Wife has a new commuter which she says is over geared.
Current cranks are a 150mm cheapo set with integrated chainring, my spare cranks are 170mm.
Any armchair mathematician interested in telling me how if I fit the longer crank it'll give more leverage and hence reduce the effective gearing before I shell out on new chainrings?
Crappy rear wheel with cheap 6 speed freewheel is being replaced with an 8 speed Alfine so I'm trying to do a lot of gear conversions & my brain has given up.
This will allow you to work our the ratios , I can’t explain the maths / physics
https://www.sheldonbrown.com/gear-calc.html
Silly question, but why does the bike have 150mm cranks? If she's particularly short, 150 to 170 is a massive leap.
One revolution at 150 is one revolution at 170 so looks like I'll have to use Gain Ratios which is a new one one me.
It has 150 because... er, no idea. Both her other bikes have 170mm and this has a higher BB so I've no concerns about changing.
If it is over geared then lower the gearing and with 150 cranks it is easier to spin a lower gearing. Does the Alfine give a lower gear?
I am lowering the gearing but need new cranks to do that.
Alfine can give (almost) whatever gearing you like.
My understanding is that crank length will have no effect on gear inches
Your understanding is correct, crank length does have an effect on effort though.
In answer to the original post, if we assume the limiting factor here is that in bottom gear, standing on the crank, she's not going anywhere, without factoring in cadence and power, those 20mm longer cranks would give the same benefit as a 13% lower gear.
Not a lot really, but every little helps.
It has 150 because… er, no idea.
My best guess is because most commuter bikes have the saddle set too low so that people can reach the ground from the saddle. With 1" shorter cranks, you can have the BB 1" closer to the ground, so the pedal will be 2" closer to the ground at the top of the stroke (compared with 175mm cranks). This will reduce the angle of your knee when you are pedaling, so should make it much easier if you have the saddle set too low. Downside, obviously, is that you need to run lower gearing to get up hills.
What length cranks should she be using?
What is her preferred cadence?
I use 170mm on my bikes but I'd go to 165mm if it wasn't expensive as I prefer spinning faster. A mate I cycle with does about 85-90% or so of my rotation rate when he is clipped but on flats I'm almost doing 2 rotations to his 1. (We were even discussing if he should change crank length when he rides flats)
My kid has 145 and though those do feel small for me I just spin faster.
The commuter on the turbo has 152mm and we both use it...
My XC has dual holes at 145 and 170 .. when the lad rides on 145mm he is a LOT faster over sustained effort. This is massively noticeable on roads and he spins much quicker
This is just my personal experience but it does seem to fit the theory of "you just spin faster to compensate".
@onzadog Thanks, 13 doesn't sound much but its probably a couple of teeth. I'll get the calculator out.
@Hols2, I think its more likely because its a very cheap bike. The BB height on this 20" bike is the same as on her 29er.
@stevextc, she should be using whatever I've got in the spares and cadence is a new word to her. Its only a 5km commute so as long as she doesn't have to get off it doesn't matter.
170 versus 150 will definitely be noticeable with all other things being equal on the gearing side but will probably still feel over geared anyway. Will also be a much bigger rotation which will be more noticeable if she has ridden 150 for a long time and may or may not prefer it.
I prefer shorter cranks as the smaller rotation and less extreme angle for knee at top of stroke is better for me. I also spin up to 200 RPM down hill where long cranks become a liability...
All her other bikes are 170s, she's done maybe 20km on the 150s so I cant see the change upsetting things too much.
Ah, that will explain why she feels it is over geared. If always used 170 and then use 150 they will feel harder work in same gears.
OP, if you live within travelling distance of SO18, you are more than welcome to salvage the Alfine drivetrain off my old Saracen Pylon 8, IIRC it has a 38T chainring and an 18/19/20T sprocket.
Just before the frame developed a terminal seat post crack around 2014, I ordered a new sprocket, but I've no idea where it is and whether I ordered a smaller or larger sprocket compared to what had been on their since purchase in 2008.
While the hub gear may have siezed in ~6 years of sitting in the garage (hoarder), the crankset should be ok, which should be the 170mm Truvativ Isoflow 1.0G 38T.
The gear inch range (with 38T and one of 18/19/20 sprockets) will be very similar to the 11-34 cassette I've recently fitted on my 22-speed road bike when used with the 34T ring, 27.0-83.5 (my road bike) vs 27.0-82.8 (38/20 Alfine combo).
I think its more likely because its a very cheap bike. The BB height on this 20″ bike is the same as on her 29er.
Yes, cheapness is generally a major part of it.
@kerley, It feels over geared because it is over geared. 30.7 Vs 19.7 gear inches City Vs Gravel bike.
@n0b0dy0ftheg0at, Thanks but I'm in Sheffield & already have 2 unused 8 speed Alfine & 2 unused 3 speed Sturmey Archer.
If I was commuting 5km each way for a week on 175mm cranks ve my 170mm I'd be a cripple by the end of the week..
presuming its square taper last set I bought were £15 used off ebay... 104 BCD alivio's then fit chainring of choice for gearing
In that case I'm glad you're not my wife.
Looks as if I stick a 42 on there now it'll drop the gearing enough to use the cheapo freewheel setup while I rebuild the alfine which will need a 16 tooth.