How much difference do shorter cranks make for IM bike and/or run?

Nov 23, 22 5:45

Post #1 of 32 (3157 views)

I'm getting back into tris after spending 4 years racing crits/rr/tts. My current bike is a gen 2 Speed Concept with 170mm cranks, which has served me well (I'm 5'11") I feel very comfortable on it and can hold aero position for multiple hours. My previous stent into tris included sprints and olympic distance, but next year I'll be doing IM Florida. The bike will undoubtedly be my strongest discipline (plan on between 4:40-4:55). It seems that because of the BB90 bottom bracket, the shortest cranks I would be able to fit on the SC would be Shimano/SRAM 165s.

Question 1: Would switching to a shorter crank benefit me either during the bike or help with my run?
Question 2: If so, would switching to 165 make a meaningful difference, or do I need to switch to something smaller?
Question 3: If I need something smaller than 165, does anyone know of one that will fit a SC with BB90?
Question 4: If there isn't anything smaller than 165 that will fit the SC, would going smaller be worth selling the SC to get a different bike?

Thanks in advance for the help.

Last edited by: dhoose: Nov 23, 22 5:46

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

Nov 23, 22 6:16

Post #2 of 32 (3132 views)

1. Potentially both
2. Possibly. It depends on your current fit.
3. Look at rotor. You will need a crank that is 24mm to fit the shimano bb90 bb.
4. Doubtful

Unless you enjoy testing things on your own, I would suggest finding a reputable fitter that has a dynamic fit bike where he/she can easily adjust crank length so you can see how it feels and how it opens up your hip angle. It also can allow you to get in a more aero position. You can then decide what crank length you like. Remember that once you go shorter cranks, your seat needs to be raised, pushed rearward slightly, pad y needs to be raised, and pad x needs to decrease to match your current fit.

I'm about your size (5'10") and I have had a couple SC's. I'm running 165mm cranks. I was running 172.5mm cranks years ago but have found the 165mm cranks enabled me to be in a much more comfortable and aerodynamic position. I also have hip impingement issues so the 165 cranks have helped with that too.

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

Nov 23, 22 7:14

Post #3 of 32 (3066 views)

What crank are you currently running? If your current crank has a 24mm spindle then the sky is the limit. If not then you can get an adapter. Same goes if it’s GRX or DUB. Check out Wheels MFG, they’re pretty good about having parts to make parts fit.
I’m 6’1 and went with 160 arms on my TT bike and 170 on my roadie. Just pay attention to the finer adjustments to what Steve said about adjustments to this and that.
As to fit, my hip flexors have never felt better while in position.

Re: How much difference do shorter cranks make for IM bike and/or run? [stevej] [ In reply to ]

Nov 23, 22 7:23

Post #4 of 32 (3045 views)

Thanks, guys. Unfortunately based on my research, Rotor won't work (spindle isn't long enough) and DUB won't fit either.

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

Nov 23, 22 7:24

Post #5 of 32 (3042 views)

Rotor 155mm cranks.

Re: How much difference do shorter cranks make for IM bike and/or run? [jimatbeyond] [ In reply to ]

Nov 23, 22 7:34

Post #6 of 32 (3020 views)

Belay my last...Rotor will work!

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

FindinFreestyle

Nov 25, 22 7:49

Post #7 of 32 (2785 views)

Not that height is the best proxy, but assuming ballpark 730-760mm saddle height, the most common self selected crank length at your size is 160mm. The difference between 170 and 165 is significant. Rotor basically fits in any bike and goes down to 150mm. The crank is integral to approaching your most aerodynamic posture while maintaining aeorbic power.

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

Nov 25, 22 10:14

Post #8 of 32 (2720 views)

I’m a big believer in my 165’s at 5’10”.

Sure, aero is slightly more comfortable, but the big advantage is in less strain on brick days and racing.

The combo of slight impingement at the top of the pedal stroke that would create a little hip rocking and then hammering up the legs running would create a lot of discomfort in my hips and lower back.

I converted one bike and eliminated the pain, but then occasionally rode another bike and would wake the next day in pain. Over the next year every bike got 165’s and a similar bike and I am pain free.

Re: How much difference do shorter cranks make for IM bike and/or run? [Matt J] [ In reply to ]

Nov 27, 22 6:40

Post #9 of 32 (2495 views)

My fitter told me to take my 165mm cranks off my Tri bike, put them on my road bike, and buy a 155mm Rotor crank for the Tri. The idea being that the shorter crank (172.5 to 165 on road and 165 to 155 on Tri) was to open the hood up more and spin faster. I was skeptical but damn if he wasn’t right. Feels smoother with the higher cadence and I feel like I can sit in aero position more comfortably. I am 5’10” with a 29.5” inseam … short legs.

Re: [ In reply to ]

Nov 27, 22 7:52

Post #10 of 32 (2462 views)

I’m a guy who doesn’t have access to test things and do things by feel and stuff. I used to have 175mm crank and switched to 165mm. Everything remained the same. All I can tell you is I PRed in bike leg in 70.3 race recently and still had the decent run so I would say go for it. I used 175mm crank because the it came with the used bike. I rode like that for several years and decided to try 165mm because ST said so. I’m a guy who leaves things alone if they are not broken but shorter crank was one thing I had to try because my knee came up pretty high. I’m 5’8 and ride size 51 frame.

Re: [s13tx] [ In reply to ]

Nov 27, 22 9:01

Post #11 of 32 (2427 views)

You should try 155mm cranks.

Re: [jimatbeyond] [ In reply to ]

Nov 27, 22 10:40

Post #12 of 32 (2382 views)

jimatbeyond wrote:

You should try 155mm cranks.

I would like to but I don’t want to spend more money on my bike. Enough is enough I guess.

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

Nov 27, 22 10:59

Post #13 of 32 (2373 views)

https://www.youtube.com/...b_channel=PeakTorque

There are a million videos on what is right and wrong about long or short cranks by every level of YouTube expert.
Came across this one and couldn't resist muddying the waters.....👼

Re: How much difference do shorter cranks make for IM bike and/or run? [Marksf] [ In reply to ]

Nov 27, 22 11:58

Post #14 of 32 (2346 views)

Marksf wrote:

My fitter told me to take my 165mm cranks off my Tri bike, put them on my road bike, and buy a 155mm Rotor crank for the Tri. The idea being that the shorter crank (172.5 to 165 on road and 165 to 155 on Tri) was to open the hood up more and spin faster. I was skeptical but damn if he wasn’t right. Feels smoother with the higher cadence and I feel like I can sit in aero position more comfortably. I am 5’10” with a 29.5” inseam … short legs.

It's about time to change my chainrings and I am currently riding a 165mm 50/34 compact SRAM Force 22 GXP. With a 30" inseam I have seriously been considering 160mm crank arms and would not be opposed to trying 155mm in order to raise my seat a bit. This would open my hip angle on the up stroke which I think would be an improvement. Apologies to the OP but I have a couple of questions as well.

1. 10mm or 5mm reduction in crank arm length would be a pretty big change. Would I need to go to standard gearing (53/39) on the new crankset?
2. Neither 155mm or 160mm crank arm length cranksets are easy to find with 24mm spindle required for bb90 bottom bracket. Where can I find one?

Last edited by: Calvin386: Nov 27, 22 12:06

Re: How much difference do shorter cranks make for IM bike and/or run? [Calvin386] [ In reply to ]

Nov 27, 22 12:06

Post #15 of 32 (2337 views)

Calvin386 wrote:

Marksf wrote:

My fitter told me to take my 165mm cranks off my Tri bike, put them on my road bike, and buy a 155mm Rotor crank for the Tri. The idea being that the shorter crank (172.5 to 165 on road and 165 to 155 on Tri) was to open the hood up more and spin faster. I was skeptical but damn if he wasn’t right. Feels smoother with the higher cadence and I feel like I can sit in aero position more comfortably. I am 5’10” with a 29.5” inseam … short legs.

It's about time to change my chainrings and I am currently riding a 165mm 50/34 compact SRAM Force 22 GPX. With a 30" inseam I have seriously been considering 160mm crank arms and would not be opposed to trying 155mm in order to raise my seat a bit. This would open my hip angle on the up stroke which I think would be an improvement. Apologies to the OP but I have a couple of questions as well.

1. 10mm or 5mm reduction in crank arm length would be a pretty big change. Would I need to go to standard gearing (53/39) on the new crankset?
2. Neither 155mm or 160mm crank arm length cranksets are easy to find with 24mm spindle required for bb90 bottom bracket. Where can I find one?

I am no expert, but for me I had to raise my seat (started at 8mm and ended up about 11mm with the fitter) when I shortened my crank because the bottom of your pedal stroke is not as far. This also caused me to move the seat some. Basically, when you shorten your crank you will need to take a look at your seat height and front/back position. Lastly, the length of your crank is independent of your gearing. I did find that I spin much faster with the 155mm…. About 90-95 rpm.

As for finding a short crank, did you look at the Rotor 3D modular system?

Re: How much difference do shorter cranks make for IM bike and/or run? [Calvin386] [ In reply to ]

Nov 27, 22 12:28

Post #16 of 32 (2328 views)

You could shorten your cranks by 20mm and be fine.

Rotor Aldhu 24.

Re: How much difference do shorter cranks make for IM bike and/or run? [Marksf] [ In reply to ]

Nov 27, 22 12:34

Post #17 of 32 (2321 views)

I did look at rotor. I could only find 165mm with a 24mm axle to accomodate the bb90 bottom bracket

I assume you moved your seat back a little with the 10mm change to crankarm??

Re: How much difference do shorter cranks make for IM bike and/or run? [jimatbeyond] [ In reply to ]

Nov 27, 22 12:40

Post #18 of 32 (2315 views)

jimatbeyond wrote:

You could shorten your cranks by 20mm and be fine.

Rotor Aldhu 24.

Ahhh... was looking at VEGAST... Thank you

Rotor is compatable with Trek SC? No adapters or spacers?

Last edited by: Calvin386: Nov 27, 22 12:43

Re: How much difference do shorter cranks make for IM bike and/or run? [Calvin386] [ In reply to ]

Nov 27, 22 13:27

Post #19 of 32 (2265 views)

Just get the standard 24mm axle.

Re: How much difference do shorter cranks make for IM bike and/or run? [Marksf] [ In reply to ]

Nov 27, 22 17:48

Post #20 of 32 (2164 views)

Marksf wrote:

Calvin386 wrote:

Marksf wrote:

My fitter told me to take my 165mm cranks off my Tri bike, put them on my road bike, and buy a 155mm Rotor crank for the Tri. The idea being that the shorter crank (172.5 to 165 on road and 165 to 155 on Tri) was to open the hood up more and spin faster. I was skeptical but damn if he wasn’t right. Feels smoother with the higher cadence and I feel like I can sit in aero position more comfortably. I am 5’10” with a 29.5” inseam … short legs.

It's about time to change my chainrings and I am currently riding a 165mm 50/34 compact SRAM Force 22 GPX. With a 30" inseam I have seriously been considering 160mm crank arms and would not be opposed to trying 155mm in order to raise my seat a bit. This would open my hip angle on the up stroke which I think would be an improvement. Apologies to the OP but I have a couple of questions as well.

1. 10mm or 5mm reduction in crank arm length would be a pretty big change. Would I need to go to standard gearing (53/39) on the new crankset?
2. Neither 155mm or 160mm crank arm length cranksets are easy to find with 24mm spindle required for bb90 bottom bracket. Where can I find one?

I am no expert, but for me I had to raise my seat (started at 8mm and ended up about 11mm with the fitter) when I shortened my crank because the bottom of your pedal stroke is not as far. This also caused me to move the seat some. Basically, when you shorten your crank you will need to take a look at your seat height and front/back position. Lastly, the length of your crank is independent of your gearing. I did find that I spin much faster with the 155mm…. About 90-95 rpm.

As for finding a short crank, did you look at the Rotor 3D modular system?

Crank length is part of the gearing between foot and wheel. A shorter crank means that in order to maintain the same foot speed you will run a slightly higher cadence (as you noticed), so for the same bike speed you would need to be in a slightly lower gear. I agree it is unlikely that in practice you will notice this through a required change in gearing (though you may end up being one sprocket further up in the cassette). You certainly don't want to get bigger chainrings though - that would be the opposite of what is required!

Re: How much difference do shorter cranks make for IM bike and/or run? [jimatbeyond] [ In reply to ]

Nov 27, 22 23:51

Post #21 of 32 (2035 views)

New Ultegra 160mm crankset for 269€. Quite cheap vs aldhu. Found it at bike discount.de, I don't know the cost to ship in the US.

Re: How much difference do shorter cranks make for IM bike and/or run? [dhoose] [ In reply to ]

Nov 28, 22 1:58

Post #22 of 32 (1986 views)

Our JCOB cranks are a 24 /24 axle and will work, they are available down to 145mm in Std. Mid compact and compact gearing.

Re: How much difference do shorter cranks make for IM bike and/or run? [Calvin386] [ In reply to ]

Nov 28, 22 2:51

Post #23 of 32 (1968 views)

Calvin386 wrote:

jimatbeyond wrote:

You could shorten your cranks by 20mm and be fine.

Rotor Aldhu 24.

Ahhh... was looking at VEGAST... Thank you

Rotor is compatable with Trek SC? No adapters or spacers?

I made this move last year, BB90 with C-bear ceramic BB (24mm internal diameter) and ordered a 24mm alu Rotor axle with 155mm Rotor Aldhu crank arms and the MAS Aero spider. Best investment I made.

I bought the bike with 170mm cranks but found a lady willing to swap out the 165mm Ultegra crank on her small sized road bike for the 170's that were on my Speed Concept. Felt good but now with the 155's it's a lot better. I'm 5'7" with only 30" inner leg length (seat height now is 704mm) so quite short legged and the short cranks really allow me to spin more smoothly. No more/a lot less of a dead spot in my pedalling.

The required spacers to fit it to your BB are in the same box the 24mm Rotor axle is shipped in.

Last edited by: Tri_Joeri: Nov 28, 22 2:52

Re: How much difference do shorter cranks make for IM bike and/or run? [John Cobb] [ In reply to ]

Nov 28, 22 3:56

Post #24 of 32 (1920 views)

John Cobb wrote:

Our JCOB cranks are a 24 /24 axle and will work, they are available down to 145mm in Std. Mid compact and compact gearing.

hi john - can you give me the cliff's notes version of what you think the benefits are of shorter cranks for a triathlete?

____________________________________
https://lshtm.academia.edu/MikeCallaghan

http://howtobeswiss.blogspot.ch/

Re: How much difference do shorter cranks make for IM bike and/or run? [rmt] [ In reply to ]

Nov 28, 22 6:26

Post #25 of 32 (1840 views)

rmt wrote:

Marksf wrote:

Calvin386 wrote:

Marksf wrote:

My fitter told me to take my 165mm cranks off my Tri bike, put them on my road bike, and buy a 155mm Rotor crank for the Tri. The idea being that the shorter crank (172.5 to 165 on road and 165 to 155 on Tri) was to open the hood up more and spin faster. I was skeptical but damn if he wasn’t right. Feels smoother with the higher cadence and I feel like I can sit in aero position more comfortably. I am 5’10” with a 29.5” inseam … short legs.

It's about time to change my chainrings and I am currently riding a 165mm 50/34 compact SRAM Force 22 GPX. With a 30" inseam I have seriously been considering 160mm crank arms and would not be opposed to trying 155mm in order to raise my seat a bit. This would open my hip angle on the up stroke which I think would be an improvement. Apologies to the OP but I have a couple of questions as well.

1. 10mm or 5mm reduction in crank arm length would be a pretty big change. Would I need to go to standard gearing (53/39) on the new crankset?
2. Neither 155mm or 160mm crank arm length cranksets are easy to find with 24mm spindle required for bb90 bottom bracket. Where can I find one?

I am no expert, but for me I had to raise my seat (started at 8mm and ended up about 11mm with the fitter) when I shortened my crank because the bottom of your pedal stroke is not as far. This also caused me to move the seat some. Basically, when you shorten your crank you will need to take a look at your seat height and front/back position. Lastly, the length of your crank is independent of your gearing. I did find that I spin much faster with the 155mm…. About 90-95 rpm.

As for finding a short crank, did you look at the Rotor 3D modular system?

Crank length is part of the gearing between foot and wheel. A shorter crank means that in order to maintain the same foot speed you will run a slightly higher cadence (as you noticed), so for the same bike speed you would need to be in a slightly lower gear. I agree it is unlikely that in practice you will notice this through a required change in gearing (though you may end up being one sprocket further up in the cassette). You certainly don't want to get bigger chainrings though - that would be the opposite of what is required!

Thanks... that is kind of what I was thinking but with three forces (arm length, chain ring, cassette) working together, I wanted to make sure I wasn't missing anything

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