I have tried to search for info on this, but having little luck. Only one video from Unicycle.com.
I am changing ISIS cranks for the first time. I am almost ready to go, but wanted to double-check how folks go about choosing spacer size.
I believe the video says to use calipers to measure. They say there is a 10 mm gap, so they use 8 mm spacings (if my memory serves me). Is this something everyone does, or can I get away with using a ruler? Is the formula just N-2, with N = number of mm space? I don’t (yet) fully understand the point of the spacers, but I have a feeling once I get inside there - I’ll get it. I know it is a buffer between the cranks and the bearings (or I think).
Thanks for any insight,
I bought two of the variety packs of spacers, so I just need to know
Not just a buffer. Spacers prevent the cranks from being over squished initially. 2mm should be fine, I think 2-5 is common, better to start on the small side to extend crank life. Just use a 2mm Allen key to measure if you don’t have a caliper. Over time, the crank will loosen and then you will need a smaller spacer to give the crank more room to squish on since it is a slightly tapered design. I’m not sure if my wording is good on this, but hopefully I portrayed the general idea.
If you remove and reinstall your cranks, or the spacers were too long to start with.
Once installed with the correct pre-load, I’ve never known the interface to wear/deform over time such that smaller spacers are then required.
I’ve replaced many a crank and spacer, but I know from experience that many unicycles come withe the wrong length straight from the factory…
Have you used venture cranks? Maybe the spacers I used was too long but it has happened to me. Last day of an offroad 161km ride, I had 3 UPDs on the last day and realised my cranks were a bit loose. Before the last day, everything was fine.
I have. Were the cranks installed on a unicycle as new or were the spacers measured to fit when they were first installed?
I didn’t measure them before install. It all felt tight and secure before the last day of the ride though. I definitely couldn’t tighten them sufficiently that day so I think the interface on the cranks wore a bit. I did the same too before on a ~20km ride on a 36er but that was my error (riding without stoppers - I couldn’t find them)
So my 36er has a set of damaged Spirit cranks on it (from installing on a Schlumpf hub without spacers) and when I put them on by hand there’s zero space between them and the spacers.
One day I’ll get some spacers the correct length and re-adjust the disk to suit, but I’ve not got around to that yet.
You wouldn’t guess that they’re damaged from riding it as they’re not quite damaged enough to move, especially not with how much I have the crank bolt torqued down
If the crank bolt somehow came loose then they would start to wobble and eventually fall off though.
A word of advice: do not use carbon fiber spacers for ISIS crank application. The spacers will eventually crack and become ineffective. Always use aluminum for this application. Why UDC has offered these is beyond me.
 Oh nevermind, the ones I ordered from UDC are aluminum yay.
Thank you. Well, its too late at this point seeing that I ordered from UDC for this project. I guess I’ll use them and just keep an eye out? Where do you find aluminum spacers?
Thanks everyone for your tips and advice. I am still foggy on how this actually works, but when I take everything apart I’m sure I’ll get it. I’m just waiting on the crank puller at this point. Should be doing this on Saturday.
I’ve not seen this yet. Is this a theoretical, or have you seen this happen?
It has happened twice to me and once to a friend I ride with. I don’t think carbon is an appropriate material for the relatively high compression force between crank arm and bearing.
Ahh, I’ve got at least one pair installed on my freewheel, but I’ll steer clear of them in the future!
Theoretically the compressive force on the spacer shouldn’t be too large, but we all know that spacers are often too large, and everyone just torques those crank bolts down as hard as they can…
That’s almost what I do but not quite. I use a 16 inch socket handle with an 8mm bit on the end then tighten “firmly”. The ISIS F.A.Q. (search for “torque”) says torque specifications are to be provided by the bolt manufacturer. The Kris Holm assembly instructions say: Firmly tighten the bolts in the ends of the axle …
Not to say carbon spacers won’t work, but that aluminum (or steel) is far less likely to crack under compression.
Ahh yes, I’ll be sure to set my torque wrench to firmly.
It does bother me mildy that I’ve never seen a torque rating provided for any unicycle components aside from Schlumpf hubs, especially as components get lighter and such things become more critical, some (Qu-Ax RGB seatpost clamps…) could definitely benefit from actual numbers when “firmly” can be enough to break things.
Perhaps the real question is do unicyclists have calibrated arms?
Remeber to check your torque wrench from time to time. I started with a wrench set to “not enought” around 6 y.o., than around 16 the torque wrench get past the initial period of bed-in and get to a “massively firm” due to everyday train. I read online that after marriage you’d better check your torque wrench every 20 year or so to keep it at the right torque of “firm”