Road Cycling - how important is weight on level roads? - physics question

Everyone talks about the weight of road bikes and their components. Physics tells us that weight is important during climbing and acceleration. The connection between weight and climbing is well understood. However, I’m wondering if weight is also important during apparently constant speed on level ground.

Basic physics tells you that if a bike/rider is traveling at a perfectly constant speed on a flat road, weight should have no effect. Constant speed means zero acceleration and therefore no force is required to maintain that speed. (Let’s leave aerodynamic drag out of this discussion).

However, if you were to measure the speed of a bike even along a flat road, it probably fluctuates. I’m basing this on studies I’ve read that measured forces during a pedal strokes. An article I read said Lance Armstrong’s stroke was perfect – he exerts a constant force all the way around the pedals. Translating that into driving force, that means his bike is going at a constant speed. For all the rest of us, with our less-than-perfect strokes, our speed is fluctuating. Therefore, we experiencing accelerations and decceleations. We are creating an acceleration with each downward stroke. The resulting speed change may be slight but the cumulative effect of lurching the weight of bike/rider forward with each stroke could be significant

Does anyone know if the effect of speed fluctuations on a level road has been studied and quantified?

Here is my take-

Along with force, you also have momentum. It seems to me that on a perfectly level surface, if you have more weight, you carry more momentum, thus, your fluctuation in speed would be slower, and less noticable than someone with less weight. However, the person with less weight has less to accelerate, so they dont feel the fluctuation as much.

In other words, I would say that it is pretty much the same-
More weight == less fluctuation, but more force
Less weight == more fluctuation, but less force (to accelerate back to speed)

E

Basic physics tells you that if a bike/rider is traveling at a perfectly constant speed on a flat road, weight should have no effect. Constant speed means zero acceleration and therefore no force is required to maintain that speed. (Let’s leave aerodynamic drag out of this discussion).
constant speed in a straight line or around a curve :o! :D

If I remember correctly there is a direct relationship with weight and friction. If you take drag out of the picture frictions is what is going to slow you down. I think the contact area of the tires also comes into play.

If I remember correctly there is a direct relationship with weight and friction. If you take drag out of the picture frictions is what is going to slow you down. I think the contact area of the tires also comes into play.

yes, if you take friction out of the picture the weights won't matter. the force pushing up on the bike (from the road) and the force pushing down on the road (from the bike) is the same. the weight won't matter - it's always going to cancel out. in real life though, there will be more friction for a heavier bike because the tires will flatten out to accomodate the extra weight.

aero is the best way to go. but all those bikes are light anyway. and wear tight clothes.

Don't forget, us fat people are acting as bigger sails against the air.... think Ullrich, times two, in my case. ;)







But, being fatter means I'm harder to kidnap, so it all balances out.

yes, if you take friction out of the picture the weights won't matter. the force pushing up on the bike (from the road) and the force pushing down on the road (from the bike) is the same. the weight won't matter - it's always going to cancel out. in real life though, there will be more friction for a heavier bike because the tires will flatten out to accomodate the extra weight.

but the weight will increase the footprint of the tire ( i.e: 200# pushing on 100psi tires = 2 sq inches of tire to road surface area.......100# pushing at 100psi tires = 1 sq inch of tire to road surface area), causing more available surface area for friction.

Granted, it's really not much, maybe the larger person loosing a 1/4 mph

My track bike is heavier than my road bike!

My track bike is heavier than my road bike!

same here...but my fixie is a beast...I've got these steel araya 36H rims that are absolutely bulletproof...plus 400 gram 27x1_1/4 tires w/mr tuffys...its my urban assult vehicle. It rides nice, though, but i think that also might be a function of the ultra long wheelbase.

as to answer the original question, it doesn't matter a whole lot. however, there are not very many areas where it is perfectly level. so in an ideal circumstance, weight doesn't matter.

On a flat surface, you would want your extra weight in the rims and tires. This increased angular momentum would tend to keep the wheels spinning at the same speed with minimal application of pedal force. Perhaps an old set of Rolf vector comps (or other heavy aero wheel) with the air pressure set up around 110 or 120.

Basic physics tells you that if a bike/rider is traveling at a perfectly constant speed on a flat road, weight should have no effect. Constant speed means zero acceleration and therefore no force is required to maintain that speed. (Let’s leave aerodynamic drag out of this discussion).

Basic physics tells me that the force of friction is equal to the coefficient of friction times the weight of the body in motion. It also tells me that zero acceleration means forces are at equilibrium (again, we're neglecting air resistance). That does not mean that no force is required to maintain a constant speed, it means that the rider is putting out a force equal to that of the opposing friction force.

Everyone talks about the weight of road bikes and their components. Physics tells us that weight is important during climbing and acceleration. The connection between weight and climbing is well understood. However, I’m wondering if weight is also important during apparently constant speed on level ground.

Basic physics tells you that if a bike/rider is traveling at a perfectly constant speed on a flat road, weight should have no effect. Constant speed means zero acceleration and therefore no force is required to maintain that speed. (Let’s leave aerodynamic drag out of this discussion).

However, if you were to measure the speed of a bike even along a flat road, it probably fluctuates. I’m basing this on studies I’ve read that measured forces during a pedal strokes. An article I read said Lance Armstrong’s stroke was perfect – he exerts a constant force all the way around the pedals. Translating that into driving force, that means his bike is going at a constant speed. For all the rest of us, with our less-than-perfect strokes, our speed is fluctuating. Therefore, we experiencing accelerations and decceleations. We are creating an acceleration with each downward stroke. The resulting speed change may be slight but the cumulative effect of lurching the weight of bike/rider forward with each stroke could be significant

Does anyone know if the effect of speed fluctuations on a level road has been studied and quantified?

Since no road is perfectly flat and you can never leave aerodynamic drag out of any discussion like this unless you plan on cycling on the moon, it sounds like what you're actually asking is how important a proper spin is to cycling?

A heavier bike will beat the lighter bike in a downhill race.

A heavier bike will beat the lighter bike in a downhill race.

In going downhill a heavier object has no advantage of a lighter one, as far as acceletaion. In fact a lighter bike will change direction quicker and tend less to plough straight forwards less. The more momentum the longer it takes to stop.

I was thinking a wheel could be designed with small weights on springs mounted in the centre when the bike is at rest. When it speeds up the weights move outwards increasing the intertia of the wheel making it slow down less on a level road. So acceleration isn't comprimised at slow speeds but at speed it keeps it better. I have no idea if this would work or how it would effect cornering however :) Just an idea that popped into my head while riding.

Check it out yourself.

http://www.kreuzotter.de/english/espeed.htm

Note that if you switch tires, you'll also affect frontal area which has a big impact. When I went from 38 mm tires and 17mm (inside width) rims to lighter 25 mm tires and 15 mm rims, I not only saved about 4 lbs, but reduced my frontal area by about 1/2 sq. foot. The result was that my cruising speed increased by about 2 mph (from roughly 15 mph to 17-18 mph and as predicted by the program), but my top speed and speed into the wind went up more than 4 mph. All on level ground. Nothing beats cutting the weight (and frontal area) of your wheels/tires. The improvement in acceleration is not only dramatic, but makes riding "on the level" a lot more fun.

Al

rolling friction:

rolling friction = coefficient of friction(of material) times wieght

thus

Torque = radius x force

where force => rolling friction....

did i make sense?

An article I read said Lance Armstrong’s stroke was perfect – he exerts a constant force all the way around the pedals. Translating that into driving force, that means his bike is going at a constant speed.

May I know where you got the article from? I would love to read it. It's weird that Lance Armstrong's stroke exerts a constant force all the way around the pedals. The quads should exert more force on the downstroke than the hamstring and calves will on the upstroke. Lance Armstrong might spin in perfect circles, but is it really possible that he exerts a constant force through the pedalling mechanics?

Does anyone just get on the bike and ride without doing math problems?

Does anyone just get on the bike and ride without doing math problems?

It's fun, fun, fun. :D

I'm pretty sure Francesco Moser experimented with weighted wheels leading up to his hour record attempt.
Anyone know what came of it?

Well, I fooled around a bit with loaded touring. I found that a fully loaded touring bike was pretty dern heavy. The bike weighed about 7 lbs more then a racing bike and then the panniers and gear added another, oh 30-40 lbs. I learned really quickly that if I was going to stop, I better gear down because it was hard to get that mass moving again. The extra weight meant that acceleration took more effort. However, once I got up to speed, maintaining the speed was not a problem. But in effective terms, one seldom gets up to very high speeds with that much weight because you don't really want to work that hard and then lose it all on the first stop sign. So average speeds were often like 14 mph compared to 20 mph on a road bike. There were a few times when I got up to decent speed and it really was not that much of a deal to maintain the speed assuming you got up to the speed to begin with.

Now if you are talking about large riders vs small riders, that is another matter. If both riders are in superb condition, the large rider has the advantage on flat terrain. The large rider and the small rider have pretty similar wind resistance and the large rider tends to have a substantially greater muscle mass which translates to more speed. This is why large riders tend to do well in time trials but small riders do well in climbing. And in descents, the large rider does not even have to be in good shape to beat the small rider down the hill (assuming no drafting).

Assuming a perfectly level road, I would rather be on a heavier bike. It'll take longer to accelerate but once up to speed, I can coast longer if I need a break. If a gust of wind hits you, you still have all that momentum so you don't decelerate too quickly. But things don't always work in a perfect world.

Assuming a perfectly level road, I would rather be on a heavier bike. It'll take longer to accelerate but once up to speed, I can coast longer if I need a break. If a gust of wind hits you, you still have all that momentum so you don't decelerate too quickly. But things don't always work in a perfect world.

agreed. I took the tandem out with gang about a year ago. Once we got up to speed it was easy to maintain the speed and because we had better aerodynamics and could coordinate our power, we could maintain a higher speed on the flats too. On short climbs, we would speed up and use the momentum to power over the hill...we would be climbing at 25 mph! :) After a stop light, the guys would constantly be looking back to see when we were going to go by them and they would jump on our wheel...or be left behind.

agreed. I took the tandem out with gang about a year ago. Once we got up to speed it was easy to maintain the speed and because we had better aerodynamics and could coordinate our power, we could maintain a higher speed on the flats too. On short climbs, we would speed up and use the momentum to power over the hill...we would be climbing at 25 mph! :) After a stop light, the guys would constantly be looking back to see when we were going to go by them and they would jump on our wheel...or be left behind.

Hehe, that's great. On a tandem, I understand you have the weight of the bike and both riders to add on to momentum but what about pedaling? Can you have one person in 1 gear and another person in another gear if they can't go as fast so that both people can contribute power to the bike?

Does anyone just get on the bike and ride without doing math problems?

None who participate in a thread wth the title of this one I would suppose. Besides, if you use a program, you don't have to do math problems. It's the modern way.

Al

None who participate in a thread wth the title of this one I would suppose. Besides, if you use a program, you don't have to do math problems. It's the modern way.

Al

So i dont need to do math problems before each ride?

So i dont need to do math problems before each ride?

anytime is a good time for math problems. if you can't do it before the ride, don't worry...there is plenty of time during and after.

My head hurts. LOL