I just got Isis crank spindle spacers, and instead of the two I expected there are ten in the set. Do you just use one spacer per side, or do you use more than one? How do you know what is correct?
I was wrong. There are fourteen spacers, and they come in different thicknesses, from about 1/16" to about 1/4". So how does one determine proper fit?
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.
Here is a video that calls for 2mm space but the official ISIS spec is 3 to 5mm preload that is specified by the crank manufacturer.
UDC UK recommend a 3mm gap for ISIS cranks and I’ve always aimed for 2-3mm.
5mm sounds perhaps a little excessive.
I agree, 5mm sounds like a lot. Seems like it would either deform the crank more then needed or not push the crank up to the spacer when tightened. The ISIS specification states that the crank manufacturer is supposed to specify the preload for the cranks that they make. I’ve never seen a crank manufacture state what the preload should be. Seem like that should be included in details about each crank made.
How much pre-load should I design into my cranks?
This depends upon your crank material. For most aluminum alloys we have found a pre-load of 3-5mm is ideal (as measured according to drawing sheet three). The individual crankset manufacturer will have to determine their required pre-load for themselves, based on analysis and testing.
Especially for the price that they are now - they’re well manufactured so I agree it would be nice to see this information.
I’d also like to see things like torque specs given. People can just ignore them and go with “whatever feels right” if they like, but sometimes it’s nice to have a number to aim for.