Why does reducing bike weight have such an impact on speeds?

Question

I am trying to understand the physics of bike weight. I have just made a switch from an 11Kg road bike to an 8Kg one (obviously there are many more factors and differences between the 2 bikes other than weight), and have noticed the benefit of reduced weight. However I am not sure I understand it.

Let's say for example My combined weight is 8+75 for a total weight of 83Kg. The difference between this and 86Kg is minimal, but is does make a difference seemingly beyond what I would expect.

Assuming I could equalise the total weight by carrying a pack or particularly heavy bidon/saddle pack) such that the combined weight could be the same, why is the lighter bike always seemingly faster?

I have read something about rotational mass, which seems to indicate that it's nothing to do so much with your combined weight as perceived through gravity, but with the equivalent lifting effort of the moving parts, namely your wheels, and in fact it's wheel weight that is the major factor because your are in effect having to lift half the wheel in order to produce forward momentum. Is this a sort of gyroscopic affect whereby the easier it is to turn your wheel the faster you go?

Answer 1

The two major "drags" on a bike, on level ground, are the rolling resistance of the tires and the wind resistance of the bike and rider.

In general, up to a point, a tire with higher pressure will have lower rolling resistance, as will a tire with a smoother tread. Tire width also figures in, but mostly in terms of how it influences pressure and tread.

Wind resistance is affected by tire width, number of spokes, tube size, and rider position. Generally, as the "raciness" of a bike increases, tire width and spoke count decrease and rider position becomes more prone (and hence more aerodynamic). Tube size doesn't tend to vary much (and may actually increase as you go steel->aluminum->carbon) but that effect is slight and subtle differences in frame design can be more significant.

There may also be ergonomic changes -- eg, the more prone riding position, while more fatiguing, is often somewhat better for peak power output.

Bearing quality hardly figures in at all. Even cheapo bearings have very low losses, and the difference in drag between them and top-of-the-line bearings is not likely to be measurable. (The bigger difference is in durability.)

Bike weight only comes into the equation when one must "climb" (and to a very minor degree for rolling resistance), but of course it's rare to have a route that does not go up/down by 0.5% or so most of the time. That 0.5% increases the power required to hold a steady speed by about 10% over a dead-level course. But the 3KG difference between the above two bikes would amount to about a 0.4% power difference on that 0.5% slope.

Answer 2

Some of the physics are simple. To move an object of mass m up an incline requires an amount of work proportional to the weight. This is the mass times the gravitational acceleration times the displacement W = m g d. So for the heavier bike it requires more work. It also requires more average power to lift it over the same rise in a fixed set of time.

But if in your experiment with the same weights are then this physical effect is not there, it is just perception, or another physical phenomenon. Also this is limited to climbing.

The angular acceleration of your wheels is a factor as you are accelerating. As you apply torque to increase the angular velocity of the wheel, the moment of inertia is a factor. This is basically an accounting of how much mass and how far away it is from the axis. Think of a figure skater spinning with their arms out and then pulling them in to spin faster. Gravity on the wheels is a separate consideration, described above.

Another physical phenomenon at play could be aerodynamic drag. In the simplest models, this is proportional to the cross sectional area, velocity squared, and drag coefficient. Some say a heavier bike with better aerodynamic performance performs more efficiently overall. Given the combined weight with the rider this could be true at sufficient speeds.

Finally there could be a variety of fit factors on the lighter bike that make it feel easier and more efficient to you. It may even be that the fit puts you in a more aerodynamically advantageous position. But you will feel better on a properly fitted bike.

Try the new bike with the old wheels and a loaded bag. If it still feels faster, it must be because it fits you better.

Answer 3

The following is junk science:

Dynamic friction on moving surfaces comes into play - I'd put $ on it that the 8kg has better quality components and therefore bearing than the 11kg bike..... Nothing to do with weight, but in cycling weight and quality is inextricably linked

Mechanical losses in a bike powertrain are tiny - and when you're running fast on the level most of the work is against air resistance, which means you only get the square root of the gain as a change in speed. So the difference between a pretty good bike and an excellent in this area will be something like the sqrt of 1.01...

Tires are another factor - I'm definitely faster on my light carbon fiber road bike than my heavy steel commuter bike, but the road bike has lightweight 23mm tires @ 110psi, compared with much heavier (puncture resistant) 32mm tires at 80psi on the commute bike so I suspect that most of the difference is in the wheels (and a more aerodynamic position on the road bike). – Johnny Oct 21 '13 at 19:51

Semi junk. The commuter bike may have higher RR tires, because a racer will be booted for speed and a commuter for puncture protection and tyre durability - but when tire design is the same, then wider is LOWER rolling resistance!

Stiffness also factors in. The stiffer the BB area, the less power lost to frame flex. How much, I couldn't say. But the reduction of mass for climbing, increased stiffness and reduced friction for increased power to the wheels combined probably add up to something noticeable

Yes, it's something that bike magazines tell you is a reason to buy their advertisers more expensive CF bikes. But lab tests haven't shown it. In fact, flexier frames can actually be faster especially in climbing - because the flex makes the frame act as a spring smoothing power transmission

http://janheine.wordpress.com/2011/02/27/a-journey-of-discovery-part-5-frame-stiffness/

http://bikemagic.com/how-to/mountain-bike-maintenance/stiffness-and-the-sean-kelly-paradox.html

...Super stiff bikes are excellent for elite sprinters, but for everyone else an expensive and uncomfortable waste of money.

The angular acceleration of your wheels is a factor as you are accelerating.

It's a factor, but an incredibly tiny one. Of all the stupid things cyclists believe about performance, the idea that wheel weight has a large effect is probably the stupidest because everyone has seen evidence to the contrary. When you work on your derailers and spin the pedals is there any great resitance to the wheel coming up to speed? No, it's instant. And if you wear a pair of leather gloves you can stop a wheel moving at the rpm for 20mph with a brush of your hands. Both these things are because the energy required to make the wheel spin are tiny. (There's a formula for this - you were supposed to have learned it in highschool!)

The worst case for a wheel is that it requires double the energy of non-rotating mass for kinetic energy. So for 2kg wheels, a 10kg frame and a 50kg rider, that's 4 to 10 to 50. The effects of wheel mass become even smaller when you consider a realistic case with resistive forces.

Answer 4

I am trying to understand the physics of bike weight. I have just made a switch from an 11Kg road bike to an 8Kg one (obviously there are many more factors and differences between the 2 bikes other than weight), and have noticed the benefit of reduced weight. However I am not sure I understand it.

..So you haven't noticed a difference due to weight at all. You have assumed it, because that's what bicycle marketing tells you to do.

My guess is that you experienced a mixture of placebo and rolling reduction resistance - tires of the same width and category can vary by about 50% in RR.

Answer 5

It actually doesn't have a big impact on speed. Keep in mind when you're riding it's not just the weight of the bike you're dealing with. It's also the weight of the rider and whatever you're carrying. When you add that up losing a few pounds is rarely more than a total weight reduction of 5%, usually more like 1-2%.

If you think dropping a pound off a bike makes a huge difference then next time you're out riding toss a full water bottle off your bike and see if you suddenly become significantly faster. You won't. Race results bear this out. Take a look at the average speed of the 2012 Tour de France and the 1982 Tour de France. During that 30 year gap the average bike dropped more than 6 pounds (25-30%!) in weight yet average speeds have gone up by less than 1mph. Also keep in mind bikes have also become more aerodynamic, teams have become more coordinated in their tactics (thanks to radios and live TV feeds in team cars), riders now train with greater precision (no power meters back in '82) and riders come into the Tour better rested due to a shorter racing schedule.

Light bikes feel lighter largely due to a placebo effect. Also if a bike is faster it's hard to say if that's due to it being lighter. Only way to really measure that would be to take two absolutely identical bikes and add weight to one and then do a double blind test with data being captured on a power meter. People have done these studies and the results showed that weight isn't nearly the big deal most of us think it is.

All that said, lighter bikes are more fun. They just are. ;)