Forum home Mountain biking forum MTB workshop & tech

Replaced cranks, is this too big of a gap?



Replaced my cranks as someone had crossed threaded a pedal (someone being me).
I thought it would be a good idea to upgrade in the process. Bought some Sunrace narrow wide cranks to replace the triple shimano cranks on there (upgraded other parts in the process) but I'm concerned this gap is too big.

The chain line would be really pushing it in the lowest gears (bigger rings on the cassette).

Do I need a different bottom bracket? Return the cranks and get more specific ones?

Any advice will be appreciated.
(Trek marlin 5 2020)

Posts

  • reaperactualreaperactual Posts: 564
    edited 30 September
    When you say changed for Sunrace Narrow Wide cranks and other parts in the process? Is it correct to assume you've upgraded to a 1x10 or 11 and changed the cassette, etc. too?

    I've seen gaps that size on square taper cranks but it's not important as long as you have the optimal chainline for your bike's rear frame dropout standard and the cassette you're running.

    You need more offset for correct chainline if going for 10,11 and 12 speeds compared to 7,8,9 speed cassettes.

    The only thing to worry about is the final position of the chainring and although unlikely, clearances from the frame.

    If chainline is wrong then a different length spindle bottom bracket would be the easiest and most cost effective option.
  • photonic69photonic69 Posts: 1,039
    Did you change the bottom bracket cartridge as well? As far as I know Square Taper is Square Taper so there shouldn't be any changes in gaps etc. What is the gap size on the drive side of the bottom bracket axle?
  • lesfirthlesfirth Posts: 1,077
    Square taper bottom brackets have different tapers and lots of different lengths.
    Sorting out what you need when you change things is a nightmare.
  • reaperactualreaperactual Posts: 564
    edited 30 September
    lesfirth said:

    Square taper bottom brackets have different tapers and lots of different lengths.
    Sorting out what you need when you change things is a nightmare.


    J.I.S. or I.S.O? Shimano and Sunrace cranks more than likely J.I.S. Accurately measure chainline, compare overall spindle lengths, choose correct length give or take 0.5mm, divide or multiply by two depending on the longer or shorter length required? 🤔
  • When you say changed for Sunrace Narrow Wide cranks and other parts in the process? Is it correct to assume you've upgraded to a 1x10 or 11 and changed the cassette, etc. too?

    The only thing to worry about is the final position of the chainring and although unlikely, clearances from the frame.

    If chainline is wrong then a different length spindle bottom bracket would be the easiest and most cost effective option.

    Yes, Managed to pick up some cheap 1x12 Sram SX parts and I sold off the other parts so it was cost neutral. New rear wheel for the cassette as well.

    The Bottom bracket remains the same bottom bracket (VP BC73, 73mm, threaded cartridge) So im guessing I would need a bottom bracket that has a 73mm shell but with a shorter taper.

    Appreciate the help and im not normally this dense (in a past life i only had road bikes that had HT2 BB's)
  • Did you change the bottom bracket cartridge as well? As far as I know Square Taper is Square Taper so there shouldn't be any changes in gaps etc. What is the gap size on the drive side of the bottom bracket axle?

    Drive side is exactly the same gap. It was easier to get a photo of the non drive side.
  • reaperactualreaperactual Posts: 564
    edited 30 September
    If you don't mind changing the whole set up to Hollowtech II then that would be the easiest solution except for the obvious cost. New BB and Non Boost HT 2 crankset is 49mm chainline, job done for around £120ish?

    Your trek Marlin 5 is non Boost. Correct chainline for 7,8 and 9 speed cassette is 44mm. Add 5mm offset required for 12 speeds.

    Get your chainline at 49mm and it will be the best it can be. You need to know what it is now, You may not be that far off currently with the new Sunrace crankset?

    Best quality square taper BB would be Shimano but get whatever has a 73mm shell width, ideally one that has just the right spindle length to get your chainline to 49mm, remembering that spindle measurement is the full length so you will need to take into account that you are adjusting width by half the amount per side. New square taper BB around £15ish?
  • Thanks Reaper. Full of knowledge. I'll measure it when I'm next in the garage and order the correct BB if necessary.

    At the moment its all cost neutral so I couldn't justify getting a new HT2 chainset and bottom bracket unless I absolutely had to.
  • 😎👍
Sign In or Register to comment.