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When is 39/27 not a 39/27???

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When is 39/27 not a 39/27???

Old 09-29-07, 12:15 PM
  #1  
Donegal
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When is 39/27 not a 39/27???

I'm still looking for the answer. I am somewhat of a wrench and have built my last carbon screamer from scratch. Aegis carbon frame, Campy Compact 10 speed, Mavic Ksyrium SSl's, Ouzo Pro peloton fork w/2 degrees extra rake,K-wing bars, all the toys.

I recently went to the mountains with a friend on my new scoot, Kuota Kredo w/Dura-Ace, which I bought already built up. I am a fairly strong rider with very strong legs. 600 lb. leg presses are no problem. I was riding w/my usual partner, a personal trainer who just finished an Olympic length triathalon. While riding uphill in the smallest gear 39/27, I felt like I was 2-3 gears higher. I have ridden hills before, I live in North Georgia. I climbed 2 hills with somewhat strong 7.5-15% grades. I noticed my partner was spinning comfortably 60-70+ rpm and I was struggling 30-45 rpm with both of us in the smallest gear. I know they were the same because I maintain his bike and put the 12-27 cassette on it.

This disparity kept up for 3 hours until I finally cracked. When he came back for me and pulled me up the hill, we kept comparing cadences and I was allways 10-18 rpm lower even at a crawl. I haven't pulled the crank yet, but have counted teeth 3 times. 39. I could understand if this crankset was 42 or 44 but I count 39. I have to spin the cranks and count, but I am going to take them off and count again.

Is 39/27 allways 39/27? I would think so. The diameter of the gears must be the same, the links of the chain fit the gear. Any ideas???? So far, the only problem I found was the way the rear derailleur was mounted. It was obviously mounted w/tension on the spring and was off far enough that the tensioning screw was totally useless. I noticed that the roller on the derailleur had no clearance to the big cog. I could see some possible chain bind there which would account for the difficulity to pedal. It's the cadence that doesn't really make sense. Like I said, I crawled up the hills, one pedal stroke at a time. The level of difficulty was so great, my low back was threatening to fail. I am better on the flats and rollers, but this was ridiculous. Would welcome ideas.
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Old 09-29-07, 12:19 PM
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Same crank arm length?
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Old 09-29-07, 12:32 PM
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The fact that your friend was going faster at a faster cadence does not prove anything about relative gearing, maybe you were just having a bad day. If you want to compare gearing and cadence the two of you should ride a flat road in the lowest gear and compare cadence and speed.

Al
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Old 09-29-07, 12:44 PM
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My friend wasn't going faster though. I was on his wheel. We rode the remainder of the hill with me hanging onto his tire (wheelsucker) for dear life. Another poster mentioned crank length, his are actually longer 175 vs. 172.5, minimal and longer should be slower not faster. Maybe you're right, I might have had one of those days. I am a believer that somedays you are bulletproof and others, you just plain suck. I did fall prey to a stomach bug the next 4 days and couldn't eat.

I know he is stronger, it's the cadence that has me bugged. I have since fixed the derailleur mounting and am going to tear the cranks off and check teeth and bearings. Thanks.
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Old 09-29-07, 01:08 PM
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I think what the OP is trying to say is why was his buddy spinning in a 39/27 at 70 rpm while he was dogging it in the same gear (39/27) at 45 rpm, YET THEY WERE STILL GOING THE SAME SPEED!

I have three suggestions:

1) rear tire pressure, was your friend's tire pressure low for some reason (giving him a smaller rear wheel diameter)?
2) was he on 650s instead of 700s?
3) were you really on the 27? Or was your limit screw on the rear derailleur preventing you from getting into the 27 and you were really on the 25, and just didnt notice it?
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Old 09-29-07, 01:51 PM
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Crank length wouldn't matter here, that doesn't affect gearing, just effort level and whatnot. Tire pressure and small variations in tire size among manufacturers wouldn't give you a 20% or more difference.

What is most likely going on if you're actually in the same gear is that his pedal stroke is rounder and cleaner, so he's actually pedaling all the way around at a constant speed while you're coasting slightly at the dead zones (cranks vertical). This is easy to do if you're really hammering/grinding it out. I'd think you'd hear it given that mavic freehubs are pretty loud, but maybe not.
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Old 09-29-07, 03:25 PM
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The longer pedals add some additional leverage but make no difference to cadence or gear ratios.
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Old 09-29-07, 03:40 PM
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How accurate can you be in these circumstances about your own cadence, let alone someone elses? Granted, you may have some gadget that counts this, but it defies logic that two identically geared bikes were traveling at the same speed with the riders making different number of strokes per minute, if they were in the same gear. It isn't possible, assuming they have the same wheel diameter. Small diameter differences couldn't account for the cadence difference either you quote either (70 / 45) , so tyre pressure isn't the cause. Could it be that you just weren't well and it felt like you were pedalling slower and with more labour?

If that isn't it, then I'd check that the bikes are identically geared, and that yours is changing through the full range under load.

I think ZAC hits the issue above. Most likely your pal was in a different gear to you. I'd put it at 95% that this is the answer. You'd likely struggle to keep with him too if he was spinning as you say, because obviously he will exert more power with less exhaustion spinning than can be achieved by a similar rider working at a slower cadence.

Unless you looked at the block to check you were in the gear you think you were, there could have been some problem with your gear change. Maybe it stuck a gear or two higher, regardless of what you did with the shifter.

Last edited by EvilV; 09-29-07 at 03:54 PM.
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Old 09-29-07, 04:44 PM
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Originally Posted by EvilV View Post
............ but it defies logic that two identically geared bikes were traveling at the same speed with the riders making different number of strokes per minute, if they were in the same gear. It isn't possible, assuming they have the same wheel diameter.


+1- Not possible.
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Old 09-29-07, 07:52 PM
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Different size wheels or tires would make a small difference. Wrong number of teeth on the rear, or not being in the gear you thought you were would be a bigger issue. Perhaps a combination of several factors. If all else fails, turn both bikes upside down and compare them side by side.

Someone posted something about a skip-tooth sprocket, which I've not seen, but might throw your for a loop if present.
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Old 09-30-07, 02:09 PM
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Are you sure your friend doesn't have a compact crankset?
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Old 10-01-07, 12:26 PM
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Originally Posted by sfrider View Post
Are you sure your friend doesn't have a compact crankset?
+1
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Old 10-01-07, 12:34 PM
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The difference in rpm that the OP mentioned is huge. His partner had to be pushing a smaller chainring. It can't be a matter of different sized tire or wheel diameter because of the huge diff in rpm noted. I'm going with the compact crankset theory too.
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Old 10-01-07, 06:00 PM
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If your riding partner really has 39/27 low gear, there are two possibilities.

1) Smaller wheel. But based how much of a difference you say there was in cadence, it would have to be a pretty small wheel, and much smaller than 650C, so this is not likely.
2) Internal geared hub with low less than 1:1. OK, not likely, since if you have internal gears, why also run a multiple freewheel.

Much more likely is that you are mistaken on the 39/27 and he has a compact or even a triple.
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Old 10-01-07, 07:06 PM
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Originally Posted by roadfix View Post
It can't be a matter of different sized tire or wheel diameter because of the huge diff in rpm noted.
+1
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Old 10-01-07, 08:58 PM
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Sounds like your derailluer may be not staying in the lowest cassette. If your small pulley is rubbing when it is in the lowest gear you need to shorten your chain. For a good method see Sheldon Browns web site. If you have a 39/27 then it is the same everywhere even in Australia or in the chronosynclastic infandibulum. Good luck.
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Old 10-02-07, 02:50 AM
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Originally Posted by Deanster04 View Post
...If you have a 39/27 then it is the same everywhere even in Australia ...
Except that it turns backwards in the southern hemisphere, right?

I'm going with compact crankset on the friend's bike and the OP's inability to estimate cadence.
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Old 10-02-07, 10:20 AM
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Allright, the difinitive way to determine if you have the same gear, measure the roll out.

Put one crank up against a good reference on the bike, like the leading edge of the crank lined up with a bolt on the front der. Mark the contact patch of the front wheel on the ground. Turn the crank and guide the bike in a straight line until the crank comes back to precisely the place it started and make another mark where the contact patch of the front tire is. Do the same with your friends bike.

If the distance is the same with both bikes, you have the same gear.
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Old 10-02-07, 10:52 AM
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tire size could make a difference, but not that huge.
I have two bikes that I ride regularly, if I match cadence with my friend on one bike I match him with speed, if I match up on the other bike I'll pass him in a few stroke, gears are the same, but tires on the 'faster' bike are 27x1-1/8, matching bikes both have 700-25's

39/27 will be 39/27 regardless of the crankset, 1.44:1 is 1.44:1, rim/tire size is the only thing that will change effective gearing if both bikes have the same gears.
If you hadn't counted them and just 'felt' that they weren't right, I'd blame it on the altitude.

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Old 10-02-07, 01:33 PM
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If it happens again, swap bikes for a mile.
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Old 10-02-07, 01:40 PM
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Sounds like your bike may have locked out the lowest gear? You may have been in a 39x25t while your buddy was in 39x27t. Check while riding to make sure you really are in your lowest gear. I know on some early alloy & carbon frames with standard-diameter tubing, the frame were really soft and auto-shifted into a higher gear when flexed uphill. Annoyed the heck out of me as I'd be struggling and not know why. Then I'd look down and find that it had shifted 1-2 gears higher.
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