Reborn



difference between 175 and 170mm on my hardtail was about 1.5-2 min per 47 min lap, that's why I'm worried.
 
Originally Posted by dot .

difference between 175 and 170mm on my hardtail was about 1.5-2 min per 47 min lap, that's why I'm worried.
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Bottom line, put on the crank length that works best for you.
 
Originally Posted by JoelS79 .
So, unless my memory of the physics/geometry is horrible (which it could be), you won't lose power, but you might have to exert a little more to get the same speeds.
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The same speed requires the same power output. The length of crank is not a factor in the power output required to maintain a given speed.
 
Originally Posted by Alex Simmons .




The same speed requires the same power output. The length of crank is not a factor in the power output required to maintain a given speed.
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But you don't say that any given person will show the same times with different cranks because it won't be true.
 
In last month I've made three 10k test rides on 5k stretch of a flat road, one on a hardtail with 36 slicks, and two on my road bike. Times were: 15:52 for a hardtail, 16:00 and 15:47 on the road bike. Road bike times seem ridiculous to me although y road bike doesn't have any aero stuff and wheels are ksyriums. While I like my time on the mountain bike road bike times puzzle me. I can't understand why I'm so slow.
 
There is science to suggest that most users actually gain power going to shorter cranks. The science will surprise most. I am a day or so away from having a video to explain what is going on and why.
 
Originally Posted by dot . In last month I've made three 10k test rides on 5k stretch of a flat road, one on a hardtail with 36 slicks, and two on my road bike. Times were: 15:52 for a hardtail, 16:00 and 15:47 on the road bike. Road bike times seem ridiculous to me although y road bike doesn't have any aero stuff and wheels are ksyriums. While I like my time on the mountain bike road bike times puzzle me. I can't understand why I'm so slow.
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Over a 10k course, you're averaging 23.5 mph (37.8 kph) on a mountain bike with slicks? Extremely impressive. It's also a very respectable speed for a road bike, yet you feel as though there is a problem. Didn't you say you are new to road biking? I have a feeling you are optimally sized and positioned on the mountain bike, but could use improvement on your road bike. But, you countered by quickly suggesting the problem is not a fit/position issue. Why?
 
To minimize comparing apples to oranges, please describe the following for each bike during your test above:
  1. tire size
  2. crank length
  3. chainring gear most used for your test
  4. cassette gear most used for your test
  5. cadence
 
Originally Posted by dot .



 
But you don't say that any given person will show the same times with different cranks because it won't be true.
 
In last month I've made three 10k test rides on 5k stretch of a flat road, one on a hardtail with 36 slicks, and two on my road bike. Times were: 15:52 for a hardtail, 16:00 and 15:47 on the road bike. Road bike times seem ridiculous to me although y road bike doesn't have any aero stuff and wheels are ksyriums. While I like my time on the mountain bike road bike times puzzle me. I can't understand why I'm so slow.
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Unless you are measuring power, then times matter little as a comparison. You are not comparing apples with apples.
 
When I say same speed = same power, I mean when all else is equal.
 
You are comparing different bikes, different set ups, different environmental conditions etc etc.
 
 
 
Over a 10k course, you're averaging 23.5 mph (37.8 kph) on a mountain bike with slicks? Extremely impressive. It's also a very respectable speed for a road bike, yet you feel as though there is a problem. Didn't you say you are new to road biking? I have a feeling you are optimally sized and positioned on the mountain bike, but could use improvement on your road bike. But, you countered by quickly suggesting the problem is not a fit/position issue. Why?

 
To minimize comparing apples to oranges, please describe the following for each bike during your test above:
  1. tire size
  2. crank length
  3. chainring gear most used for your test
  4. cassette gear most used for your test
  5. cadence
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I actually only asked first about crank length because I remembered my experience with 170mm cranks. The fit issue bothers me too. Since I bought the road bike I've never felt uncomfortable and my speed doesn't change regardless of the positions I tried. When I bought the bike I went straight to Greece for 2 weeks and rode in the mountains there without any issues with the longest ride of 7 hours. Nonetheless I never felt that I'm faster in comparison to my XC bike. It keeps bugging me and I can't understand why. I kind of believe that road bikes should be faster.
 
My bikes:
 
XC Bike: Al GT Zaskar '09, size M (ETT 597mm + stem 90mm), 36mm slicks Kenda Kwick Roller. Cranks 175mm, gears used: 44x12 all the time. Candence - speed varied from 34km/h to 43.5 km/h and so did cadence (from 78-79 to 100-101, within my comfortable range). Wheels: XT 775/DT Comp/XC717 rims.
 
Road bike: Carbon Merida Scultura '09, size 54-56 (ETT 575mm + stem 100mm), conti GP4000 23mm or Vittoria Rubino pro 23mm. Cranks 172.5, gears used: I don't know, there are too many gears to choose from for an MTB-er (compact crank 50-34 and 11-25 cassette), but cadence range was the same as for the XC bike. I can only suggest that it was 50x15. Last time I didn't shift at all. Wheels: ksyrium elite.
 
PS Ooops. I was wrong, my road frame is actually 555mm long which is correct sizing for a 56cm frame.
 
Originally Posted by Fday .

There is science to suggest that most users actually gain power going to shorter cranks. The science will surprise most. I am a day or so away from having a video to explain what is going on and why.
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I would call that science if they were giving there their test riders cranks with length measured in percents of their leg length and with length change of max 5 mm, not 30-40 mm.
 
Quote:
Originally Posted by Alex Simmons .

[COLOR= #f00]The same speed requires the same power output[/COLOR]. The length of crank is not a factor in the power output required to maintain a given speed.

 
FWIW. It appears that you are presuming the same physiological efficiency for all cadences ... which is probably not the case.
 

 
 
 
Originally Posted by dot .
I actually only asked first about crank length because I remembered my experience with 170mm cranks. The fit issue bothers me too. Since I bought the road bike I've never felt uncomfortable and my speed doesn't change regardless of the positions I tried. When I bought the bike I went straight to Greece for 2 weeks and rode in the mountains there without any issues with the longest ride of 7 hours. Nonetheless I never felt that I'm faster in comparison to my XC bike. It keeps bugging me and I can't understand why. I kind of believe that road bikes should be faster.
 
My bikes:
 
XC Bike: Al GT Zaskar '09, size M (ETT 597mm + stem 90mm), 36mm slicks Kenda Kwick Roller. Cranks 175mm, gears used: 44x12 all the time. Candence - speed varied from 34km/h to 43.5 km/h and so did cadence (from 78-79 to 100-101, within my comfortable range). Wheels: XT 775/DT Comp/XC717 rims.
 
Road bike: Carbon Merida Scultura '09, size 54-56 (ETT 575mm + stem 100mm), conti GP4000 23mm or Vittoria Rubino pro 23mm. Cranks 172.5, gears used: I don't know, there are too many gears to choose from for an MTB-er (compact crank 50-34 and 11-25 cassette), but cadence range was the same as for the XC bike. I can only suggest that it was 50x15. Last time I didn't shift at all. Wheels: ksyrium elite.
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FWIW. I'm going to speculate that the difference may have MORE to do with (what 'i' perceive to be) the inefficiency of the KOPS positioning that your Road bike was probably set up with ...
 
So, compare the saddle-to-pedal position on your bikes ... adjust your Road bike's saddle to replicate your MTB's saddle position ... and, ride the "course" again.
 
 
 

 
 
Originally Posted by dot .XC Bike: Al GT Zaskar '09, size M (ETT 597mm + stem 90mm), 36mm slicks Kenda Kwick Roller. Cranks 175mm, gears used: 44x12 all the time. Candence - speed varied from 34km/h to 43.5 km/h and so did cadence (from 78-79 to 100-101, within my comfortable range). Wheels: XT 775/DT Comp/XC717 rims.  
Road bike: Carbon Merida Scultura '09, size 54-56 (ETT 575mm + stem 100mm), conti GP4000 23mm or Vittoria Rubino pro 23mm. Cranks 172.5, gears used: I don't know, there are too many gears to choose from for an MTB-er (compact crank 50-34 and 11-25 cassette), but cadence range was the same as for the XC bike. I can only suggest that it was 50x15. Last time I didn't shift at all. Wheels: ksyrium elite.
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Using the information above and plugging it into Sheldon Brown's Gear Calculator, your gain ratio on each bike is practically identical at 6.5 (meaning: for every unit of distance your pedal moves, your bike moves 6.5 units). Your ride times over the 10k course also reiterate this "sameness" between your road and mountain bikes. If you have enough power, you could always shift your road bike to the 14, 13, or 12 tooth gear for more speed (gain ratios of 7.0, 7.6, and 8.2). But because of your mountain biking background, maybe your body is only used to a maximum sustained gain ratio effort of 6.5.
 
Beyond that, it boils down to why a road bike should be inherently faster than a mountain bike. Off hand, I can think of only 2 reasons: tire friction and aerodynamics. A road bike typically has thinner tires and puts you in a more aerodynamic position. The road tires you mentioned are good race tires, as long as they are inflated properly. Which leaves aerodynamics as the next obvious culprit. Are you sure your road bike position is optimal?
 
Originally Posted by Scotty_Dog .

But because of your mountain biking background, maybe your body is only used to a maximum sustained gain ratio effort of 6.5.
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is sounds so riduculous! If I put 2.35" studded tyres on my bike, I will use 44x16 gear because of rolling resistance.
 
Originally Posted by alfeng .

 
FWIW. It appears that you are presuming the same physiological efficiency for all cadences ... which is probably not the case.
 

 
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Physiological efficiency could affect the energy input (kcal) per unit time, but not the power output to the cranks, if speed is given as the constant (which it was) and all external resistances are fixed.
 
I'd always used 175. Now, I have 172.5 on one bike, and 175 on another. Hard to say that I notice a difference to be honest.
 
YMMV of course ...
 
Originally Posted by bing181 .

I'd always used 175. Now, I have 172.5 on one bike, and 175 on another. Hard to say that I notice a difference to be honest.
 
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Same experience here, 175mm on the TT bike, 172.5mm on the road and cross bikes, 170mm currently on the track bike but have ridden 165s and with the seat adjusted for the same knee angles the power from one bike to another is the same at least when I'm riding the base bars on the TT bike, in the aero bars I tend to lose a bit but power vs. torso angle is another discussion. Jumping from one bike to another is no big deal and I don't really notice the crank arm length in and of itself.
 
Still YMMV so ride whatever length makes sense to you...
 
-Dave
 
A small update.

I bought rollers this summer and tried riding them a couple of times when it was raining. My avg. speed on rollers was 37.5-38 km/h. After I started this thread I made some changes to my position (moved saddle back 3 mm, lowered seat by 1 mm, lowered handlebars by 15mm, put a 10mm longer stem). On tuesday I rode rollers again and did the same routine. Wow, my avg. speed increased by 3 km/h! Pity, I won't be able to test myself on the road since the winter season has come.
 
http://mywheelsandmore.com/cranksets/bike-crank-arm-fit.htm

http://www.nettally.com/palmk/crwives.html

http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

Google was just invented and I found it first.
 
On my race only bike the cranks are 172.5mm on my training bike they're 175mm.
The set up dimensions are the same on both bikes.

I have long legs so apparently I should have 177mm cranks (by some websites advice) or at the least 175mm.
Does that 2.5mm make any different to my speed or climbing faster etc....NO.

Adjust your position so it's the best for you and get used to the crank length you have.

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