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advice on chainset replacement

hoysy7hoysy7 Posts: 19
in MTB workshop & tech
hi
I am running a Shimano XT FC-M8000, 36x26T, 175mm CRANKSE with
Shimano SLX CS-M7000, 11-42T CASSETTE but i find i run out of gears for speed on the flats just need a bit of advice of what to change to give me more gears for speed .spinning out at 19mph , hardly ever use the climbing gears so not bothered about losing to many of them could i just change the cranks or the cassette or will i have to change both thanks

Posts

  • steve_sordysteve_sordy Posts: 2,438
    I confess to being a bit puzzled by your numbers.
    I have a 11-46t cassette and a single front gear of 34t and I spin out at 30mph. This is at the point where I can feel the bike running away with me and the gears are overtaking my pedals, I can feel the clonk. I don't know what my cadence is , but I don't want to pedal any faster.

    So in other words my 34t driving the 11t on the rear spins out at 30mph. My wheels are 29er. If my wheels are at the other extreme, say 26", then with that gearing, I would spin out at 26/29 x30mph = 27mph.

    If I had your big front ring, I would spin out at 36/34 x 30mph = 32mph (with a following wind).

    Both cases suggest that you should be going faster.

    The only way I can make the numbers work is if you are using the small front ring (26t) and are on a 26er, in which case I get 20.6mph. (26/34 x 26/29 x 30mph)

    That is pretty close, but surely you are not trying to go fast using the small front ring? Hence my puzzlement.

    Anyway, the answer is to fit larger front rings, or just the one and go single speed. There is no need to change the cassette as well. If you fit a larger big ring, you will need to check your chain length as it may be too short. If it isn't, then it is too long now.
  • JBAJBA Posts: 2,852
    As Steve says, if you are spinning out at just 19mph something is wrong.
    I have a 32T chainring and an 11-42 cassette on my full sus and spin out at around 28mph.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • steve_sordysteve_sordy Posts: 2,438
    JBA said:

    .......if you are spinning out at just 19mph something is wrong.
    I have a 32T chainring and an 11-42 cassette on my full sus and spin out at around 28mph.

    Using the same approach as my above post, that is exactly what I would have predicted. You have 32t, I have 34t. (32/34 x 30mph = 28.2mph).
  • hoysy7hoysy7 Posts: 19
    ok thanks for the help i will simplify this forget about the speed i spin out on i must have it wrong ,what i do know is with my chain on the front big chainring the only gears i use are the bottom 3 cogs on my cassette .so what should i be looking for if i want more gears to increase speed the bike i have is a cube 2017 pro hardtail 29er thanks
  • pblakeneypblakeney Posts: 24,700
    The 11 at the back is about as small as you can go so look for a bigger ring assuming one will fit your cranks and the derailleur can handle it.
    The above may be fact, or fiction, I may be serious, I may be jesting.
    I am not sure. You have no chance.
    Veronese68 wrote:
    PB is the most sensible person on here.
  • JBAJBA Posts: 2,852
    edited April 2020
    You could try swapping the chainrings to 28/38 if your front derailleur has the capacity and there is enough clearance at the chain stay.
    That will give you a few extra mph.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • Charlie_CrokerCharlie_Croker Posts: 1,662
    Link --> https://www.cube.eu/uk/2017/hardtail/ltd/cube-ltd-pro-2x-blackline-2017/
    If I’m right you’ve got one of these, I also believe the above calculations are correct. I don't know the max capacity of your front derailleur but without changing that component too I can't see it being much greater than what you have.
    A look at Sheldon Browns gear calculator shows at 90rpm cadence you should be hitting about 26mph. How are you measuring your speed, is it accurate

    https://www.sheldonbrown.com/gear-calc.html perhaps you need some cadence work



    Perhaps it’s the wrong bike for your chosen application and a different one would be a better answer here
  • steve_sordysteve_sordy Posts: 2,438
    What wheel size do you have?
  • hoysy7hoysy7 Posts: 19
    Charlie_Croker said:

    Link --> https://www.cube.eu/uk/2017/hardtail/ltd/cube-ltd-pro-2x-blackline-2017/
    If I’m right you’ve got one of these, I also believe the above calculations are correct. I don't know the max capacity of your front derailleur but without changing that component too I can't see it being much greater than what you have.
    A look at Sheldon Browns gear calculator shows at 90rpm cadence you should be hitting about 26mph. How are you measuring your speed, is it accurate

    https://www.sheldonbrown.com/gear-calc.html perhaps you need some cadence work



    Perhaps it’s the wrong bike for your chosen application and a different one would be a better answer here

    went out today and i did manage about 29 mph and your right it is probaly the wrong bike for what i want to do but until i can get a gravel bike i was hopeing i could maybe get away with maybe a small upgrade somewere to help me till i got my new bike
  • hoysy7hoysy7 Posts: 19
    steve_sordy said:

    What wheel size do you have?

    29inch
  • The RookieThe Rookie Posts: 27,811
    I'm a bit of a spinner and can manage 33mph on 30/11 (29er), but your speed isn't far removed. The only choices are
    1/ Bigger front chainring
    2/ Convert to an XD driver hub and thus a 9t top gear (although at that size it can feel lumpy)
    Bigger chainring (and longer chain) is the obvious choice, you can get away with a 40t with your existing 26T (14T is considered the max difference you can get away with) and will likely need to move the front mech up the seat tube to cope.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • steve_sordysteve_sordy Posts: 2,438
    hoysy7 said:

    ..............I went out today and i did manage about 29 mph ...............

    So 19 mph was a typo?
    If you can do 29mph, then all is well, there is nothing wrong. Pretty much all you should do is to fit a larger front ring.
  • hoysy7hoysy7 Posts: 19
    The Rookie said:

    I'm a bit of a spinner and can manage 33mph on 30/11 (29er), but your speed isn't far removed. The only choices are
    1/ Bigger front chainring
    2/ Convert to an XD driver hub and thus a 9t top gear (although at that size it can feel lumpy)
    Bigger chainring (and longer chain) is the obvious choice, you can get away with a 40t with your existing 26T (14T is considered the max difference you can get away with) and will likely need to move the front mech up the seat tube to cope.

    thanks for the advice
  • hoysy7hoysy7 Posts: 19
    steve_sordy said:

    hoysy7 said:

    ..............I went out today and i did manage about 29 mph ...............

    So 19 mph was a typo?
    If you can do 29mph, then all is well, there is nothing wrong. Pretty much all you should do is to fit a larger front ring.
    cheers
  • david37david37 Posts: 1,313
    hoysy7 said:

    steve_sordy said:

    hoysy7 said:

    ..............I went out today and i did manage about 29 mph ...............

    So 19 mph was a typo?
    If you can do 29mph, then all is well, there is nothing wrong. Pretty much all you should do is to fit a larger front ring.
    cheers
    and start racing
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