides. There’s no point having different sizes on the two sides. You might need to do one side up rather tighter in the first instance, but all will be just fine. The tighter crank will just deform slightly more this time.

1 Like
35

Then which side do I measure? Doesn’t really make too much sense to me. What is the detriment to using two different sized spacers? What does it hurt?

36

I would use whatever size is required on each side, no matter if they happen to be different or not. Spacers are cheap and it makes sense to use the correct size.

1 Like
37

And the interface on aluminium cranks is malleable, and will be perfectly happy being smushed on slightly more.
Using different length spacers just forever means you’ve got to remember which side is which, or keep measuring, and you’ve got to buy two sets of spacers as the come in pairs of equal lengths.

One crank will of course be slightly closer to the centre of the wheel than the other too, but I suspect very few people (if any) would be able to tell that.

Whatever you do it’ll likely be better than most unicycles seem to be from the factory, manufacturing tolerances seem to mean that spacers are rarely the perfect length on new unicycles.

1 Like
38

Right I agree.

39

I don’t have to remember anything, though. I have a set of calipers and just take the 10 seconds every time I change cranks to measure the gap. I dunno; it doesn’t sound like a big deal to me BUT I don’t change cranks everyday. If I had to change cranks constantly I’d probably cut corners.

I appreciate the notes and discussion. I’m still learning :smiley: