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Big difference in crank length between trainer and real bike

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Big difference in crank length between trainer and real bike

Old 04-28-22, 07:32 AM
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Big difference in crank length between trainer and real bike

I have inseam length of 83 cm and crank calculators tell me I should use 170 mm crank.

I have 170 mm crank length on my real bike (sounds good right??). But on my stationary indoor bike trainer, the crank length is only 145 mm.

Everything else fit-wise is identical and I spend roughly the same amount of time on the trainer as on the real bike per week. However, it seems I can hold more intense efforts (~160 bpm heart rate) for longer periods on the trainer. The same effort on the real bike, I can feel my quads getting sore sooner. The same effort on the trainer while similar intensity in terms of breathing and heart rate, feels significantly easier because my legs feel fresh for longer.

I've done some research and read that shorter crank does make things feel easier and less fatigue for the leg muscles.

However, I still would like to adapt to the 170mm crank of my real bike and I suspect my short crank trainer won't help much. Increasing the crank length of my trainer is not possible. It's the dirt cheap variety with single piece crank like in BMX but totally incompatible. I can't install 150 mm crank on my real bike either because 150 mm cranks are simply not available where I live, even 160 mm crank are very hard to come by. But I doubt the 160 will make much difference, given my proven better performance with even shorter crank.

I've wondered if lowering the saddle on the trainer could stimulate similar effort to my real bike with 170 mm crank??
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Old 04-29-22, 10:30 AM
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Seems as if you posted that your "real bike" had 165mm cranks. Hard to keep up....
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Old 04-29-22, 10:57 AM
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The measure that matters is power, not heart rate. Until you can measure it in both scenarios, all you know is your heart is working harder with shorter cranks. But efficiency is providing the most power for the least cost in resources. Also, are you tracking your pedal RPM? It has been known forever that riding a given speed and therefor power in a lower gear lowers the load on the legs but raises heart rate and oxygen consumption. This may be what's happening on your trainer with the shorter cranks helping you spin faster.

Remember power, what you are after ultimately, is the force you apply to the pedals times the pedal RPM times the crank length. If you keep the same RPM and pedaling force, you will only be doing 145/170 = 85% as much work on the trainer. (Can you change the trainer cranks to 170? I would if I could. Edit: I see you answered this. I'd consider this trainer a temporary and not ideal solution unless you are training for BMX short crank riding.)

Last edited by 79pmooney; 04-29-22 at 11:01 AM.
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Old 04-29-22, 11:44 AM
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High rpms raise your HR without increasing your power or speed. Don't worry about it. Buy a set of resistance rollers and get rid of the trainer. Alternatively, increase the resistance on your trainer so your legs hurt pretty good after an hour at 90 cadence. Gradually over a couple of months, try to increase your time at that power to 2 hours.

Another kind of "fun" thing to do is to use a low resistance on the trainer and take your cadence up to 130 or so or to where you start to bounce on the saddle - and hold it until your legs can't take it anymore. Try to get that high cadence time up to 40' without a break. Your high cadence HR will gradually come down as you get more efficient. See, there's lots of fun to be had on a trainer, even if the cranks aren't the right length!

Basically, I wouldn't worry about it. My wife rides 149 cranks on our tandem and 165s on her trainer. It's not an issue of adaptation, just an issue of working hard on either machine. If your legs don't hurt, you're doing it wrong, that's my motto.
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Old 04-29-22, 12:19 PM
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You aren't going to duplicate on the trainer what you encounter on the road. Just use the crank length that feels good to you and don't worry about what a calculator says. They are often based on someone's own idea of what is ideal.

Crank length isn't so much just upper and lower leg length. Other things like your actual range of motion you have and what you consider a comfortable rpm at any particular power output matter too. It's very much a personal thing.

I like shorter cranks than my 34.5" inseam suggests. 165mm to be exact. I also tend to like a higher cadence.

Last edited by Iride01; 04-29-22 at 12:24 PM.
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Old 04-29-22, 04:41 PM
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Originally Posted by couldwheels
I've wondered if lowering the saddle on the trainer could stimulate similar effort to my real bike with 170 mm crank??
No, not really.
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Old 05-02-22, 08:56 AM
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Originally Posted by shelbyfv
Seems as if you posted that your "real bike" had 165mm cranks. Hard to keep up....
Whaaaat??
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Old 05-02-22, 09:11 AM
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Originally Posted by 79pmooney
The measure that matters is power, not heart rate. Until you can measure it in both scenarios, all you know is your heart is working harder with shorter cranks. But efficiency is providing the most power for the least cost in resources. Also, are you tracking your pedal RPM? It has been known forever that riding a given speed and therefor power in a lower gear lowers the load on the legs but raises heart rate and oxygen consumption. This may be what's happening on your trainer with the shorter cranks helping you spin faster.

Remember power, what you are after ultimately, is the force you apply to the pedals times the pedal RPM times the crank length. If you keep the same RPM and pedaling force, you will only be doing 145/170 = 85% as much work on the trainer. (Can you change the trainer cranks to 170? I would if I could. Edit: I see you answered this. I'd consider this trainer a temporary and not ideal solution unless you are training for BMX short crank riding.)
That's too bad, but thanks for the math work.

I'm mostly doing low cadence strength training on the trainer. I avoid such training on the real bike to minimize wear and among other reasons. The trainer on the other hand has wide fixed gears, enclosed drivetrain that won't mind the extra wear of high resistance training.

If I do the same type of training on the real bike, I'll hurt my quads. Anyway, I end up lowering the saddle on my trainer and did my typical high resistance training for a few days. It was really hard at first, total sufferfest but seemed to have solved the problem. I don't seem to be hurting my quads on my real bike anymore! It was the first time I went out, pushed really hard on the real bike but did not get sore quads afterwards.

Last edited by couldwheels; 05-02-22 at 09:15 AM.
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Old 05-02-22, 09:20 AM
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Originally Posted by Iride01
You aren't going to duplicate on the trainer what you encounter on the road. Just use the crank length that feels good to you and don't worry about what a calculator says. They are often based on someone's own idea of what is ideal.

Crank length isn't so much just upper and lower leg length. Other things like your actual range of motion you have and what you consider a comfortable rpm at any particular power output matter too. It's very much a personal thing.

I like shorter cranks than my 34.5" inseam suggests. 165mm to be exact. I also tend to like a higher cadence.
My cadence can vary a lot but whether I'm spinning or mashing, shorter cranks just feel easier even when standing on the pedals. It's too bad, they did not make more of the <160mm ones.
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Old 05-02-22, 09:32 AM
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Originally Posted by Carbonfiberboy
High rpms raise your HR without increasing your power or speed. Don't worry about it. Buy a set of resistance rollers and get rid of the trainer. Alternatively, increase the resistance on your trainer so your legs hurt pretty good after an hour at 90 cadence. Gradually over a couple of months, try to increase your time at that power to 2 hours.

Another kind of "fun" thing to do is to use a low resistance on the trainer and take your cadence up to 130 or so or to where you start to bounce on the saddle - and hold it until your legs can't take it anymore. Try to get that high cadence time up to 40' without a break. Your high cadence HR will gradually come down as you get more efficient. See, there's lots of fun to be had on a trainer, even if the cranks aren't the right length!

Basically, I wouldn't worry about it. My wife rides 149 cranks on our tandem and 165s on her trainer. It's not an issue of adaptation, just an issue of working hard on either machine. If your legs don't hurt, you're doing it wrong, that's my motto.
I'll start doing that low resistance spinning!

I don't like to have sore legs if I can help it. Don't get me wrong for taking it too easy. My lungs on the other hand, my workouts are intense enough to feel like I'm suffocating! Sorry, no rollers for me. I don't want to use my real bike for indoor training. I want to keep the wear on my real bike as little as possible. The fixed gear trainer never ever seems to wear. Makes training quite economical.
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Old 05-02-22, 01:34 PM
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Originally Posted by couldwheels
I'll start doing that low resistance spinning!

I don't like to have sore legs if I can help it. Don't get me wrong for taking it too easy. My lungs on the other hand, my workouts are intense enough to feel like I'm suffocating! Sorry, no rollers for me. I don't want to use my real bike for indoor training. I want to keep the wear on my real bike as little as possible. The fixed gear trainer never ever seems to wear. Makes training quite economical.
Rollers don't wear your bike at all. My tires would last forever if I didn't ride that bike outside, same with the components. No dirt, no water, that is if you keep your sweat of the bike with a sweat bra or towel.
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Old 05-02-22, 07:20 PM
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Originally Posted by Carbonfiberboy
Rollers don't wear your bike at all. My tires would last forever if I didn't ride that bike outside, same with the components. No dirt, no water, that is if you keep your sweat of the bike with a sweat bra or towel.
I'll give it a thought. Finding space to put the rollers and the bike to train is another big problem. I live in very cramped conditions.

Would the resistance in a resistance roller be high enough to simulate out of the saddle training when using the highest gear on my bike? Unfortunately, that's another problem with my bike because the highest gear on my bike is 13-34 with the 1x drivetrain.
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Old 05-02-22, 08:30 PM
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Originally Posted by couldwheels
I'll give it a thought. Finding space to put the rollers and the bike to train is another big problem. I live in very cramped conditions.

Would the resistance in a resistance roller be high enough to simulate out of the saddle training when using the highest gear on my bike? Unfortunately, that's another problem with my bike because the highest gear on my bike is 13-34 with the 1x drivetrain.
Maybe, but I wouldn't worry about it. By the time you're good enough on rollers to ride OOS, your bike situation would probably be different anyway. I mostly do seated training on mine, anything from recovery rides to Z5 intervals. I might stand from time to time just to give my butt a break. Because the there's no acceleration on a stationary device, OOS training isn't much use anyway.
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Old 05-02-22, 09:22 PM
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Originally Posted by Carbonfiberboy
Maybe, but I wouldn't worry about it. By the time you're good enough on rollers to ride OOS, your bike situation would probably be different anyway. I mostly do seated training on mine, anything from recovery rides to Z5 intervals. I might stand from time to time just to give my butt a break. Because the there's no acceleration on a stationary device, OOS training isn't much use anyway.
My alternative to OOS is setting the same high resistance I do for OOS but doing the efforts seated. With enough setback on the trainer and adjusting the handlebar to the shortest reach, it's becomes possible to push down the equivalent of your body weight without bouncing up and down on the saddle.

It would be possible on roller if the pedal resistance can be made as high as body weight.
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Old 05-03-22, 08:28 AM
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I think it a flight of fancy to think you can and attempt to make your indoor trainer mimic your outdoor stuff. Just make your indoor bike fit you comfortably within reason for the things you are able to do on your indoor trainer or rollers.

Same for your outdoor bike, make if fit you comfortably within reason for the conditions and the way you ride outdoors.

Within reason means that sometimes you might sacrifice a smidgeon of comfort to be put in a better position to produce power and efficiently get it turned into over all speed or power to climb a hill.

Personally I can climb a hill better seated. And I tend to beat my son up them when he choses to stand.
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Old 05-03-22, 08:46 PM
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Originally Posted by Iride01
I think it a flight of fancy to think you can and attempt to make your indoor trainer mimic your outdoor stuff. Just make your indoor bike fit you comfortably within reason for the things you are able to do on your indoor trainer or rollers.

Same for your outdoor bike, make if fit you comfortably within reason for the conditions and the way you ride outdoors.

Within reason means that sometimes you might sacrifice a smidgeon of comfort to be put in a better position to produce power and efficiently get it turned into over all speed or power to climb a hill.

Personally I can climb a hill better seated. And I tend to beat my son up them when he choses to stand.
Both my indoor and outdoor bike used to have identical fit until getting frustrated about sore quads during climbs on my outdoor bike and altered my indoor bike fit.

I used to ride a lot more hours on the outdoor bike and sore quads was never a problem until I have to cut back due to work hours. I am spending more time in the trainer now but my total training time is still a lot less than before.

So I think my legs has adapted to the short crank fit of the trainer. Lowering the saddle on the trainer due to short crank seemed to have restored my previous performance on the outdoor bike. It's working well so far. I'm no longer experiencing sore quads on the outdoor bike and climbing feels a lot easier now.

I also climb seated the vast majority of the time on the outdoor bike. I only do long OOS trainings on the trainer (but not lately to facilitate short vs long crank training). Only for very short durations I stand (few pedal strokes) on the outdoor bike during long climbs to "crack" my knee joints if they're starting to feel stiff.
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Old 05-04-22, 02:10 AM
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125mm crank-length user rolling through !
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Old 05-04-22, 06:23 AM
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Originally Posted by Mr Sir
125mm crank-length user rolling through !
Hope I could but having problem sourcing them. I just need "special training" setup to get used to 170 mm crank, practically by training with low saddle height on the trainer to mimic leg extension I get with 170mm crank at the top pedal stroke. Less ideal but works so far.

My personal opinion, typical crank lengths produce unnatural leg ROM / exertion compared to running on foot. I'd like to use running as baseline since we practically evolved to run. When running, the legs are more extended throughout the portion where the foot is on the ground and force is applied.

Comparing to traditional crank length fit, the legs suffer from reduced leg extension at the beginning of the power stroke. I found it's a lot easier to maintain hard efforts for longer on short cranks.
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