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Question: The physics of bike weight
CREATED: 08/21/10 by markhakd REPLIES: 17
markhakd's ravatar markhakd    JOINED: 10/8/09    POSTS: 220
Question: The physics of bike weight
POSTED: 8/21/10 9:33 PM

Okay, so I know there are plenty of incredibly smart people in the club (it complements our athletic prowess and good looks quite well), so somebody should be able to provide a good answer to this question:

Is there any difference, in terms of bike speed, between losing 5 pounds of body weight and buying a bike that is 5 pounds lighter? Let's assume that losing the body weight does not affect the amount of power you generate, and let's assume that the bike is unchanged other than its weight.

Or stated another way that is perhaps a bit more realistic, is there any difference between removing 1 pound of clothing from what you are wearing and removing 1 pound of water from your bottle? (Okay perhaps it's not that realistic, but it is pretty funny picturing a cyclist stripping off some huge mound of clothing while riding.)

Anyway, my sense is that I would barely notice the body weight difference, but the bike weight difference would change my speed dramatically. My brother (quite a smart guy himself) argues that the wheels don't know (or care) what 5 pounds is missing, there will be the same resistance and speed characteristics in either case. My gut says that he has to be wrong, but I can't come up with any physics-based reason as to why.

Any physics experts out there who can come to my rescue? :)

Thanks,
Kevin

bad ford's ravatar bad ford    JOINED: 2/28/09    POSTS: 184
RE: Question: The physics of bike weight
POSTED: 8/21/10 10:55 PM

The physics equations don't ask how you distribute the weight. If it's not in the equation, it's not in the solution.

I suppose it's possible that if the weight was in your legs, your legs wouldn't have to lift the weight with each revolution. Likewise if the 5 pounds was in the pedals, vs. the frame.

But there is no difference between removing 5 pounds of fatback and 5 pounds of frame from a physics perspective.

hppeabody's ravatar hppeabody    JOINED: 9/17/09    POSTS: 193
RE: Question: The physics of bike weight
POSTED: 8/22/10 1:23 PM

I'm curious about all of this myself. What I seem to find is that I always get passed on the downhills by these women and men that are twice my body weight (ok, maybe an exaggeration, but you get the idea..). Today I was pedaling as hard as I could, in my highest gears, on a long downhill, and getting as aero as possible, hitting about 32-34 mph and this lady just flies by me without even pedaling!!! Now, I get it that I have a very entry level road bike, not a tri bike, but her bike didn't look much different than mine so that should not have had anything to do with it. I of course caught her relatively fast on the uphill but still...I mean, i guess gravity is the answer there but it seems counterintuitive to the light bike helping go faster argument.

hamburger1971's ravatar hamburger1971    JOINED: 8/13/09    POSTS: 46
RE: Question: The physics of bike weight
POSTED: 8/22/10 2:47 PM

I'm not sure if it's true on a bike, but on a car the "unsprung weight" - that is, wheels, tires, brakes - can have a dramatic effect on how the car performs. With bikes, maybe the rotational mass (read: wheels) has a bigger impact than just the pure rider/bike weight?

karlaj's ravatar karlaj    JOINED: 2/28/09    POSTS: 201
RE: Question: The physics of bike weight
POSTED: 8/22/10 5:47 PM

To question #1: A pound is pretty much a pound. Extra rotational weight is slightly more penalizing (heavy wheels are worse than light ones), but not much.

Q2: Why does the other girl descend faster while you catch her on the uphill? Yes, weight (gravity) is the answer. I'd rather have your problem than hers. The difference is that downhill, it's all wind resistance and a heavier rider goes faster (but not really that much). Whereas uphill, wind resistance is much less (lower speeds) and power-to-weight ratio dictates how fast you can climb. So it's not really the bike, it's your (lack of) extra weight.

-Jason

bsull184's ravatar bsull184    JOINED: 2/6/10    POSTS: 283
RE: Question: The physics of bike weight
POSTED: 8/22/10 6:06 PM

My expert advice is to put on 10 pounds and only race flat courses.

merlinkim's ravatar merlinkim    JOINED: 2/28/09    POSTS: 294
RE: Question: The physics of bike weight
POSTED: 8/22/10 9:47 PM

Kevin - you can answer the question you asked yourself with an easy experiment: Go to Hains Point, carrying two bottles in your jersey pockets, none on your bike. Time a short loop (or any other distance). Rest, remove bottles from jersey pockets, place in bike cages. Repeat. Note any difference. Do the same experiment on a hill. Do the same experiment on a rutted dirt road. Note differences. If you want more useful answers, you'll have to be willing to modify your assumptions. (weight vs. power, etc.)

Hilary - Getting passed on a downhill can happen because of several things: 1) your initial speed at the crest of the hill was slower; 2) your friction (aero-ness, tires, hubs) is higher; 3) your mass is lower, so you have less momentum to counteract forces slowing you down; 4) you're on the brakes more; 5) you're steering a slower line; and 6) your top-end power is lower. I would say that you always want to improve all of the above except for (3). What's the best ROI to get faster on downhills will vary.

gwcrew2000's ravatar gwcrew2000    JOINED: 2/28/09    POSTS: 127
RE: Question: The physics of bike weight
POSTED: 8/23/10 12:43 PM

I think Jason answered it best, the rotational weight of a wheel does make a difference vs. the weight of the bike or your body. Here is a good link about the benefits of lighter wheels http://www.ultracycling.com/equipment/wheels.html

abbyjs's ravatar abbyjs    JOINED: 4/7/09    POSTS: 699
RE: Question: The physics of bike weight
POSTED: 8/24/10 3:07 PM

This discussion is confirming my novice opinion on where to spend money on my next bike: components. I have argued that it's a lot cheaper to lose a pound off myself than it is to lose a pound off my bike... and I think it's true.

But components do seem to make a big difference. Like that time I demo-ed a Trek Madone... and won my AG on the bike. I love my Roubaix, but the Madone had better components, hands down. Better shifters, better derailleurs, better wheels. I'm not saying my engine didn't cook that course, but the machine was killer.

And Hillary: yeah, I'm going to cream you on every downhill. I work hard to keep a good power-to-mass ratio so I can climb well, but at 5'11" and 140, I've got 6 inches and *at least* 35 pounds on you. I can hit 40mph on Persimmon Tree Road if I try. ;)

karlaj's ravatar karlaj    JOINED: 2/28/09    POSTS: 201
RE: Question: The physics of bike weight
POSTED: 8/24/10 8:07 PM

Abby,
The machine may be killer, but like you said, it was the engine. I also like the idea of putting $ to components, but that's b/c I hate my chain dropping when I go to my big ring or miss-shifts when I hit an uphill, not b/c they otherwise speed me up. So, I think I disagree that components make a "big difference".

I'll put it this way -- how would components/wheels make a difference in a race? Did your chain fall off often during other races on your Roubaix (a la Schleck)? Is there usually alot of sprinting and your wheels were too heavy to keep up with the accelerations in the past? That's where components come into play. So the whole bike (Trek) might make you faster (lighter weight, better positioning), but components play a very small role in your bike split.

-Jason

abbyjs's ravatar abbyjs    JOINED: 4/7/09    POSTS: 699
RE: Question: The physics of bike weight
POSTED: 8/24/10 8:30 PM

Funny you should ask... I dropped my chain twice at IronGirl...

karlaj's ravatar karlaj    JOINED: 2/28/09    POSTS: 201
RE: Question: The physics of bike weight
POSTED: 8/24/10 9:26 PM

Yikes. Now I see where you're coming from. I suppose that might slow you down. ;-)

merlinkim's ravatar merlinkim    JOINED: 2/28/09    POSTS: 294
RE: Question: The physics of bike weight
POSTED: 8/24/10 9:37 PM

Kevin - If you don't think you would notice dropping five pounds in body weight when riding, put two 2-liters of soda in a backpack and tote that around on your next bike/run workout. I'd be interested to hear what happens.

Abby -- if you want new components because you're dropping chains or mis-shifting, the answer may be new cables ($50-ish + installation), not new components. Or it could be just an adjustment. (Literally, two turns of a screwdriver could fix it.) New components won't make you more efficient than current equipment, if your current components are well-maintained. The weight difference between Red/Dura-Ace and Rival/105 is not very much, and if you don't keep your chain clean, your cables slick, and gu/gatorade from gunking up your housings, you're just as likely to drop a chain or miss a shift with the high-end as the lower-grade stuff. That said, Red and DA [i]feel[/] nicer, if you can spring the $$ for it.

Dollar-for-dollar, diet, training, and maintenance aside, best bang-for-buck upgrades are clothing, fit session+proper (for you) aerobars, and aerohelmet.

Also, note that the Madone has a more aggressive geometry than the Roubaix, which was expressly designed for comfort over long distances. If you found the Madone to 'feel' faster, you should be able to talk to the shop about duplicating that position on your Roubaix.

With wheels, it all depends on what you're after. I'm adding weight to my rear wheel, in the form of a cover, so I can go faster (in theory) for IMWI. Were I racing Savageman 70 this year, my setup would be completely different (though I'd keep it the same for the 30).

-ihan

vatriathlete    JOINED: 2/28/09    POSTS: 682
RE: Question: The physics of bike weight
POSTED: 8/24/10 9:49 PM

exactly, proper adjustment and maintenance of your bicycle and such will make a world of difference.

the more expensive components are usually made with better materials so "in theory" they should be better than cheaper components, but that assumes that you keep it in top shape and not neglect it.

and as one bike shop salesman said to me, "if you think you'll go faster with a better bike, you probably will".

abbyjs's ravatar abbyjs    JOINED: 4/7/09    POSTS: 699
RE: Question: The physics of bike weight
POSTED: 8/24/10 10:39 PM

Well, I try to at least clean my bike well, including taking a toothbrush to the derailleur about every other week, but adjustments are over my head. I'm a novice.

My chain dropping is probably user error, or as Christina Raia would put it, "irresponsible gearing." That is... trying to get massive mph on a downhill and not knowing the course well enough to get into a low gear for a hill in front of me.

I will also allow for some superstition, that for my unlucky 13th triathlon, it was friggin' Murphy's Law out there. I hope that my karma is all clean for the DiamondMan Half!

merlinkim's ravatar merlinkim    JOINED: 2/28/09    POSTS: 294
RE: Question: The physics of bike weight
POSTED: 8/24/10 11:16 PM

Remember that operator happens to us all, including Schlek in this year's TdF and Chrissie with her CO2 in Kona.

PotomacTriathlete2009's ravatar PotomacTriathlete2009    JOINED: 9/19/09    POSTS: 514
RE: Question: The physics of bike weight
POSTED: 8/25/10 4:06 AM

Your gearing can make a difference on downhills too, can't it? If you have a bigger front ring, you should be able to go a little faster if you're pedaling downhill.

Then again, you would have to take care that your rings and cassette were appropriate for the entire course (flat or hilly). I know there are all of those gear formulas out there but I've never looked into that too deeply.

karlaj's ravatar karlaj    JOINED: 2/28/09    POSTS: 201
RE: Question: The physics of bike weight
POSTED: 8/25/10 8:44 AM

Now we're getting a little off topic into gearing/aerodynamics/efficiency instead of weight, but hey...

Depending on the hill, pedaling downhill can be just fine (slight declines) or nearly useless (steep). You're fighting so much wind resistance at 30+ mph, you'd have to put alot of power to go just a little faster (try it). If the decline is gentle, you're probably not maxing out your cassette going 25 mph no matter your chainrings (unless it's a mtn bike). So IMO, no, gearing doesn't make too much difference at our humble novice level.

-Jason

ps. good discussion going

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