So I am upgrading my crankset thanks to scoring a set with a pioneer dual sided on eBay. I currently ride a 53/39 on 175 cranks. I have an 11/32 cassette on the rear. The new cranks are 170 which I got to allow myself to be a little more aero as a 6’5” guy to open up the hip angle and flatten the back a bit. Should I still get 53/39 rings or a little something smaller? I’m Pittsburgh based so it’s hilly but I’ve never know anything other than the 53/39 so any advice/guidance would be great.
Sorry I don’t have a good answer to your chain ring question but I’m curious how you end up liking the shorter cranks. I’m the same height and have thought about trying shorter cranks
@FrankTuna I will let you know. I had been tossing it around for a while but then when I won the bid for a shockingly low price I figured no time better than now. I have 172.5 on my gravel and I notice it being a little more comfortable (granted it’s got 38’s vs 26’s lol) from what I have researched it allows you to raise your seat and get higher in the back which then makes you more aero not that at our height and 260 pounds it matters that much but hey marginal gains right lol
the basic physics of levers says if the cranks get shorter you should change the chainrings by the same percentage to keep the leverage the same.
Dr. Jim Richards, the professor who has done the most studies on crank lengths says that because of the way your joints work your legs don’t generate the same amount of leverage over the whole stroke and shorter cranks clip off the less powerful parts at the ends of the stroke. So to compensate you only need to gear down about half the percentage.
John Cobb, who has been setting up time trialists and triathletes on shorter cranks for years and years, says in working with them they found they needed bigger gears - the riders who had 34/50 compact setups wanted 39/53 gears!
So since those points suggest both smaller and bigger, then consider:
You have gears in the back so you’ll self-select the gearing you need until you run out, so if you don’t spend a lot of time at either end of the cassette it probably doesn’t matter.
5mm of 175 is 3% so if you go by physics you could go from a 53 to a 50 and ignore what Cobb says.
Going by Richards you’d have to find ~1 tooth different stuff which you probably can do but it seems like not a lot, because, well, it’s not.
A single shift in the back is going to be about 10% which absolutely swamps any chain ring fine tuning.
At 6’5", the change as a percentage of your leg length is even smaller so any general recommendations based on shorter people will apply less!
So if at this point you’re totally confused, I’d agree with the above poster that it’s a good opportunity to make the gearing work better for you. If your area is hilly and your 53/11 is untouched by all means go compact.
I was thinking about splitting the difference and doing a 36/52. I do climb in the big ring a good amount unless it’s a big hill or a sharp one. Seems like the 36 would be a great addition for the tough climbs and 52 would still give me high power when I want it
I run a shimano 105 52/34, with 11-32 in the back. I swapped the 36 out for a spinny 34 climbing gear because I’m so out of shape these days lol. It works very well so far, can be a little finicky to set up but if you’ve adjusted gears in the past you’ll be fine. I’ve found that I only seem to spin out 52-11 if I have a tail wind and a slight downhill during a vo2 max interval. I’m about 192lbs with around 250 ftp right now if that helps
If you have a headunit connected to your di2 setup it will write the gears to the fit file. There are tools (wko5) which will let you look at what you’re using.
Find what gear is an optimal chainline and how often you are away from it.
My guess is also that you probably won’t notice the difference in power/leverage, even if the feel is slightly different. Is the bolt hole configuration the same between your old & new cranks? If so, just swap the rings over to the Pioneer cranks & go for a few rides first. You might not need to buy anything more.
If they’re incompatible then yeah you’ll have to buy rings. As for options, I run a 52/33 without trouble on a bike that has a Shimano 105 crank with the X110 bolthole configuration. The delta is 3 teeth out of spec, but it works fine for me. As we all know Shimano is pretty conservative with their recommendations. There’s nothing but convention saying you can’t mix & match chainrings, but yes if the gap is too wide you could end up with shifting problems or find yourself with not enough overlap for those in-between speeds on lumpy terrain, meaning operating with a crossed chain a lot. Having said that I see a lot of riders here at just riding in the big ring, stomping on climbs, & only using the small ring when the climb gets over about 5%. Crossed chain more often than not. Depends how you ride. If you want to keep a 53 up front, you could probably go down to a 34 or 36.
I went from 172.5 mm cranks to 165 mm cranks to allow me to move more freely. (In an aero position, my knees would massage my stomach.)
When I computed my gearing, I assumed that my body would prefer keeping the force roughly constant. To produce the same power with shorter cranks at the same pedal pressure, my cadence would have to increase accordingly. Hence, I would have to choose slightly easier gearing where the difference corresponds to the relative difference in crank lengths. For me that was spot on.
In detail: in my case, we are talking a 4 % difference in crank length. I anticipated that when I chose my gearing: I shifted everything down by about 1/2 gear. E. g. I opted for a 42-tooth chainring instead of a 44-tooth chainring on my road bike (coupled to a 10–36 cassette). That was bang on the money. On the trainer in high gear (= high flywheel speed = high inertia = simulation of going fast on the flats), my self-selected cadence have shifted up from 95-100 rpm to 98–103/104 rpm. That lines up very well with the 4 % difference in leverage.
So if you were to do the same, the relative difference between crank lengths is about 3 %. That would suggest you size down to 52/36. However, if you don’t use your top gear very often (climbing gears >>> top gears), I’d even consider a compact crank.
@drwelby
Thanks for your post, if something is worth doing, it is worth overdoing. I’m sure this post will be linked to regularly. (At least it should!)
I hope the change works out for you. I made the change a few years ago to 170 from 175 to see if all the hype was valid. In my use case it is not. I did not like the feel and went back 175. Alternatively a good riding buddy did go shorter and will never go back. It seems to work for most people but my feeling is it has the most benefit for triathletes and TT riders. With respect to your thought it will aid in a flat back, I cannot see how. Crank length is independent of upper body angle. Handlebar height and reach affect this but if being able to ride lower without your legs hitting your chest then yes this will help.
