Mother_of_birds

I've just got an indoor smart trainer set up and the road bike I have on it came with 175mm crank arms (Crankset - Shimano Sora 9 speed 50/34T). I'm 5'3 (160cms), inseam approx 74cms (not sure if supposed to be measuring with or without bike shoes on?) so according to all the calculations and charts out there, its coming up with anything between 152.5 and 160mm. I have quite good flexibility/mobility so I can ride the 175mm but I definitely can tell I'm compensating in order to do so. SO, I went online trying to find Shimano Sora 9 speed 50/34T in either 155 or 160 only to discover they only go down to 165mm. I know almost nothing about bikes so I don't have a clue where to go from here. Please can anyone advise what options I have or is the 165mm the best I can get?

big john

I'm not an expert on crank length but I think someone your height should be using a shorter than 175. If you can get a 165 that would probably work for you. How much difference does it make? I don't know but some people seem to think it's important and some don't seem to care about a few mm either way.

I think if you wanted to use something like a 152.5 you would probably have to have it custom made. If you are trying to address a specific issue it might be worth it. But if you're just spinning along on a trainer I think the 165 would be fine. Probably a 170 would be fine.

Mother_of_birds

I bet you know a lot more than me lol I'm completely new to all this. Do you think it's worth the money to reduce by only 10mm going for a 165mm? I'm only going by the online calculators and videos which so far for my size have suggested max 160mm. I didn't know if maybe I could get a kids one or something? But I have no idea on how interchangeable they are. The only reason I think it's an issue is at the top of the pedal stroke it feels like I lose connection with the bike and have to chase it to catch up if that makes sense. When seated I can adapt to it and make the full stroke which is OK for now but in long term it's not good. When I try to stand though I can't compensate so it's like each rotation is broken at the top so not able to do a complete stroke.

Carbonfiberboy 

My 5'1" wife got along fine for years with 170 cranks. I finally got her a pair of 151 custom cranks which she does like better. I'm 5'6", 30" (76.2mm) inseam and have 175 cranks on our tandem which work fine for me. Certainly 165 will be better for you. Assuming that knee pain is not an issue for you, the effect of too-long cranks is mostly that one is more comfortable at lower cadences. The advantage of lower cadence/long cranks is that one's heart rate and pedal force is a bit lower at any given power, though I think that's not really much of an advantage.

Jack Tone 

Found this on Amazon:

https://www.amazon.com/Shimano-9-Spe...410044&sr=8-46

Mother_of_birds

Not had any issues with knee pain just the feeling of loosing connection with the bike that I attempted to describe in reply above.

Mother_of_birds

Thanks, that's the one I've been looking at but not sure it's worth it to just reduce by 10mm? Was hoping to try 155 or 160mm

Jack Tone 

If you're 5'2", it might make a huge difference..

big john

I rode with a friend today who is not much taller. She has been riding for close to 40 years and raced as a Cat 2 years ago. She has never been concerned with using super short crankarms. She's using 170s these days and she spins faster than most.

sdimattia

I'm 5'2" with a 29in/73.6cm inseam (barefoot). Been riding with 165mm cranks with no issues for years.

Jean_TX

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The 165’s would have more effect than just 10mm :

Think about the circle that the pedal makes in one complete revolution – the circle for the 175’s has a diameter of 350mm whereas the circle for the 165’s would have a diameter of 330mm . The 20mm difference in diameters is a tad more than 3/4 inch.

So height wise, with the 175’s your foot has to be raised/lowered ~3/4” more than with the 165’s. And fore/aft wise, with the 175’s your foot has to travel ~3/4” more than with the 165’s.

bruce19

Your crankset/arms are basically a 1st class lever. What that means is that the longer the lever (crankarm) the easier it is to move the BB. However, the longer the crankarm the more the distance in a complete rotation. With respect to cycling, I don't think there's much of an advantage one way or the other. The differences are rather small.

Tomm Willians

I’m 5’-8” with a 30” inseam and rode on 172.5 cranks for a very long time without any issues (at least that I was aware of) until I started using aero bars. At that point it felt like my knees were much too close to hitting my chest so I went to 165 cranks and wish I’d done so from the start.
I have not noticed any loss of leverage with shorter arms however my cadence feels more natural.

Mother_of_birds

That makes it seem a bit more worthwhile thank you for explaining

Mtracer

175 mm cranks would generally be considered much too long for someone the OP's height. I'm 6' 2" and 175's are too long for me. Oddly, they bother me when riding out of the saddle. Seems it forces my knees up too high and I get some knee pain. But while I'm tall, I have a long torso and therefore relatively short legs.

To the OP, I'd give those 165 mm cranks a try. That cost is pretty modest and while that range from 165 to 175 mm seems small, that is pretty much the available range crank sizes. Cranks longer and shorter are less common. Also, keep in mind that various measurements on bike frames may change by 5 mm or less from one bike frame size to the other. So, it's a bigger difference than it seems.

Having said that, as I understand it, people with no abnormal physical issues don't tend to be very sensitive to crank length. But you're on the short end of the spectrum currently using a crank length for people on the tall end of the spectrum. You're having an issue that you believe is due to crank length, and your cranks are generally considered too long for your height, so it makes a lot of sense to me to change to the shorter cranks.

PeteHski

It is best to focus on the diameter of the pedalling circle relative to your range of leg motion, rather than the crank leverage. Unless riding a single speed, gears compensate for differences in crank arm leverage, so a shorter crank is not a problem unless the lowest gear on your bike already feels too high. However, if you are struggling to pedal larger circles with a long crank with short legs there is nothing you can do about it. So if in doubt, shorter cranks are a good bet and have little or no downside.

So another vote for the 165 mm cranks here. For the OP's height they will probably still be on the long side of ideal.

RJC1811

I just switched to 165mm cranks and couldn’t be happier. I’m 5’9” and getting old. Past few years I’ve had some lower back pain from cycling. I also just had a knee replacement in January. I had 172.5 cranks which are pretty common but after reading a lot about crank length came to the conclusion there would be no downside to shorter cranks. It was going to cost in the $150-$200 range for new Shimano cranks so I thought why not upgrade to a power meter while I was doing it.

I went with the Power2max crank based system with 165 cranks. It was an easy swap and cost $590. So I justified it to myself in thinking I upgraded to a power meter for only $390. The only downside is that now I see just how little power I’m putting out. I think they only list 160 or 165 cranks as their shortest but after chatting with on of their guys he said they did have 155 available but didn’t have it on their website yet.

The knee recovery is a long process so still not 100%. I absolutely love the shorter cranks. It seems like it has eliminated any pelvic rocking. My lower back is happier and once warmed up the knee works well within that range. I also improved my riding position a bit and can ride a bit lower. Next step now that I have some miles on and everything is functioning well is to go and get a bike fit. It’s been years since I had one done and I’ve probably changed everything since then.

eduskator

175mm and 5'3'' does not go along together, unless you are disproportionate. I am 6'2'' and ride 172.5mm.

bruce19

Crank arm length determines diameter of pedaling circle. Although I would agree that the actual difference in force needed is negligible.

bruce19

I haven't read all the posts but this just occurred to me. If you change crank arm length you will have to adjust seat height.

Iride01 

With 175mm crank arms on the bike, I have to wonder if the bike is really too big for you anyway. Usually don't see that long a crank until you get to the larger size road bikes. Or is this a mountain bike you put on the trainer?

I suppose if it's a used bike you bought then the previous owner may have put longer cranks on it. If you know that person, then ask if they still have the original cranks.

It's also entirely possible that you might find a used bike of the correct size for you with the size cranks you want for about the same as what new cranks will cost you. Depending on how good you are at finding deals and the market in your area.

RJC1811

If you are concerned about it changing your effective gearing, there are calculators online that factor crank length with front and rear gearing to come up with meters per crank revolution. It’s the most accurate way to compare gearing combinations. I didn’t bookmark the site, it was more curiosity on my part on how my effective gearing had changed.

It actually worked out better than I had hoped. I have a compact crankset 34/50 and an 11-32 cassette. Everywhere I ride is hilly and I live near the top of the hill. I really appreciate those low gears to make that final climb if I have pushed too hard earlier in the ride and don’t have much left in the tank. So anyway, going from 172.5 to 165 made it so my lowest gear would be equivalent to 34/31 so just one tooth off in back which is fine with me. Where I picked up the big gain (besides a better pedal stroke) was on the other end. It effectively gives me a 53/11. With the compact crankset I would run out of gears on the downhill and just have to tuck in and coast. Now I can pedal further into the decent. Unfortunately I don’t have the power to use that gearing on the flats.

Sy Reene

Yes, and potentially saddle setback

bruce19

Does this help?

https://www.bikecalc.com/gear_speed

Carbonfiberboy 

This is interesting - I learned a new term, "gain ratio." That's the ratio between how far your feet move vs. how far the bike moves per pedal revolution. I found 3 calculators which will show gain ratio as well as the usual results:

https://mike-sherman.github.io/shift/
Bicycle Gearchart Calculator | yojimg.net
https://patricktaylor.com/bicycle-gear-inch-calculator/

I think I like Mike Sherman's best. Bookmarked.