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Obviously the next step is to phone middleburn but just checking if anyone else has had similar problems to me... (LBS could shed no light and didn't have a 118mm for me to try)
Basically, Midddleburn recommend a 113mm axle for thier rs7s, which is what I have had fitted for quite a while using un72 and un73 bbs. These have finally run out so had a un55 in reserve which I have tried to fit.
When I do the cranks up on the un55 bb (73 shell btw) the lockring touches the flange of the bb before the crank is tight. Even just putting the crank on the bb without doing it up leaves only about a 2mm gap. The tapers look fine (the cranks are less than 12 months old) and nothing else has changed.
Help! Do I just need a longer BB?...
Isn't axle length dependent on frame rather than crank?
but yes, a longer axle sounds like the solution.
(I assume the old BB's were def 73x113 and the new one also measures 73x113?)
Nothing odd like un55 is iso vs un72 is jis is it? I can't remember which is more tapered, but it could result in the crank pulling on further if its the right / wrong combo
I think it's a problem with clearance on the 73mm shell.
68mm I've never had a problem with but I've seen it come a bit close.
Obviously a 118mm axle moves the chainline out 2.5mm which TBH isn't enough to cause major problem.
If you are running SS or 1x you could tweak middle ring position to compensate.
Axle length is dependent on the crank in theory.
The problem I guess is that your crank isn't a shimano one so it's probably that it has a longer taper than Shimano design their BBs for - a longer axle will probably sort it though obviously that'll throw out your chainline a little and widen the q-factor.
I had an issue the the tapers becoming worn (possible over tightening) over the years with some RS7 cranks.
A longer axle (115mm IIRC) resolved it as I wasn't the original owner.
I had the same issue on some newly purchased RS7s and they were swapped out straight away dealing with Middleburn direct.
Pending if they were yours from new I would give Middleburn a call.
Thanks for replies so far - old bb was def 73x113. Running ss, so can adjust a bit at the back if necessary.
Not sure about iso/jis as nothing on the bb or packaging. Waiting on a call back from sjs where I bought the cranks from at the moment, to see if they have encountered similar problems...
It's worth calling Middleburn direct. They have always been really helpful in the past.
I have had similar issues, but generally only with older cranks where the taper has worn over time. Or with some of the older chainsets where the chainline was designed to be narrower. With the new cranks, the spider/lockring are designed to sit further out.
Slight hijack, what are the current choices of square taper BB's? Are there any sources out there of anything other than poverty spec ones?
And is there any way of determining axle length for a frame/crank without trial and error?
UN55 is great - call it poverty spec if you like but it's got a hollow axle and good bearings. You can pay a lot more for a little bit of weight saving and no other real benefit IMO.
In theory, the manufacturer will spec an axle length for a set of cranks. What ones are they? Someone will probably know.
also worth bearing in mind that there are two thicknesses of lock ring for the middle burn spider, the thinner one is fine and dandy for me, the thicker one does the same thing as you describe, so it now sits on a fat bike with a massive axle length so not a problem
Those Shimano are all JIS so not a problem. I think it is just a close thing with a 73mm shell. When I was using st RS7s on a 113mm axle I had less than 1mm clearance and probably should have gone for 115mm.
You're not greasing the tapers are you? 😉
Isn't axle length dependent on frame rather than crank?
No and yes 🙂
Producer should suggest what BB axle length is suggested to get on with particular chain line.
After that you should either check or measure chain line in the frame and that should solve the problem.
Never had any problems with UN55 and they were giving me proper chain line as declared with various cranks (RF, Sugino, Middleburn RS1, RS3 and RS7).
I would say probably your tapers are going a bit tired...
Cheers!
I.
I've got RS7's and 8s on 113mm UN5X's with no issues (73mm and 68mm), they do rub like that if I try a 110mm though, and it's close with a 113 but never actually made contact.
Are you sure you've not got a BB intended for an e-type front mech?
Stronglight tripple 172.5mm road/toruing cranks. I've no idea which model though, and apparently according to google there are at least 2 likely ones with the same name but different designs.
fwiw I have RS7s on a Shimano 68x107mm, all ok.
Looks like Tazzy wins the expert award
Speaking to M-burn, there are indeed 2 thicknesses of lockring and I appear have the thicker one. Here's hoping that solves the problem...
Edit: nope, look off the lockring and it's about 4mm - off to SJS tomorrow then, to see if they can shed anymore light...
Had this issue with my wife's RS7s. Couple of minutes in the vice with a file sorted it out.
My RS7s were on a 68x107mm crank and that was proper tight clearance - super tight on the chainstay too but never an issue.
Here is hoping the RS7s and a 118mm BB work nicely with the Rooster chainstays . . .
Basically, Midddleburn recommend a 113mm axle for thier rs7s
Blimey! I never knew this.
Ive been running my SqT RS7's for years on a 122 BB (and plan to continue on my Rooster).
Update: SJS have been very helpful. It seems the tapers are probably not within tollerance, so they are going back to middleburn