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Crank length, cassettes, gear ratios, etc

 
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Tin Pot




Joined: 08 Jul 2013
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PostPosted: Thu Mar 16, 2017 7:48 pm    Post subject: Crank length, cassettes, gear ratios, etc Reply with quote

What is the lowest gear ratio you can get with common retail components on a compact 165mm or 155mm chainset?

Had a Tri fitting done and one of the main question marks left is which crank length I should go with on my new build bike.

(See the painful backstory: http://www.tritalk.co.uk/forums/viewtopic.php?t=99679&postdays=0&postorder=asc&start=0 )

Apparently my comfortable fit is fairly aggressive on 172.5mm, and with shorter cranks, 155mm, I can get a little more out of it, e.g. back angle drops from 22 to 19.

I couldn't tell much difference in comfort or output, so it's really a question of whether I can get cranks and the gear ratio I need...

I think I was grinding up the hills of Bolton on 172.5 34/27 at a ridiculously low cadence last year.
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jibberjim




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PostPosted: Thu Mar 16, 2017 8:13 pm    Post subject: Reply with quote

33x36 on 650c would give you about 40% improvement on 34x27 on 700c and should be easily available. Obviously 36 would mean big jumps so it would depend on what you want.

33x32 would still see about 20% improvement providing plenty of options and not needing much in the way of special difficult to source or expensive kit.
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Tin Pot




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PostPosted: Thu Mar 16, 2017 8:52 pm    Post subject: Reply with quote

jibberjim wrote:
33x36 on 650c would give you about 40% improvement on 34x27 on 700c and should be easily available. Obviously 36 would mean big jumps so it would depend on what you want.

33x32 would still see about 20% improvement providing plenty of options and not needing much in the way of special difficult to source or expensive kit.


33? I've not seen any crankset with a 33

I'm not familiar with Shimano or SRAM, but I'm under the impression they only go as short as 165, and as low as 34/32.

So I might get a degree lower or so, but does a 32 cassette over come the change to a 165 crank?

In fact is a degree or two a significant benefit?
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jibberjim




Joined: 15 Aug 2008
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PostPosted: Thu Mar 16, 2017 9:49 pm    Post subject: Reply with quote

Tin Pot wrote:
33? I've not seen any crankset with a 33


It's rare, but it's perfectly and not that expensive, if it's worth it over the very common 34 given the tiny difference is up to you - spa cycles or xxcycle will sell the chainrings, they both generally have most stuff.

Tin Pot wrote:
I'm not familiar with Shimano or SRAM, but I'm under the impression they only go as short as 165, and as low as 34/32.


34x36 is perfectly feasible too, and yes Shimano and SRAM only go 165 I believe, but rotor go smaller, and there's plenty of shorter square taper options that take standard BCD's

Tin Pot wrote:
So I might get a degree lower or so, but does a 32 cassette over come the change to a 165 crank?


Just changing to 32 vs 27 is much bigger difference than 172.5 to 165, it'll still gain you about 10%
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Cat5 in the Hat




Joined: 14 Mar 2011
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PostPosted: Fri Mar 17, 2017 8:20 am    Post subject: Reply with quote

jibberjim wrote:
Tin Pot wrote:
33? I've not seen any crankset with a 33


It's rare, but it's perfectly and not that expensive, if it's worth it over the very common 34 given the tiny difference is up to you - spa cycles or xxcycle will sell the chainrings, they both generally have most stuff.

Tin Pot wrote:
I'm not familiar with Shimano or SRAM, but I'm under the impression they only go as short as 165, and as low as 34/32.


34x36 is perfectly feasible too, and yes Shimano and SRAM only go 165 I believe, but rotor go smaller, and there's plenty of shorter square taper options that take standard BCD's

Tin Pot wrote:
So I might get a degree lower or so, but does a 32 cassette over come the change to a 165 crank?


Just changing to 32 vs 27 is much bigger difference than 172.5 to 165, it'll still gain you about 10%


SRAM Red goes to 162.5. Rotor do 155.

No amount of 32 vs 27 is going to solve a biomechanical issue with a closed hip angle.
For various reasons I am also looking to move to a shorter crank to open the hip angle. Certainly on the TT bike and perhaps on the road and CX.
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mattsurf




Joined: 28 Sep 2016
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PostPosted: Fri Mar 17, 2017 3:53 pm    Post subject: Reply with quote

I use a 170mm or 165mm crank, and to be honest can't really tell much difference, however, 172.5 immediately feels too long and 175 feels horrible. My inside leg is 780mm so using the fromula inside leg * 0.216 gives me a theoretical crank of 168.5mm

I see no difference in cadence, power or speed between 165 and 170mm cranks

In terms of gearing, I really don't notice, the difference between 165 and 170 is 3% the difference between a 27 and 28 tooth cassette is 3.7%. Even then I am not sure that the comparison is right, because the cadence between a short and long crank will be identical for any given speed, a longer crank has a slightly longer lever, but a shorter crank may give a slightly better pedal efficiency if you have short legs, so it is entirely possible that the efficiency and leverage improvement cancel each other out.

In terms of cassette, most standard length derailleurs will take a 32 cassette, so you could get away with a compact chainset (50/34) and an 11-32 cassette
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mattsurf




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PostPosted: Fri Mar 17, 2017 4:16 pm    Post subject: Reply with quote

Another option would be to go for a 46/33 or 48/33 chainset on a 110 bcd spider.

with an 11-32 and 46/33 your range is 34
with an 11-32 and 48/33 your range is 36

capacity of 6800/5800 GS derailleur is 37 so should be fine and both are less than 16 tooth difference on chainset


http://www.acycles.co.uk/specialites-ta-x110-11-speed-chainring-shimano-110mm-black-grey-16597.html?gclid=CJfaibP13dICFcYp0wodz8wATg
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Tin Pot




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PostPosted: Fri Mar 17, 2017 10:05 pm    Post subject: Reply with quote

Just remembered my back wheel is Campagnolo so I think the only reasonably priced cassette is a Miche 11spd 11-30.

But from everything I hear so far I dont really need to worry about changing the gear ratios to compensate for crank length shortening.

Not sure why I'd want to lose the top end from 50 down to 48 or 46 for a single tooth less on the granny ring.
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Cat5 in the Hat




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PostPosted: Sat Mar 18, 2017 8:57 am    Post subject: Reply with quote

You may not notice any difference in cadence/speed/etc between say 172.5 & 165, however you'll be a gear out (can't remember if it is higher or lower) to match the output.
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mattsurf




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PostPosted: Sat Mar 18, 2017 11:21 am    Post subject: Reply with quote

Tin Pot wrote:


Not sure why I'd want to lose the top end from 50 down to 48 or 46 for a single tooth less on the granny ring.


Shimano quote max difference between inner and outer ring on chainset of 16, if you wanted to go to a 33, the difference would be 17, some people do us a bigger drop - eg 53/36 without problem.
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Tin Pot




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PostPosted: Sat Mar 18, 2017 7:37 pm    Post subject: Reply with quote

mattsurf wrote:
Tin Pot wrote:


Not sure why I'd want to lose the top end from 50 down to 48 or 46 for a single tooth less on the granny ring.


Shimano quote max difference between inner and outer ring on chainset of 16, if you wanted to go to a 33, the difference would be 17, some people do us a bigger drop - eg 53/36 without problem.


Gotcha.
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stenard




Joined: 04 Sep 2013
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PostPosted: Mon Mar 20, 2017 11:54 am    Post subject: Reply with quote

mattsurf wrote:

In terms of cassette, most standard length derailleurs will take a 32 cassette, so you could get away with a compact chainset (50/34) and an 11-32 cassette

I still cant figure out why the SRAM Red rear mech (it's an older 10sp version) on my TT bike wont accept a 28t cog. All the tech docs says it should, but no amount of b-limit screw adjustment will allow it to accept one.

I'm contemplating Wales later in the year, and if I use the TT, I really don't fancy having a 25t as my largest ring. Will be tempted to stick a long cage cheaper SRAM mech on it for that race, with maybe a 30+ cassette, just so I know I can keep spinning.

Relevant to Tin Pot's original question is factoring in any front mech restrictions. The same TT bike rubs badly on the smaller cogs if in the little ring on the front, as I put a set of compact rings on for another race last year to make up for the lack of anything more than 25t at the back. Semi-compact and standard double are fine. It is apparently a known restriction of the frame and that particular front mech that you can't cross-chain little-little on a 34. That's workable though, as I just switch to the big ring once it begins to rub.
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Tin Pot




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PostPosted: Mon Mar 20, 2017 12:40 pm    Post subject: Reply with quote

stenard wrote:
mattsurf wrote:

In terms of cassette, most standard length derailleurs will take a 32 cassette, so you could get away with a compact chainset (50/34) and an 11-32 cassette

I still cant figure out why the SRAM Red rear mech (it's an older 10sp version) on my TT bike wont accept a 28t cog. All the tech docs says it should, but no amount of b-limit screw adjustment will allow it to accept one.

I'm contemplating Wales later in the year, and if I use the TT, I really don't fancy having a 25t as my largest ring. Will be tempted to stick a long cage cheaper SRAM mech on it for that race, with maybe a 30+ cassette, just so I know I can keep spinning.

Relevant to Tin Pot's original question is factoring in any front mech restrictions. The same TT bike rubs badly on the smaller cogs if in the little ring on the front, as I put a set of compact rings on for another race last year to make up for the lack of anything more than 25t at the back. Semi-compact and standard double are fine. It is apparently a known restriction of the frame and that particular front mech that you can't cross-chain little-little on a 34. That's workable though, as I just switch to the big ring once it begins to rub.


Same here.

In contradiction though, I've just rebuilt my roadie (Veloce 10spd) and small/small works fine...it's big gear to smallest sprocket I'm struggling with (!)
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Half Iron '17 7h39!, 6h28 '16 5h53
Olympic '16 3h18 '15 3h33, '13 3h36
Sprint '16 1h17, '14 1h40, '13 2h01
Half Mara '16 2h04, '14 2h07
10 Mile TT '16 00:26:30
Trail 10K '16 54:01 '13 54:46
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