I have read a zillion posts on crank length. I really don’t get it. I do understand that a smaller crank would allow me to lower my seat a little bit. But I don’t get what it means based on power.
I presently ride a P2C with 175 crank. I am 5’11" but have 35" inseam. I am considering purchasing a 170 or 172.5 instead of a 175 in a new crank for bike I am building. The outside ring will be 53t and inside 39. But I am replacing the 53 with a solid 54. The cassette I am planning on is 11-26.
In simple terms what would a 170 or 175 get me? Lower seat? More or less power or depends?
As EH said, you’ll have to raise your seat. At the bottom of the stroke, the pedal will be 5mm (if you swap 170’s in place of 175’s) HIGHER. The shorter crank means the pedal is closer to the bottom bracket (the center of the pedal’s rotation), ie 5mm higher from the ground. Meanwhile, the other pedal will be 5mm lower, again closer to the bottom bracket.
Assuming you move the seat up 5mm to maintain knee again at the bottom of the stroke, the net effect on the top of the stroke will be 10mm.
The net leverage effect is rather small, changing from 175 to 170 is less than 3% shorter lever arm, but there is a decrease. Having a shorter lever (crank arm) may actually allow a rider to generate more power because on the effect on hip angle. If you have a less than ST ideal physique (read any belly at all) the additional room at the top of the stroke may help.
It’s really simple and it’s really complex. The simple part is, the seat needs to move to compensate for the new pedal position, assuming the seat height was correct to begin with. That, in turn, changes the drop to the bars, which can be compensated for with a spacer of equal height to the change in seat height, assuming you want to maintain upper body position. The complex part is how that interacts with pedal stroke and power generation.
it gets your thight squishing up into your stomach/chest less
you must be a monster to need a 54T front =)
I have read a zillion posts on crank length. I really don’t get it. I do understand that a smaller crank would allow me to lower my seat a little bit. But I don’t get what it means based on power.
I presently ride a P2C with 175 crank. I am 5’11" but have 35" inseam. I am considering purchasing a 170 or 172.5 instead of a 175 in a new crank for bike I am building. The outside ring will be 53t and inside 39. But I am replacing the 53 with a solid 54. The cassette I am planning on is 11-26.
In simple terms what would a 170 or 175 get me? Lower seat? More or less power or depends?
if you plan on transfering your P2C coordinates, you will have to RAISE your seat AND your aerobars to maintain your fit coordinates.
Ewan
You should also move your seat and bars aft 5mm to maintain the same orientation of your leg to the pedal in the “power stroke” portion of the pedal stroke.
I am blessed to be able to buy this gear and play. Some buy lake houses, fast cars, girlfriends or whatever.
I choose to buy tri gear. I also am learning. So would agree I don’t have a ton of cents but I am getting there. Plus the more gear I purchase and play round with and most important to me, test, I will become pretty well educated.
This forum is a fantastic resource. But it still could use more real data comparing components. I am taking power data. Different wheels, basebars, etc. I hope to contribute. I realize very difficult to test but if enough did and provide data we can see some trends.
Example. I have an H3 front wheel. I race in tri races with very low wind. Races early in the morning. I want to test this wheel compared to a Stinger 9. There is very little independent data of this sort that I have seen. I got into the group wheel purchase on the forum. I want to see what is the difference of the 80mm clincher front with latex tube and Bontrager wing versus Stinger 9 tubular.
So yes I purchase lots of stuff and definitely have a lot to learn but over the next 6 months I will get to learn a lot. I don’t expect to get a lot faster from the gear but faster from the training playing with the gear and maybe a little faster from the gear.
while there may not be any independent data on that subject, there is defendant data and since both wheels come from the same manufacturer that shouldn’t matter so all you have to do is go to their website and look at the plot.
So I am far from a Monster, with a FTP around 250, but I am very glad I’ve put a 54T on my ride. Please tell
me why you think this is a bad a idea. Looking up my last two rides, I averaged 89 and 90 RPM so I
am not grinding away at 70 RPM.
I live in Colorado and so I am often riding descents that last for miles and I can end up with significant time
over 30Mph…I feel a lot more comfortable at 95-100RPM in the Big ring than I do at 110+. It seems perfectly
rational to me, more comfort, more top end and a miniscule weight penalty.
I think I prove out my advantage when I race, and I pass dozens of people on the downhills … going faster is the
whole point, right?
It is not exactly clear to me how HED tested. Was the wheel moving or still? Also in what fork? I want to test in my real life situation. I know it won’t be perfect but five days doing my TT 20k course with power data on one wheel and five on the other. Great workout and might see a trend I believe in.
Also the data I have seen on the H3 would indicate that it is not the faster option versus Stinger, 808, 1080, etc unless you have higher wind yaw. A wind yaw I have not seen this year in the seven races I have done.
That is exactly why I was going to the 54t. I honestly don’t know how others do it. I am not comfortable turning high RPMs down a hill. I am not even talking about a steep hill. Something like -5% grade for a mile. I would be spinning my legs off. Also allows me to much more easily clear rolling hills at a decent speed.
I think I prove out my advantage when I race, and I pass dozens of people on the downhills … going faster is the
whole point, right?
Passing people on downhills while pedaling is using up a lot of power when it is not doing you a lot of good. I also pass a lot of people going downhill during races, but I do it in a full tuck while not pedaling at all. Generally the people I pass are pedaling away furiously.
You are correct; pedaling 110 rpm downhill does not feel safe. That’s why you should not even be pedaling at all.
I average (not so much this year, but past years) around 24mph on hilly tris and I don’t even pedal hard past 25 mph. Above 30 and I’m back in the tuck. If you are pedaling hard past 25 mph and not averaging 30mph then you are wasting a lot of energy.
Chad
I think I prove out my advantage when I race, and I pass dozens of people on the downhills … going faster is the
whole point, right?
Passing people on downhills while pedaling is using up a lot of power when it is not doing you a lot of good. I also pass a lot of people going downhill during races, but I do it in a full tuck while not pedaling at all. Generally the people I pass are pedaling away furiously.
You are correct; pedaling 110 rpm downhill does not feel safe. That’s why you should not even be pedaling at all.
I average (not so much this year, but past years) around 24mph on hilly tris and I don’t even pedal hard past 25 mph. Above 30 and I’m back in the tuck. If you are pedaling hard past 25 mph and not averaging 30mph then you are wasting a lot of energy.
Chad
Actually - according to some extensive threads here by Coggin, Lakerfan and others - steady power output throughout the course of the bike is the most efficient and fastest way to T2. Depending on the distance I try to average between 220-180 W up and down hills with a cadence of 85-90 resulting in a low VI - usually around 1.02-1.03
If you coast down hills - you are probably one of the people I pass, pedaling at a normal cadence and at my target power output.
And that says nothing about why a 54T is or isn’t a bad thing…
Feel free to send any used & abused cast-offs my way though…