Bottom bracket width

[thousandwords]

Has anybody ever ran into clearance issues (with chain stay) with an 128mm spindle bottom bracket and had to go wider?

That's what I would like to order for a new build and I am a little anxious it might not clear. I have a spare 131 mm splined spindle and if I lay that thing across the stays it's tight...they apparently designed that particular frame for a massive tire clearance, although I do not know the exact numbers.

Thanks.

[Mark W]

That'll possibly depend on the cranks. Some cranks have much less offset/Q-factor and will cause more problem than others. If it's a trials frame, most are designed around the use of a 128mm length axle so you should be fine, but if you're using something like some Middleburns or older RaceFace cranks with minimal offset you'll possibly still encounter problems.

[thousandwords]

Thank you. I haven't decided yet on specific crank (Probably the trailtech sport lite splined) but I noticed that most if not all available cranks have an offset of 33-34 mm so so fitting purposes brand won;t make mush difference anyways (I just like the Trailtech because it comes with freewheel and bashplate preinstalled, plus Ali said in one of his bild vids that trailtech make some of the better freewheels and cranks if I remember correctly)

I will bite the bullet and buy an 128 mm BB and hope Q factor is wide enough to clear chainstays. BBs aren't super expensive (at least not the Comas that is the only available 128 splined BB on Tarty) so if I have to start digging for a wider BB later I won't cry too much lol.

 

[thousandwords]

BB and cranks came today (man Tarty ships fast!) so I had my answer. Between the width of the crank head (where it interfaces with the isis splines) and the flare of the crank arms, there is an enormous amount of offset, so the crankarms clear the chain stays be a good 3 cm or so. In other words I could have gotten a much narrower BB, even an 108 and still clear the chainstays by a good margin. Will probably other another BB because now I worry about chain line lol, the freewheel is so much out with an 128 that the rear hub sprocket will be very far off medially.

[aener]

Sprockets sit really far out on 116mm hubs - you'll be fine there unless you went for an unusual hub. The chainline on mods has basically always been shit, but it's good enough for what we use them for. A 128mm BB on a T-Pro with a fixed rear hub has proven itself to work fine on many, many counts.

Also note that a lot of the apparently excessive clearance is there to account for the rear slave pistons that you currently don't have attached. They protrude a decent amount, and will close up that gap considerably. It will still clear - don't worry about that - but it won't look so over-the-top when you have a brake on the frame.

[thousandwords]

Aener- thank you so much! Great points, and needless to say, once the HS33 calipers were in, the extra clearance was welcome, as you pointed out. Will stick with the 128 mm!

[Mark W]

The cranks will bed in and sit a little further on the axle after you use them for a bit, so that'll rob a little clearance too. If you did want to go shorter then you could probably get away with a 122mm, but that'd be about it realistically.

Chainline isn't a huge deal in all honesty - it's never great on trials bikes on the whole, but it's worth bearing in mind you'll generally have a bit of float from the chain links on the sprocket and freewheel teeth (especially if you have a 1/8" chain) so that helps smooth things out. Ultimately though you only need to see how insane chainlines get on modern 1x MTB setups to see what you can really get away with! I'm not sure I'd want to go as extreme on a trials bike as the chainline at the furthest ends of a 1x12 setup, but a few mm either way on a trials setup won't be a deal breaker.