Smaller chain ring or shorter cranks ... or won't it matter?

[Bristolian]

I have a bit of a quandary that I can't get my head around. My Specialized Allez if equipped with 3x9 Shimano Sora gears, with 50/39/30 chainrings, 175mm cranks and an 11-32 cassette. I'm finding it difficult to keep on top of the gears when using the big chain ring on even very gradual slopes (never mind the hills!). My cadence, and therefore, speed drops slowly and I find myself having to change down into the lower gears every 30 seconds or so. I don't have the same problem when riding on the middle chainring but then I struggle to maintain the much higher cadence that results.

I've mulled this over for a while and was thinking that life would be easier with a slightly smaller 48 tooth big chainring but then someone said why not fit shorter cranks on the basis that it takes less effort to push shorter cranks around than longer ones. Is that even right? If the answer is shorter cranks then I would most likely opt to convert to a 2x set-up with 50/34 rings and 172.5mm (or 170mm) cranks.

My understanding of the mechanics of pedalling isn't up to par so now I can't figure out which would give me the best result ... or maybe neither will - my head is spinning Can someone please point me in the right direction?

 

[presta]

why not fit shorter cranks on the basis that it takes less effort to push shorter cranks around than longer ones

No it doesn't, the shorter the crank the harder you have to press on the pedal all else being equal. A shorter crank is a higher gear ratio: more force over less distance.

 

[Fastpedaller]

You have a large spread of available gears, no need to change it. There is also no issue with using the middle chainring - I've been cycling over 50 years and my big ring isn't used very often. The big ring is for downhills!

 

[EckyH]

I'm finding it difficult to keep on top of the gears when using the big chain ring on even very gradual slopes (never mind the hills!). My cadence, and therefore, speed drops slowly and I find myself having to change down into the lower gears every 30 seconds or so. I don't have the same problem when riding on the middle chainring but then I struggle to maintain the much higher cadence that results.

"Top of the gears" means 50:11 or one gear lower? That would be real high gears (which I couldn't use even in the flat for more than 30 seconds).
We don't know at which speed you're cycling and which gears in particular you mean. It would be helpful if you could provide us that information.

If I get that right than one reason might be the gap between your expectation of yourself and your actual fitness in regards to strength endurance (power for high gears) and speed endurance (high cadence in lower gears). Both have to be trained.

But there are other factors which can lead to difficulties: people with shorter legs may find it easier to cope with shorter cranks. If you have the possibility to borrow a shorter crank with a compatible shaft, then give it a try. I switched from 170mm cranks to 175mm cranks on my commuter bike and I don't notice that much difference.

E.

 

[cyberknight]

I have used 175 mm cranks and it felt like it was harder to keep on top of the gear compared to 170 mm

 

[mickle]

There's a general move in certain areas of the cycling world to the use of shorter cranks. Its a very welcome and interesting development which goes against years of accepted 'wisdom'. However I've not seen anyone highlight what seems to me to be two obvious consequences of fitting shorter cranks: firstly that gear ratios will be affected, a shorter crank being the equivalent of installing a larger chainring (a ten percent shorter crank is a ten percent shorter lever), and secondly, that the bottom bracket drop really wants to be lowered by a corresponding amount to keep the pedal height in the same place at the bottom of the stroke. I'm not aware any manufacturers offering any frames with lower BB heights specifically designed to accommodate shorter cranks. Some mountain bikers are experimenting with 150mm cranks. An increased saddle height of 25mm is quite a big deal.

To answer the OP. Road bikes have gear ratios suitable for the use of professional cyclists riding in a peleton. For the vast majority of regular mortals that big chain ring is a complete waste of metal. Fit a smaller ring If you want to equip your bike with reasonable ratios that you're more likely to use. If you can be arsed.

 

[screenman]

I have six bikes here, one running 150 cranks, one 165 the rest 172.5. The saddle height is slightly different, I have never felt the need to lower the bottom bracket, hearing wise I have not got a clue I just pedal, switching from one bike to another feels no different. The reason I have experimented with different lengths is to try and relieve a hip problem, this was at the advise if a physio trained bike fitter. In time all my bikes will have shorter cranks.

 

[screenman]

I should add that a 75 inch year ratio remains the same, no matter what length crank you have on.

 

[ianrauk]

For the vast majority of regular mortals that big chain ring is a complete waste of metal

For my tourer/commute bike I changed from a regular compact to a super compact and it's probably one of the best things I've ever done. I'm now using more of the middle of the cassette as apposed to higher. It feels far more natural as I do have a high cadence.

 

[Dogtrousers]

I have a bit of a quandary that I can't get my head around. My Specialized Allez if equipped with 3x9 Shimano Sora gears, with 50/39/30 chainrings, 175mm cranks and an 11-32 cassette. I'm finding it difficult to keep on top of the gears when using the big chain ring on even very gradual slopes (never mind the hills!). My cadence, and therefore, speed drops slowly and I find myself having to change down into the lower gears every 30 seconds or so. I don't have the same problem when riding on the middle chainring but then I struggle to maintain the much higher cadence that results.

Hang on, you say you find it difficult to keep on top of gears in the 50T, but struggle with high cadence in the 38T, right?

But there is a significant overlap between the two. It's not like there is a set of high gears and a totally distinct set of low ones.

50/28 is 42" (low end of 50T) and 38/14 (high end of 38T) is 72".

 

[roubaixtuesday]

I should add that a 75 inch year ratio remains the same, no matter what length crank you have on.

Yes, but the force you need to push a crank is directly proportional to crank length for a given power output.

How this translates to how it feels I don't know; I would hazard a guess not much on the flat but probably quite a bit if it's very steep (where you're limited not by power output but by gradient). Speculation though.

 

[Dogtrousers]

I should add that a 75 inch year ratio remains the same, no matter what length crank you have on.

Are you channelling Sheldon
https://www.sheldonbrown.com/gain.html

 

[wafter]

I find shorter cranks more comfortable and think they're easier on my crap joints but as mentioned they give less leverage so produce less torque for a given pedal load, so effectively give a higher gear ratio.

I think there's still potential benefit generally from reducing joint articulation.

In any case the range of crank lengths available commercially is very small - Shimano typically do 165-175mm which is only about 6% difference; meaning the pedal would require 6% more force for the same torque at the crank / equivalent to a 6% higher gear ratio; which on a wide-range cassette like an 11-32 is about half the difference between two adjacent sprockets.

Personally I'd look into how you're using the gears; spacing between 50 and 39T sprokets is pretty modest by modern double standards at a bit more than 25% (if shifting from 39-50T) so you should have a lot of overlap in available ratios on the cassette between both.

Personally my Fuji remains in the middle 36T chainring nearly all of the time which provides a perfect range with the 11-34 cassette, and I only use the big ring on fast descents. Your 39T is obviously even larger, however you may be a stronger rider / travelliing faster since you're on a road bike.

Are you using the whole cassette in the middle ring? When you hit a hill and down-shift from the 50T, are you also up-shifting on the cassette?

 

[Alex321]

I have a bit of a quandary that I can't get my head around. My Specialized Allez if equipped with 3x9 Shimano Sora gears, with 50/39/30 chainrings, 175mm cranks and an 11-32 cassette. I'm finding it difficult to keep on top of the gears when using the big chain ring on even very gradual slopes (never mind the hills!). My cadence, and therefore, speed drops slowly and I find myself having to change down into the lower gears every 30 seconds or so. I don't have the same problem when riding on the middle chainring but then I struggle to maintain the much higher cadence that results.

I've mulled this over for a while and was thinking that life would be easier with a slightly smaller 48 tooth big chainring but then someone said why not fit shorter cranks on the basis that it takes less effort to push shorter cranks around than longer ones. Is that even right? If the answer is shorter cranks then I would most likely opt to convert to a 2x set-up with 50/34 rings and 172.5mm (or 170mm) cranks.

My understanding of the mechanics of pedalling isn't up to par so now I can't figure out which would give me the best result ... or maybe neither will - my head is spinning Can someone please point me in the right direction?

No, shorter cranks will if anything make it harder. The longer the lever you have, the easier it is to pivot it.

When you change from the big to the middle ring, you should also change "up" a couple of gears on the cassette (I.e. To smaller/harder cogs). With a 9 speed cassette, you may only want to go one gear in the "other" direction.

I always do that on my 50/34 11-32 (11 speed) setup, and if you look at di2, that is usually set up to do that automatically.

50 to 39 isn't even quite as big a jump as 50 to 34, so you should find doing that solves the issue.

With the range of gears you have, there should always be a gear in which you can maintain the cadence you want, until you hit a steep enough hill that you just can't pedal quickly, or you spin out on a downhill.

 

[Alex321]

There's a general move in certain areas of the cycling world to the use of shorter cranks. Its a very welcome and interesting development which goes against years of accepted 'wisdom'. However I've not seen anyone highlight what seems to me to be two obvious consequences of fitting shorter cranks: firstly that gear ratios will be affected, a shorter crank being the equivalent of installing a larger chainring (a ten percent shorter crank is a ten percent shorter lever), and secondly, that the bottom bracket drop really wants to be lowered by a corresponding amount to keep the pedal height in the same place at the bottom of the stroke. I'm not aware any manufacturers offering any frames with lower BB heights specifically designed to accommodate shorter cranks. Some mountain bikers are experimenting with 150mm cranks. An increased saddle height of 25mm is quite a big deal.

Technically, it doesn't alter the gear ratio at all - one revolution of the pedals still produces exactly the same distance travelled.

But it does make it feel like a harder gear, because of the lever effect you mention.

And bottom bracket wouldn't be lowered, it would be the saddle you drop a bit to keep the same distance between your bum and the pedal at the bottom of the stroke. When manufacturers supply bikes with varying crank lengths, that normally goes along with different seat tube lengths.

To answer the OP. Road bikes have gear ratios suitable for the use of professional cyclists riding in a peleton. For the vast majority of regular mortals that big chain ring is a complete waste of metal. Fit a smaller ring If you want to equip your bike with reasonable ratios that you're more likely to use. If you can be arsed.

Most modern road bikes come with 50/34 chainset as default, while the professionals will tend to be using 52 or 53 big rings. Bikes with lower end groupsets, such as the sora that the OP has will often even come with a triple chainset.

And a 50t big ring is certainly not a waste IMO. I spend most of the time when on the flat or downhill in the big ring. On the flat I'll usually be 3-4 sprockets up from the smallest (11t), depending on wind (and my level of tiredness). On even a fairly gentle downhill, I will usually end up using the 11t sprocket.

And I will still usually be in the big ring on hills up to about 4-5% so it really isn't a waste (I am a 65 year old non-competitive cyclist).