login
Using power to calculate gearing
-
No new posts
 2009-09-21 4:29 PM |
Veteran  196   Arlington, VA |  Subject: Using power to calculate gearingI was fooling around with analyticcycling.com to help me figure out gearing choices for IMLP next summer. If my lowest gear is 39x25 at a cadence of 75 (I prefer to spin at 100 but trying to do the calculations with the low-end for the steepest hills), the resulting speed is 9.1 mph. I used the Speed for Given Power calculator to figure out how many watts are required to climb an 8% grade, which I think is the steepest on IMLP. I used the default .5 effective frontal area and drag coefficient. I changed the weight to 86 kg for me, my bike and accessories. I've never used a power meter but entered 160 watts for running the calculations at 0% grade. The resulting speed is 21.1 mph. If I change the grade to 8%, I need to increase the power to 300.2 watts to climb an 8% grade without slowing below 9.1 mph (or 75 rpm at 39x25). If I switched to a compact with 34x25 gearing at 75 rpm, the required power to maintain 75 rpm at 8 mph decreases to 259 watts, but still well above the 160 watts I'd prefer to average. To maintain 160 watts at 75 rpm, I'd have to average 5.0 mph, which would require a 30x34 gear, which is almost beyond granny gear territory. So I guess there's no way to maintain constant power and still get up an 8% grade on a normally geared road bike. On the plus side, at a 4% grade, I'd be in my 39x25 cruising up at 160 watts at 9.1 mph waving as everyone passes me, knowing I'll catch them on the run after they bonk. |
| 2009-09-21 5:50 PM in reply to: #2418515 |
Master 1529  Living in the past | Subject: RE: Using power to calculate gearing OK, I saw no question in your post but that's not going to stop me from commenting....love the techie stuff as much as the next nerdy engineer, but wow! From your stated goal of 'figuring out gearing for IMLP', I believe you have overthought the problem. Simple suggestion - go with a xx/27 cassette and spend the next 9 months training to improve your power output. The goal to maintain constant power is unrealistic and unnecessary in my opinion. You are right that there is a practical limitation to gearing on a road bike. As I geared for MOO my options were limited by how big a rear cog my deraileur could handle - a 27, although you wouldn't be the first to ride an IM with a mountain bike, if your goal of constant 160 watts is really that important to you. |
| 2009-09-21 6:19 PM in reply to: #2418515 |
Not a Coach 11473 Media, PA | Subject: RE: Using power to calculate gearing On an 8% grade, you have little choice but to increase your power if you want to maintain a reasonable cadence. But, obviously, a 34x25 gives you more flexibility. That's what I would suggest you ride. It's what I used. But I also think there is very little in LP at 8%. LP has long climbs--very little steep increases in grade. As an aside, it's not optimal to maintain iso-power unless you are riding on a flat, windless course. You should go (somewhat) harder on hills and easier on descents. Similarly, you should go (again, somewhat) harder into a headwind and easier with a tailwind. You should always expect to be increasing power up a hill. But, since at LP they are mostly very long, it should be only a moderate increase over your 'target' power level for the whole race. |
| 2009-09-21 6:20 PM in reply to: #2418515 |
Champion 19812 MA | Subject: RE: Using power to calculate gearing You probably should rethink your upper limit for your power for hills. Ideally you keep your watts under zone 4 or at least that was my plan for my IM. I redid my gearing so I could keep my watts under 140 which seems ridiculously low but helps not burn matches and sets up a better run. Goal to keep your VI as low as possible is key. My VI at IMLP was 1.17 and at Canada was 1.04 before I got sick. The more you spike your power up hills the higher your VI and more energy you waste with little return. Tech stuff is fun to play with. |
| 2009-09-21 6:41 PM in reply to: #2418515 |
Veteran 196 Arlington, VA | Subject: RE: Using power to calculate gearing My post was all theoretical--I have no plans to ride a mountain bike or flat wattage, but the calculations show that constant power output is nigh well impossible over a hilly course. Edited by kcb203 2009-09-21 6:43 PM |
| 2009-09-21 6:44 PM in reply to: #2418741 |
Champion 19812 MA | Subject: RE: Using power to calculate gearing kcb203 - 2009-09-21 7:41 PM I have no plans to ride a mountain bike or flat wattage, but the calculations show that constant power output is nigh well impossible over a hilly course. You can ride flatter power with practice and purposing watching your power up hills. What it means if folks fly by you up hill, but once you crest the hill they are often toast and just coast where you can use your power to crest and start the down hill and pass them all back using less over all power. Lot of folks can do IMLP under 1.09 VI....stronger cyclists even lower. Ride it like you want just something I have learned from various sources about how to execute a good bike to set yourself up to run well. |
|
|
|
View profile
Add to friends
Go to training log
Go to race log
Send a message
View album
CONNECT WITH FACEBOOK