I changed from a 172.5 to a 175mm crank on my road bike. How much lower should you figure the seat height should go for this type of change? Is forward and back positions also a factor? I have a 34" inseam and have been told that the 175mm is better suited for my leg length.
Thanks.
Lower it 2.5mm… Maybe forward a couple mm also, but depends on feel (it’ll move forward some with lowering the seat post).
thanks.
I will give it a try tomm.
When you go lower by 2.5 you will close the top end by 5 mm. This will cause the hip angle to close. Some end up moving forward a fair amount (more than the 2.5) to open the hip angle back up. I will be getting a pair of 175 mm powercranks shortly and will replace the 172.5’s that I have currently and I will let you know how much fore and aft I end up fiddling around with in addition to lowering.
Dev
I would lower you saddle by the length of you increased. It won’t be very noticeable though. I’ve seen people write they could tell when they changed the saddle by a couple of millimeters, but I hardly believe it. It takes a change of at least 5 mm or so before I can tell any difference.
Hopefully the guy who told you a 175mm was better for you than a 172.5 set was not the guy who sold it to you at the bike shop. 2.5 mm is insignificant and will barely be noticeable. Crankarm length discussions are like religious or political arguments, but I have met very few long-legged folks who tried out a proportionally longer arm and did not prefer it.
With an inseam like that you could probably go to 180mm or perhaps 185 before you really take advantage of the range of motion in your long legs. I’m about half an inch longer and use 190mm to good effect. My rode bike has 195s that I really like for climbing at somewhat lower rpms.
Manufacturers don’t want to make a wide range, but there are a few places you can buy cranksets beyond the normal 170 to 180 range.
Chad
I’ve got 175s on my road rigs, and 172.5s on my tri. I can feel the difference, though the change in position may have something to do with it.
When I switch from riding the road bike at 76ish to the tri bike at 82ish it always feels different–and the cranksets are both 190mm. I would be very surprised if most people could tell the difference in 2.5 mm if you switched out their crankset in a blind test. A person could easily have their cleats positioned differently and it would make more of a difference than 2.5 millimeters.
Chad
I got the DA crankset from a friend. He went to compacts and was not using them. Based on your comments, a 172.5 really would not be ideal for my 34 inch inseam anyway. Like you said, the 172.5mm is the most commonly one found on most bikes sold.
So by putting the 175’s on I am going in the right direction. I have used 175’s on my TT bike for a couple of years, but not for long distances too much. I have a century next weekend and plan to do some riding b4 on the road bike with the 175’s. It is getting the seat height, etc. right.
The TT bike is a P3 with very different geometry so the seat height and reach is very different than my Tarmac.