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At what gradient does a light bike become an advantage?(23 posts)

At what gradient does a light bike become an advantage?GileyD
Jan 27, 2003 3:54 AM
Yesterday I set off for my usual Sunday morning ride but about 2 minutes after leaving the house I punctured. After I'd fixed it I noticed the tyre had a large slit in it from a piece of glass or similar so wasn't really safe to ride. I went back home and got my Litespeed out (not scheduled to see daylight til Spring!!).

My Litespeed weighs about 17.5lbs as against my steel winter bike's 23lbs. On the first steep climb (probably aprox 16%) I accelerated hard out of the saddle as the road was narrow and a car behind was waiting to get by, and was reminded just how well the bike climbs / accelerates. The same effort on my steel bike would have seen a gradual increase in speed, but nothing like the same burst of acceleration on such a steep incline as I got from the Litespeed.

This got me thinking, taking frame material out of the equation (there has been plenty of discussion on this recently on this board!), at what sort of gradient does a light bike become a real advantage? On the flat and on gradual easy climbs I didn't feel any great advantage being on a lighter bike, but on (I guess) 12% + gradients it was very noticeable.

Is there any scientific answer to this, or does it come down to rider perception?
www.analyticcycling.com (nm)geeker
Jan 27, 2003 6:43 AM
anything not downhillDougSloan
Jan 27, 2003 6:51 AM
All else equal, the lighter bike will be faster any time the grade is not downhill. Simple laws of physics. I have no idea what you mean by "real advantage." For some people, that could be 1 second over 25 miles. For others, it might be 5 minutes.

Run the calculations yourself here: http://www.analyticcycling.com/ForcesLessWeight_Page.html

Doug
anything not downhillGileyD
Jan 27, 2003 8:59 AM
Thanks for the link. Not had a chance to run any calculations yet, but looks very interesting.

By "real advantage" I just meant the point (or gradient) at which less weight becomes a measurable advantage rather than a psychological one.

(When it comes to physics the only thing "simple" is me!)
measureable exampleDougSloan
Jan 27, 2003 9:18 AM
Dropping 1 kg over a 2000 meter 1 percent grade will put you .5 seconds, and 5 meters, ahead at the top (using default numbers otherwise in the calcs).

That's certainly measureable if you are racing side by side up the hill to a finish at the top.

Even with a zero grade (flat), dropping 1 kg puts you 1.3 meters ahead. Certainly races have been won or lost by that much (but of course there are many other factors, but here were are confining the discussion to other factors being equal).

Doug
measureable exampleGileyD
Jan 27, 2003 9:23 AM
Being , as I said, a bit of a dunce when it comes to physics, how come there is an effect on the flat? Wouldn't the rider's power output and maybe any aerodynamic influence be the factor here?

Thanks
measureable examplespeedisgood
Jan 27, 2003 10:06 AM
I think the lighter bike accelerates just a little faster to get up to speed and that's the difference. I did some calc's on sprinting out of a corner with 2 differently weighted bikes and there was a small difference (like 6 cm or something).
Everything equal but weight ANSWER increased rolling resistancebigrider
Jan 27, 2003 1:18 PM
that said...laffeaux
Jan 27, 2003 11:00 AM
Going back to the original question, and the "measureable" effects of weight loss (especially from the bike) versus theoretical savings, if a 1.3 meter gain is realized by droping 1 kg, imagine the effects of several grams. It's pretty close to zero.
lafeaux's point is worth expanding! nm.mainframe
Jan 27, 2003 11:34 AM
let's use his numbersDougSloan
Jan 27, 2003 11:45 AM
He inquired about a 17.5 pound versus 23 pound bike on a slope of 12%. He did not say the distance.

Let's assume 1/2 mile. Plugging in the numbers, that would put the lighter bike (all else equal) ahead by 9.5 seconds and 26 meters.

I believe the calculations are linear, so proportionally less weight difference or shorter distance will be proportionally less effect.

No one has ever claimed, as far as I know, that a few grams difference alone will make any difference anywhere. It's only when a few grams taken off a number of parts does it become significant, and, yes, it is significant.

Doug
let's use his numbersHillRepeater
Jan 27, 2003 12:59 PM
The key point is 'all other things being equal'. These equations assume a consistant and constant power output, which doesn't exist when you're out riding. It seems that these models show the theoretical 'best' improvement - assuming that the speed you're currently climbing the hill is utilizing your max power, or that you'd maintain the same power output regardless of the weight of the bike. It's kind of hard for me to type what I'm trying to say - but it seems that you'd only see these gains if you were climbing at your limits.

I think it might make more sense to calculate the power required to carry x amount of weight up a specific incline at a specific speed. Then you could determine the impact of droping weight as a reduction in needed power. ie: if dropping 5 lbs allows you to climb the local monster hill at the pace of the 'A' group with 5 watts less output, you might be able to stick with them and still have something left for the sprint.

Make any sense to anyone other than me? :)
re: models and "making sense"cyclopathic
Jan 27, 2003 1:24 PM
all models simplified, that's why they called models ;)

Problem is it's not that easy to build a model to estimate what effect on engine would have extra X sec at full throttle. Some may have an asthma attack, others would just crack and give up.

With respect to 9sec it's enough to get dropped in race. Granted if race isn't RAAM qualifier.
yeplaffeaux
Jan 27, 2003 1:10 PM
Using the original number I agree that there is a difference. I guess I was commenting more on the desire to drops a few grams (which on occasion I find myself wanting to do), and it's relative effects. When I look a a seatpost binder that is 10 grams lighter than the one I have, and only $15, it's worth considering that 10g of savings does not matter a hill of beans.

But I stand corrected that in the origial opst, the 5+ pounds is significant.
re: At what gradient does a light bike become an advantage?HillRepeater
Jan 27, 2003 7:55 AM
I'm impressed that you were able to accelerate on a 16% grade! I'm happy just to keep going forward on stuff that steep...
How do you measure/estimate the gradient? nmPaul
Jan 27, 2003 9:43 AM
here is your answerbigrider
Jan 27, 2003 11:32 AM
Rise divided by run.

If a hill raises you 100ft. in elevation over 1000ft. of road it is a 10%. 100/1000 x100= gradient in percent

If you use your cyclometer for the road distance this calculation will be ever so slightly off because the horizontal distance is slightly different than the ridden distance. That is because the length you traveled going up a hill for 1000ft. would be slightly less than 1000 ft. in the horizontal direction. With that said the math makes that point nearly negligible unless the hill is 80% {climb that one}
and furthermore....KeeponTrekkin
Jan 27, 2003 12:09 PM
... you don't need to be a surveyor. Visit:

www.topozone.com

for US Geologic survey maps in a variety of scales. I think they have the entire US.

Use the 1:25,000 scale; find your hill and its starting and finishing elevations.

Get the length of the road from one of the road map sites like:

www.mapquest.com.
The numbers don't lie, nor tell the whole story ...rwbadley
Jan 27, 2003 12:44 PM
I used to wonder about this light/heavy bike stuff all the time.

Recently I was able to get a bike that showed a four pound advantage over one I had been riding for a long time. The difference I noticed may not have been ALL due to the weight difference. I found the bike felt much lighter and more lively (this I expected) but also was more rewarding to work hard on.

This made hillwork (somewhat) easier because the bike responded to input with more feel of forward motion. Instead of slogging up the hill while seated, occasionally standing and not really 'getting anywhere'. I find I can now (heh heh) slog up the hill, stand occasionally, and feel rewarded with the bike response (and still not 'get anywhere'!)

I won't discount the numbers of light vs heavy, but the psychological aspect may be just as important...

RW
That wasn't psychological, it was realbigrider
Jan 27, 2003 1:13 PM
I think sometimes we don't trust what we feel because of the lies floating around. If your bike felt lighter and more lively and you weren't bouncing up and down but going forward..... you probably were.

The lighter bike made a big difference, and the bike was probably stiffer and didn't sap your energy bobbing. It was real.
weight vs aerocyclopathic
Jan 27, 2003 1:07 PM
if you compare light non-aero wheels vs moderately aero (Cosmics, RevX etc) the diff in 1lb becomes advantage at grade above 8-10%. All given equal lighter bike better at any grade.

Rider perception is very subjective. I've read about blindfold test group of riders and several bikes. Turned out 2lbs is about as much as you can perceive. If weight diff was under 1lbs answers were contradictory.
advantages of a heavier bikeDaveG
Jan 27, 2003 6:17 PM
The best advantage of a heavier bike is the excuses it provides. If you have a chi-chi 15lb bike and you get dropped on a climb by some wiley old veteran on an old, heavy steel bike you are totally exposed. Oh sure, you could fake a mechanical problem, a knee injury, or claim its your "easy day" but nobody's going to buy that. On the other hand, if you are riding that heavy bike you can dismiss getting dropped by the light bike with a "who wouldn't be faster on that?". Should you actually drop the light bike with your tank then you are confirmed as a cycling stud. With the heavy bike you can't lose!
Stiffness?PODIUMBOUNDdotCA
Jan 27, 2003 8:50 PM
Could it be the stiffness that makes it accelerate faster? Weight plays a role but I think its more the difference in stiffness with the bikes I've ridden.

Nick
PodiundBound.ca