Bicycle Mechanics - hillclimbing bike question...
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08-24-07, 12:19 PM
Am building a bike specifically for riding in the Pittsburgh Dirty Dozen(dannychew.com). Was wondering if there is an advantage for climbing w/shorter cranks? Normally, I use the standard 170mm, but have been thinking of going to 165 or even 160. Would think the tighter stroke would make everything more efficient when climbing. I'm not worried about speed...just want to finish the ride. The frame/fork is an old Cannondale SR300 53cm aluminum. Planning on a drivetrain using a Sugino 103mm bb, Bulletproof bmx cranks, 40t Rocket Ring single, 30x12 8 sp cass(a friend built me a set of wheels using an 8 sp hub), sram pc-68 chain, Ultegra 9 sp rd, bar-end shifter.
My commuter rigs are set up very similar to this except w/standard cranks. Thanks alot to any and all who answer.
Also, I've got an old Bio-pace 42t elliptical chainring in my parts bin...don't have any idea where it came from, but was maybe thinking of using it w/a 32x12 cassette instead of the Rocket Ring.
08-24-07, 12:27 PM
Most climbing is done at lower cadence so longer cranks may provide some advantage. MTBers add 5mm for the hills. Ive never heard of selecting shorter cranks to tackle hills.
08-24-07, 12:56 PM
I'm not familiar with the terrain, but
Is ground clearance an issue?
What size bike? (rim)
How tall are you?
08-24-07, 01:47 PM
Thanks guys. Just went all over the net and found my answer in a related article @sheldonbrown.com. Turns out that all things being equal in terms of gear inches, a 5mm increase in the length of the cranks will lower the ratio by 3%. So, I need to do exactly the opposite of what I thought. 175mm cranks are what's needed. The rims are 700 Sun MZ-14 and I'm 5'11, 185 lbs. Ground clearance isn't an issue, but I went w/a smaller frame size(my commuters are 56cm) as I'll need to be able to sling it pretty hard from side to side while climbing. The terrain is brutal. Potholes, no shoulder, narrow streets, cobblestones, etc. 13 hills in 50 miles. One of them is a 37% grade @ 300 feet, but most are over 20% and long. There's a website - iheartpgh.com which lists 15 hills in and around the 'Burgh and I know at least 7 are in this ride. Read some of the articles @ dannychew.com. They'll explain the history of the ride way better than I can. Thanks again for you responses.
08-24-07, 02:21 PM
"Turns out that all things being equal in terms of gear inches, a 5mm increase in the length of the cranks will lower the ratio by 3%. So, I need to do exactly the opposite of what I thought. 175mm cranks are what's needed."
Depending on leg length, you MIGHT be better off using a lower gear and a bit more cadence with shorter cranks. It just depends on ones "body geometry".
In my case, I'm nearly 60 and 6' with arthritic knees. I simply can't spin longer cranks very fast because the range of motion hampers me. Using short (for me) cranks, I can up my cadence more than enough to overcome their "torque deficit". Simply put, using a lower gear and short cranks, I can go noticeably faster, than long cranks and a higher gear.
08-24-07, 07:21 PM
A 40x30 chainring and cog gives a 36" low gear which may not be low enough. I'm from Pittsburgh and am familiar with most of the hills and my son-in-law has ridden (and completed) the Dirty Dozen. You don't get much momentum to start several of them so a very low low gear may be in order unless you are REAL strong.
Consider a 34T or lower chainring and a double crank. You do have to ride between the hills you realize.
08-24-07, 07:42 PM
I'm 55. No knee probs as yet(knock on wood). Evenly distributed in terms of weight/leg length. Decided on 40x30 low. The 'official' limit for this ride is 39x32. It used to be 42x24:eek: Using the tables on the sb.com site for factoring in crank length w/gear ratios it came out like this: Same size wheel/tire combo w/different length cranks/same gears per 1" of circular motion
165mm = 2.707" tire travel per 1" crank travel @ 40x30
170mm = 2.627" etc.
175mm = 2.547" etc.
It's confusing, but it seems when using this formula you get a higher travel rate in your tires w/shorter cranks. Don't know how that translates into resistance when climbing a 20% grade, but am going to find out. Just spoke w/a friend who's going to lend me a pair of 165mm cranks he's not using right now. Will swap them out w/t ones that are on my commuter(170mm) and see if the same hills I climb everyday are easier or harder. Probably should've done this in the first place :rolleyes:
08-24-07, 08:08 PM
Just posted and saw #6. Lived in Pittsburgh for over 10 years and rode part of the DD in training for a tour once. Never participated in the ride, however. Told a friend of mine here about the ride. He checked it out via the website, called me back and that was that. He said, "Dude, we're in!" Yes, I'm aware you have to ride in between the hills.:) The cluster I'm planning to use goes 30-26-22-20-18-16-14-12 w/a single 40t Rocket Ring on the front. Maybe a 39. You're allowed to do switchbacks...which I'll probably have to do, but as long as you don't 'dab' it counts. It's the honor system.
08-24-07, 10:58 PM
Something to consider (besides gears) is that a lighter tire is easier to pedal up hills. You can also "adjust" gear ratio by "tweaking" tire size.
A 26x2" tire is 4% larger than a 26x1.5"
1 tooth on a 12T cog is 8.3%.
I'd be interested what you find out between the 165 & 170mm cranks. Do you have a speedometer/elapsed time?
08-25-07, 01:35 AM
Was wondering what you're all talking about and found this article from last year's DD:
I naively thought the hills we have out west were rough, but there's some crazy grade out in Pittsburgh. I've done a century that netted 5400 feet, but the worst grade was around 16% for less than 1 mile.
Best of luck.
08-25-07, 07:36 AM
Thanks for the link and good wishes Smurf Hunter. I'd never seen that particular article. It pretty well captures the feel of the hardcore, subversive nature of the ride. It's not sanctioned and there're no medals. Just pride in one's own ability to overcome obstacles. I keep teeling my friend who's all fired up to do this, "You have NO idea how angry the hills are on this ride":D There's a few in this area(mid-tn) that compare, but not like there.
Yeah Bill, I'm training w/700x35mm airless tires and a 48x26-12 gear cluster. 170mm cranks. Getting some high grip 700x23 Bontragers for the ride itself. The speedometer/et suggestion is a good one. In building this bike I'd planned on getting a commputer rig that measures several things at once. Cadence, mph, etc. I just had some minor surgery and will be back on the bike next week so I'll post about the 165mm vs 170mm asap.
08-25-07, 01:37 PM
I recently read an article about this, which I now cannot find (online). However, from testing different crank lengths on different height riders, the author concluded that optimum efficiency was achieved with the crank length that the rider was accustomed to rather than a predetermined rule. As with most studies, I'm sure someone else has produced a paper that contradicts this.
If I find it again I'll post a link.
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