Classic & Vintage - Crank length...

Are the pedals on a large-framed road bike further off the ground when compared to a smaller frame? In other words, should I look for 175mm crank arms for my 6'5" son-in-law? The frame is a 26" fillet-brazed Schwinn Super Sport...

THANKS!

I'm not the frame design expert, but as most chainstays are almost parallel to the ground, the bottom brackets would be at similar heights for the same wheel size.

BB height from the ground is based on frame geometry more than the frame size.

^ this. i just built up a 'rally' frame with short stays and a slightly higher bb. it's really a lot of fun around town. and i can more aggressively pedal through turns. it's great. it's not the frame i'd choose to do over 50 miles, but on city streets, it's hard to beat.

i have 170/171mm arms on all my bikes. i'm not sure if 5mm would make a big difference either way. thoughts?

BB's are not all the same distance from the ground,

I have 2 bikes in question that have 10mm difference in crank length, i would like to swap cranks but the frame geometry does not allow this

Some great info on crank length here:
http://www.nettally.com/palmk/crwives.html

and of course, Sheldon 'Cranky' Brown:
http://sheldonbrown.com/cranks.html

Do you know the original crank spec for the bike?

Crank length will match the rider, with subjective preference. Standard sizes are 170, 172.5, 175 and there's not a whole lot of difference between 3cm change. A very short-legged rider might use a 165, a 35+ inner leg measurement would translate into a 175 or longer.

Geometry dictates BB height but FYI the bb height across sizes of a specific model will often be the same, whether it's a 48 or a 61. With much longer cranks, BB height can be a factor, but 175 is standard spec for a large bike. I'm not familiar with the SS, but you can measure the drop. How old is the bike? I thought the older bikes had Ashtabula cranks.

I believe to a large extent crank length is a personal issue. I am not a good spinner never have been so I perfer a 172.5 crank but wether it make a real difference in the speed or effort put out on a 30 mile ride I have no idea. In general I think taller riders with longer legs sometime perfer longer cranks. If you can find a pair a reasonable price that fits the current set up it might be worth trying. But usually only the top 2 or 3 groups came with crank length options.

BB drop equals wheel diameter subtracted by BB height. I varies according to frame geometry and seldom by frame size. It follows that you can lessen your BB drop with larger rims/fatter tires.

My Trek 620 is 61cm and I still have to turn carefully with 175mm crankarms. Racing pedals often have upswept cages to alleviate the problem. Fixie riders (if they know what they're doing) prefer short crankarms to keep the cadence up and to keep pedaling in tight turns.

If all dimensions of bikes changed in direct proportion to certain dimensions of their riders, then yes, would certainly want longer crank arms for a taller person (or a person with longer legs). In the real world, however, most bicycle components are standardized (wheel size, for example) and only a few parts are made in a variety of sizes to suit riders of different sizes.

Crank arms are neither one nor the other; virtually all are 170 mm plus or minus 5 mm. It is barely enough to make any difference at all.

That said, your son in law will probably appreciate the thought behind longer crank arms, even if they don't actually give him any practical advantage (and, in my opinion, they won't).

I'm not the frame design expert, but as most chainstays are almost parallel to the ground, the bottom brackets would be at similar heights for the same wheel size.

That is not true for 700c or 27" wheeled bikes.
In general, from a frame design perspective for a roadbike the Drop to the crank center from the axle center will be from 60 to 75 mm, (3").
Small bikes often have higher bottom brackets actually as that is one way to achieve a short seat tube while limited by the basic dimensions of the front end and keeping a level top tube.
If the Schwinn has a bottom bracket center height of at least 10.625" from the ground with the bike vertical and tires inflated, No Problem. You could actually get away with 10.5" but and 175 mm cranks but depending on the pedals and riding style could catch a pedal in a turn. Wider pedals being more of a problem.
The Schwinn probably has a higher bottom bracket unless you have traded 27" wheels for 700c.

Some small builders go for 8 cm of "drop". In the pre narrow clip less pedal days, this would not be competitive. Often though bikes with this feature descend well.

There are four dimensions that determine pedal clearance: BB Drop, Wheel Diameter, Crank Arm Length, and pedal spindle length. The BB drop (vertical distance between a horizontal line drawn through the front and rear dropout centers and the center of the crank) of my 24" 1973 Super Sport frame is 75mm. On a production model like the Super Sport, the BB drop is the same for all frame sizes, so the remaining variables are the wheel diameter (including the tire size), crank arm length, and crank spindle length (the spindle length only comes into play when you're leaning into a turn).

On the Super Sport, the standard Astabula crank arms are 170mm. According to the Schwinn specifications, the same crank (p/n 56 018) was used on all frame sizes in 1973, the rims are 27", the tires are p/n 62 203 (27" x 1-1/4"), and the pedals (p/n 57 730) are all the same for all frame sizes.

So, no, there is no difference in pedal clearance between frame sizes on the fillet brazed Super Sports.

Putting 175mm crank arms on your son-in-law's Super Sport is probably not a good idea because of the higher risk of pedal strike.

I posted these pics in a different thread a few months ago, but they seem appropriate here. Five bikes which fit me well, nominally different frame sizes but somewhat deceptive.

http://users.rcn.com/jimmuller/pics/BBs.jpg

Standovers for these frames:

http://users.rcn.com/jimmuller/pics/standovers.jpg

I would think that crank length should scale to femur length, but isn't as critical as it might first seem. (FWIW, I have a 165mm on the Bianchi, the bike with the highest BB, and 170s on everything else. I feel little difference. But I can tell that the saddle is higher.) BB height above the ground, along with Q factor, might be important for avoiding a pedal striking the ground.

In the 1950s English frame makers offered higher BB heights for path (track) bikes than for road bikes. I've always assumed that was because path bikes were fixed gear, meaning you couldn't stop pedalling when banked over, whereas on a road bike you always have the option of stopping pedalling.

I am 6'2" and rode the last year of Schwinn Super Sport as a winter fixie for at least a couple winters.
You can see a picture of one incarnation at the link below:
http://fixedgeargallery.com/2004/f/bosman.htm
That bike is fitted with 165mm BMX cranks (on the original longish bottom bracket spindle) and until I learned how to compensate I struck the pedal hard enough to slightly unweight the rear wheel in tight right turns.

I don't know how intense a cyclist the intended rider is, but most newer riders cost through turns tight enough to strike a pedal. 175mm cranks are a nice thought, but I'd use what you have readily available if it is at least 170mm.

One reason most framebuilders use BB Drop instead of BB Height (above the ground) is that using BB drop, the drop is part of the frame design which doesn't ever change, but BB height is quite variable depending on wheels and tires. For example changing the wheels on my Super Sport from 27" (630mm BSD) rims to 700c (622mm BSD) lowered my BB height by 4mm not counting the smaller cross-section of my 25c tires compared to the stock 27" x 1-1/4" tires. I use SPD pedals now instead of the OEM rattrap pedals, so the shorter pedal spindle minimizes the impact of the 4mm lower BB.

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/1973%20Super%20Sport%20CJ809208/1973SuperSportBBDrop.jpg

One reason most framebuilders use BB Drop instead of BB Height (above the ground) is that using BB drop, the drop is part of the frame design which doesn't ever change, but BB height is quite variable depending on wheels and tires. For example changing the wheels on my Super Sport from 27" (630mm BSD) rims to 700c (622mm BSD) lowered my BB height by 4mm not counting the smaller cross-section of my 25c tires compared to the stock 27" x 1-1/4" tires. I use SPD pedals now instead of the OEM rattrap pedals, so the shorter pedal spindle minimizes the impact of the 4mm lower BB.

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/1973%20Super%20Sport%20CJ809208/1973SuperSportBBDrop.jpg

I am actually surprised Schwinn went with 75 mm of drop on the Super Sport. Good for them, many Japanese bikes at this price point have higher brackets, probably to avoid pedal strike and the attending lawyers. For a bike with 27" wheels and 1 /14" tires, 175's could still be okay depending on the pedals as you mention. When I was young clipping a pedal was a concern, today, with age I am less likely to Need a high bottom bracket and short cranks on the criterium bike of my youth.

There are four dimensions that determine pedal clearance: BB Drop, Wheel Diameter, Crank Arm Length, and pedal spindle length. The BB drop (vertical distance between a horizontal line drawn through the front and rear dropout centers and the center of the crank) of my 24" 1973 Super Sport frame is 75mm. On a production model like the Super Sport, the BB drop is the same for all frame sizes, so the remaining variables are the wheel diameter (including the tire size), crank arm length, and crank spindle length (the spindle length only comes into play when you're leaning into a turn).

On the Super Sport, the standard Astabula crank arms are 170mm. According to the Schwinn specifications, the same crank (p/n 56 018) was used on all frame sizes in 1973, the rims are 27", the tires are p/n 62 203 (27" x 1-1/4"), and the pedals (p/n 57 730) are all the same for all frame sizes.

So, no, there is no difference in pedal clearance between frame sizes on the fillet brazed Super Sports.

Putting 175mm crank arms on your son-in-law's Super Sport is probably not a good idea because of the higher risk of pedal strike.

As you're the one here with specific knowledge about the SS, I'd say you've provided the best answer to the OP's question. I'd go with the original 170 spec.

Holy cow, those are some great posts, and wonderful information. Sounds like I will stick with original equipment sizing, and look for a 170mm alloy crank to replace the 4 lb. Ashtabula.

As usual, you guys are the best...thanks a ton for the input!