I've got a friend who's never ridden much distance before, just around campus on a mountain bike, who is now riding across the U.S. as part of a Habitat fundraiser. 6' tall, legs perhaps slightly long proportionally but not much. Will be riding a triple crank. But I'm wondering what would be the ideal crankarm lengh, with emphasis on preventing injury that may come with starting to ride average of 75 miles a day from a near-zero mileage base. Does the difference between 170 and 175 even matter here? 175's may be more comfortable because novice cyclists are more likely to pedal at lower rpm's, where increased leverage may be especially useful, but 170's would result in less knee-bend at the top of the pedal stroke, perhaps decreasing chance of injury. And it's quite possible that it just doesn't matter, period. But what are some thoughts here? Thanks.

Well, I always think of it as a function of leg extension. If your friend has a 34" inseam, I'd go with the 175's. I have a 29" inseam and use 170's but I could probably be fine with 165s.

Also keep in mind that 5 mm is less than 1/4 of an inch so it's pretty insignificant.

Also, I would tell him to go with a mountainbike crank, no way he's going to really need that 53t chainring.

Conventional wisdom holds that longer cranks are more likely to produce knee injuries. I've never seen any actual studies or analysis of it, though, and have always tended to discount it. We're talking about awfully small differences here, and my BS detector goes off when someone tells me 2.5mm or 5mm is the difference between safe and injured.

Having said that, it seems to me that going from almost zero miles to 75 miles per day is a recipe for injury regardless of crank length -- and if not the knees, then something else. In this fellow's shoes, crank length would be pretty far down the list of stuff I'd be worried about.

IOW, I've got a nickle that says his arse won't let him go nearly far enough for his knees to start causing trouble.

5 mm of crank length will be the least of his problems. Will there be a big comfy van he can ride in after the second day?

Seriously, he'll be fiddling with saddle height, saddle angle, reach, bar height and cleat position (probably in that order) before he'll ever think about crank length. I'm 6'3" and have ridden cranks from 165 mm to 177.5 mm, and find that Q-factor (pedal-to-pedal distance) has a bigger impact on knee comfort than crank length.

At least it's all for a good cause!

It's a she
I know, women at 6' are even rarer than men at 6'5" (my height).

Anyway, her 58cm Trek came with 175mm cranks, they started on the east coast and are in Chapel Hill tonight and tomorrow so I've been able to tune up the bike again (lots of cable stretch, interestingly) and could swap out the stock 175mm cranks for 170's that I have sitting around. But I think I'll leave the 175's on there.

The trip is smart to give them two rest days (rest from biking - they're helping with a build project) after the first four days of riding. They stop in western Tennessee for five days of building. Both good spots to give people who've developed saddle sores a chance to heal.

As for Emily, her saddle seems comfortable for her after the first 250 miles, and she's not had any problems yet, so she may be among the lucky ones. Other people on the trip, already, are not so lucky.

I'd be disinclined to change anything while she's in the middle of the ride. My educated guess is that change would be more likely to cause injury than simply using 175s. FWIW

+1

If she's not complaining about it, don't change it.

I'm 6'2,inseam 33.5" and size 12 feet.......I've always found 175s a long way around...I'm a natural spinner
I use 170s on my Miyata 610(original) and have ordered a Surly LHT with 172s.....

It doesn't need to be that hard to turn the pedals,175s seem to be on everything I guess a carry over from mountain biking.....Shorter cranks and lower gears makes for happy knees and entire body