30mm bottom bracket and 30mm spindle axle compatibility, should it go in by force? I am not sure if this is a stupid question or not. Finding compatible parts is not so easy for MTB, after quite a search I found my bottom bracket shell is not so standard, PF107.
I bought a Rotor raptor bundle, this includes a supposedly-30mm external-diameter spindle axle that should fit the Rotor bottom bracket 4130, which supposedly has 30mm internal-width. I did not buy the wrong parts, they are mentioned in the manufacturer website, even the BB says it is compatible with the axle.
However, there is no way I can put the axle through the bottom bracket, at least with my hands. They both have 30mm exactly, I would say either of them should have a slightly different measure, so that the axle can slide into the bottom bracket.
I am new to fitting MTB parts, but my road bike doesn't have this issue, the spindle goes through the bottom bracket effortlessly.
Hence the main question is: Should the axle fit by force into the bottom bracket? (I would need to apply quite some pressure, probably just like fitting the bottom bracket to the frame).
If not, would I have the same problem with other 30mm bottom brackets?
I am wondering if this is a design mistake on the bottom bracket or on the axle. If most 30mm bottom brackets have slightly more than 30mm internal diameter, then a 30mm diameter spindle can fit, and I can just return the BB-4130, go and buy another BB107 for 30mm and solve the problem.
Otherwise, if the axle is too wide (so, most 30mm spindles are slightly less than 30mm and this one is not), then it is pointless trying to fit this crankset and I should return all the products right away.
I do not have another 30mm crankset or BB to know what's the issue, my road bike uses 24mm.
Any help is much appreciated. Thanks!
There is no difference in the fit expected based on road vs mountain.
I thought I would be able to provide tolerance numbers from the original BB30 technical documents put out by Cannondale. They used to be at www.bb30standard.com, which is defunct now but can be found on Wayback. However, turns out it seems to have only ever had shell dimensions. The spec seems to have left the specifics of spindle and bearing tolerances to manufacturers.
Generally speaking, it's a pretty tight slip fit, same as you're used to. I understand that you've measured with calipers and gotten 30mm exactly for both. With higher precision tools you could see more of the story, i.e. if something is out of round or if the parts are actually requiring a press fit or what. (Note that among commonly encountered vernier calipers, some have .05mm precision and some .02mm, which could be enough to make the difference in this case, which it usually doesn't with anything bike.)
I've encountered similar situations where it took some force to get it all together, but if it was a press fit I would be unwilling to do that unless Rotor told me it was how it was supposed to be, which I don't think it is.
Hammering in the spindle is not a good idea because side loading the bearings like that can damage them. It's the peak loads from hammering that can do it, and knowing how much is too much is a guessing game. Gentle taps are probably okay, especially if it works, but I still don't love it. I believe the notion is that radial bearings are supposed to be able to handle 10% of their allowable radial loads in the lateral direction. Usually when it's tight I try to walk the spindle into the first bearing by wiggling it up and down with force behind it. I deal with a lot of BB30 and do not find myself having to hammer cranks in basically ever.