8

Edit: The bike is in a place with good mechanics but few shops. As I'm visiting the USA soon, I was thinking perhaps of getting the parts there and having someone competent locally do the repair. I don't have a workshop with pillar drills, nor tools such as crank pullers.

I use my bike for shopping etc, and am no expert. I'm looking for help with a precise description of a broken part, so I know what to buy as a replacement.

I have a crank with a stripped pedal hole. What is the specification I'm looking for to buy a replacement part? Can I buy just the part I've marked in red (which I understand is called crank + spider), or do I have to buy the green part too (which I believe is called the chainring, perhaps belt ring). I don't know what's standardised and what I need to measure or specify.

The bike is a Trek Soho DLX from 2010 with Gates Carbon Drive.

This is the crank with the stripped hole. I measured the bolt-to-bolt as about 75.5 mm. enter image description here

The crank says "Bontrager Nebula" and the back of it has numbers:

enter image description here

This is the pedal.

enter image description here

This is the stripped hole

enter image description here

Improve this question
2
  • 4
    I have to say - you've documented this question very well. The photos are clear and well-lit, and show the problem clearly. Thank you for the effort; it is noticeable.
    – Criggie ♦
    Jun 6 at 3:09
  • 1
    @Criggie thank you for the compliment: I've worked in a lot of industries and learned that the more ignorant I am of the subject, the more I have to help the experts if I want them to help me. I might not know which numbers are important or what they mean, but for certain they mean something to someone.
    – jonathanjo
    Jun 6 at 8:44

3 Answers 3

Reset to default
5

The best and simplest plan here is get the damaged pedal thread repaired with a helicoil, so you're not replacing anything.

If you were replacing things, you're replacing at least the right crank and probably the crank set. Right cranks are not typically available independent of lefts.

Gates is very sensitive to the belt line being correct. Their on-paper tolerance is +/- 1mm, but in reality it will run better the closer to perfect it is. The real conversation about how to do the job in question isn't what cranks to get, it's what steps are needed to measure and re-establish the belt line with the various options you find available. You could replace your cranks with all sorts of other 130 BCD ones, but interpreting published specs (before ordering) and taking measurements (once in hand) to get to the result you need or determine if doing so is possible is its own game. In other words, just buying new cranks isn't enough because there's nothing that says the front belt line will land in the right place if you just stick them on your existing spindle. If you were to go the replacement route then you're probably going to be looking at a bunch of cranks where the nominal front chainline with whatever the recommend BB is won't necessarily be any kind of match for whatever is on the back end of this bike.

Personally I recommend basing front end belt line setup on firsthand measurements, because using abstract published numbers to do it can lead to problems with errors stacking up from frame alignment, lack of standards for how to measure belt line as compared to chainline, etc etc. Better to have whatever array of BBs and/or spacers might be needed at hand and configure it to perfection. But best of all to keep with what you have if it's working.

Improve this answer
4
  • 1
    Thanks this is very helpful. A question: what is "BB"?
    – jonathanjo
    Jun 6 at 8:27
  • 4
    @jonathanjo BB is "bottom bracket" - not the most obvious name for the assembly of bearings and axle/spindle onto which the cranks attach
    – Chris H
    Jun 6 at 8:51
  • Is the "belt line" the distance between the plane of the front toothed wheel and the back one?
    – jonathanjo
    Jun 6 at 16:01
  • @jonathanjo Yes basically. Ideally they would be on the same plane. With chains it's measured down the center of the teeth. Putting belt components on instead will locate things differently; if the nominal chainline for a hub is 47.5mm and the crank is 47.5mm, that doesn't necessarily translate to just being able to put belt parts on and go.
    – Nathan Knutson
    Jun 6 at 16:34
3

I reckon your 75.5 is actually a little bigger. That, with 5 arms, could give you a PCD (pitch circle diameter, or BCD/bolt circle diameter - calculator) of 130mm, which is a common size.

A search on "130BCD crank" looks very promising.

You also need to know the interface between the crank arm and the bottom bracket. It's fairly likely to be square taper from what I can see in the picture. That gives you a range of options. Adding "Bontrager Nebula" to the search I find the exact crank you've got, or I'm seeing a brand called LitePro (I've never heard of them either) as a common and quite cheap option.

Just check that you get the same length as well - yours are marked 170mm, which is common.

Improve this answer
5
  • 1
    Nathan makes some good points about belt drives - I reach a different conclusion and would get the Bontrager cranks that are a direct replacement as they should give you a chainline identical to when it left the factory. But I've been very glad I didn't plan on getting some cranks helicoiled in the past (the crank puller thread was also ruined and I had to remove the left one destructively)
    – Chris H
    Jun 5 at 20:17
  • 1
    Many thanks this is very helpful. A question about BCD: My understanding was that 130BCD would have a bolt-to-bolt of 130 sin(360 / 10) ie 76.4 mm, and 75.8 / sin(36) implies a BCD of 129. I'm not criticising: I'm trying to learn! Especially many thanks for spotting the 170 mm crank measurement.
    – jonathanjo
    Jun 6 at 8:41
  • 1
    You're right, I must have made a typo somewhere. Wolftooth's guide might be more helpful anyway. It shows that the options for 5-bolt symmetric cranks are pretty limited.
    – Chris H
    Jun 6 at 8:49
  • 1
    Am I right in inferring that the spider detaches from the chainring (beltring?) by the 5 bolts? I live in a place with competent mechanics but not very many shops for parts, and am travelling soon to a few US cities. My thought was to buy the right thing, get a mechanic to fix it when I return. Re BCD: I'm sure I could be a millimetre out in my measurement.
    – jonathanjo
    Jun 6 at 9:03
  • 1
    Yes, 5 rather fiddly bolts, though the term "spider" is normally used for the bit that stays attached to the crank, while the whole of the piece that comes off is the called the chainring
    – Chris H
    Jun 6 at 10:19
2

I second Nathan's answer. Take the crank or the whole bike to a Local Bike Shop (LBS) for a proper repair.

I've done helicoils on bikes before but never on a pedal crank. The thread is 9/16" with 20 turns per inch which is not a standard number, and pretty much exists only for bicycle pedals. If you try and use the common size of 18 TPI, then the pedal simply won't thread in at all. The pedal axle is hardened and won't take a different thread easily, don't get caught out.

The taps and inserts needed to do this work are therefore quite unique and thus expensive, and so it is beyond the realm of the home machinist. Generic engineering shops are unlikely to have it unless they specifically work on bicycle repairs, and your LBS should either organise this or direct you to the good firms.

Improve this answer
9
  • 1
    A helicoil kit with 2x10 inserts is $481 NZ or about $300 USD or 275 euro. Plus one would need a pillar drill or a mill, no way this could be done properly with a hand-held drill. cycletrading.co.nz/products/… example. A common similar size is M14 and the helicoil set for that is around 1/9 the cost at $52 NZD.
    – Criggie ♦
    Jun 6 at 3:08
  • 1
    Many thanks for this. A question: what is LBS and EBS? I know about the helicoil possiblity, but am certain I wouldn't want to do it myself (no handy pillar drill), and the taps are expensive -- I suppose if my repair shop has the tools it might be practical.
    – jonathanjo
    Jun 6 at 8:25
  • 1
    Given the shape of cranks, I'd want not just a pillar drill but a decent jig or a very nice, very adjustable vice. I know I had the option of posting my cranks to get the helicoiled for around £20 per crank (call it €/$25-30) plus postage
    – Chris H
    Jun 6 at 8:54
  • 1
    I think even getting the crank off would be a challenge with my limited tools, but the prospoect of postal helicoiling certainly sounds a plausible option.
    – jonathanjo
    Jun 6 at 9:12
  • 2
    @ChrisH "riding with a loose pedal" I plead guilty your honour. "Which was probably only on one side" I think it was. I believe there is a saying about stitches made in time and their ninefold benefit.
    – jonathanjo
    Jun 6 at 12:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.