General Cycling Discussion - Tyres, rotational weight etc.

A few related questions:

I've recently installed a pair of nice and fast rolling, but rather heavy tyres. Due to their lower rolling resistance I'm probably a little faster on tarmac than I was, but I was thinking of swapping the front tyre for a lighter version (kevlar band rather than rubber belt). This would save about 350 grammes and still get some puncture protection.

Will losing the weight on just the front still make an appreciable distance (hey physics isn't my strong point ;) )?

Are there any studies on weight of tyres vs speed?

Presumably rotational weight's biggest negative influence is when climbing (not my forte) and initial acceleration, whilst on the flat under power it's less obvious and downhill it's an advantage?

Richard

Are you kid'n?350g,3 quarters of a pound less or rotational weight?Heck yes you will see a difference.

How can a tyre have a rotating mass when it is the same weight all the way around. Wouldn't the weight of the tyre be the same whether it is rotating or stationary? I can understand that wheels and tubes have rotating weight as they have heavier sections in parts and the weight is not balanced equally in all directions. However a tyre should be the same weight all the way around and wouldn't this then negate itself?

I found the following article on the net.

How much does wheel rotating weight matter?
It is a common misconception that the rotation of wheels makes their weight much more significant than non-rotating weight. The truth is that this effect only applies to wheel acceleration and even so is such a small effect as to be nearly nonexistent. Any steady-paced ride, whether it be on flat ground or up hill, does not involve acceleration and so in this case there is zero rotating weight effect. In the case of wheel acceleration, it has been shown that the weight of wheels is so small compared to the weight of the rest of the bike and the rider that the rotational effect is almost insignificant. Note that all the best professional and Olympic track sprinters today race with heaver carbon tri-, quad-, and penta-spoke wheels instead of racing with lighter weight spoked wheels.

http://www.diablocyclists.com

CHEERS.

Mark

So I might just be saving 350 grammes total weight which probably won't be that noticeable...

Originally posted by Dutchy
It is a common misconception that the rotation of wheels makes their weight much more significant than non-rotating weight. The truth is that this effect only applies to wheel acceleration and even so is such a small effect as to be nearly nonexistent.
I think you have just contradicted yourself!

I find it very noticable. You're almost always accelerating/decelerating, in varying degrees, when cycling. Likewise, when riding at a more or less constant speed, you notice the loss of the flywheel effect with lighter wheels or tires.

Any weight that you move,crank,chain,cassette,tires,rims,hubs,ect will be much more noticable then a lighter seat,bars,stem,whatever.Just thinking about that makes sence.If you can take ,lets say a half pound of rotating weight.Now go the other way,add a quarter pound weight to each ankle and ride 20 miles.Do you think you will notice that more then a half pound weight in your saddle bag?

Originally posted by shokhead
Any weight that you move,crank,chain,cassette,tires,rims,hubs,ect will be much more noticable then a lighter seat,bars,stem,whatever.Just thinking about that makes sence.If you can take ,lets say a half pound of rotating weight.Now go the other way,add a quarter pound weight to each ankle and ride 20 miles.Do you think you will notice that more then a half pound weight in your saddle bag?

Yes... but if I push a barrel of beer along the road (which seems a reasonable analogy to a front tyre) it's easier than carrying it. Is it just my poor physics?

I ride my bike,not carry it.

Originally posted by shokhead
I ride my bike,not carry it.

Good point ;)

I was given a pair of Bontrager tires for my birthday. They are less than half the weight of the Armadillos that I had. I dont feel any difference in acceleration, although my brain tells me it must be better. I do feel that the bike has more nimble handling. The box the tires came in said that their material was formulated for low rolling resistance, and I can cruise 3 or 4 km/hr faster.

OK fellow physicists. I am doing this from memory as I can't seem to locate my Halliday and Resnick, the standard introductory two-semester college classical physics book (we are in the middle of a move). So this is based on a classical physics course more than 30 years ago.

When you spin a weight above your head on a string and then let it go it flys off in a straight line. It does not keep spinning in a circle. This is centripedal (centrifigal?) force. To keep spinning in a circle you must apply something called angular momentum or force (remember, this is memory) forcing the weight to go in a circle. This is supplied through your muscles and applied through the string as you spin the weight in question. This force applies a constant acceleration. This acceleration can be more or less, depending on whether you are speeding up the rotation or slowing it down. At steady-state (constant speed) it is a constant acceleration.

So, using F=Ma (force equals mass times acceleration) you can see that as long as something is rotating (your wheels with tires and tubes, etc.) there must be a force applied. And the larger the mass or the higher the acceleration the greater the force (supplied by you, the rider) required. The end result is that the lighter the tire/wheel combination the less energy you have to supply to make the tires rotate.

This includes the crankset and other rotating parts as well. However, as the length of the crankarm is considerably shorter than the spoke length, and this length is part of the complete equations for these calculations (now, where is my H&R tome?) it is less important.

The short answer is that rotational weight makes a big difference. Believe I read a good article on same some years back that stated it is a factor of six (6) times more important to remove an ounce of rotating weight from ones tires/wheels as compared to removing an ounce from the non-rotating weight on your bike. Its all just a big energy balance. And its your energy.

There are two motions of a wheel: the forward motion of the whole bike plus the wheel, and the rotation relative to the bike.

Most of the weight of today's wheels is in the rim/tire/tube.

What all of this means is that the weight of a wheel counts for about twice its actual weight in non-wheel components in energy costs for acceleration. (See, for example, _Bicycling Science_ by Whitt and Wilson, p. 128).

So, you're lightening your bike with your new tires by the equivalent of about 700 grams. If you and the bike weigh 70 kilograms, that's a 1% reduction.

You will certainly not detect this change in weight in your perceived efforts to accelerate the bike. By comparison, a hub generator robs a rider of more than 3% of his power under typical conditions, and users of these generators widely report feeling no increase in apparent effort.

Even with a stopwatch, you'd be hard pressed to detect the difference in weight between new tires and old. Even a slight change in wind, or a slight change in your body position, or a change in your actual effort would mask the effects of such a small weight reduction.

You say small but over a long ride its not.Like i said,put a quarter pound weight on each ankle,rotational weight and do a 25 mile ride.Put a half pound weight in your saddle bag and do a 25 miler.Which do you think you'll notice?Half pound,isnt that just ,like 170g or so?

I never took physics, but someone here mentioned momentum. Once you get going, on the level or in descents, greater weight tends to keep you going. With less weight, you lose momentum faster & thus have to expend more energy.

So it seems that only in climbing would any weight reduction save energy. But weight reduction could have other benefits as noted, such as as improved handling.

Ebro, momentum is just the amount of inertia (armchair physicists, give me some latitude... I'm not even going to look for my college texts), i.e. resistance to change in state of motion. You don't get something for nothing. For the heavier wheels, you supplied the additional energy getting them up to speed, and you throw more away slowing down. So, yes, you might expend more energy while in motion, but not *more* energy than with heavier wheels.

The bigger factor, when wheels are in motion on a bike travelling, say, 20mph, is wind resistance, and particularly the resistance caused by the spokes. (They're fighting a "headwind", basically, for the entire revolution. (Paramount, did you find your physics book yet?)

We are packing to move next week, so I don't expect it to surface until the end of October.

But, being a physicist, I will stand behind what I said in my previous post. When you make something rotate you must constantly supply energy (force) as there is a constant acceleration required. With F=Ma, the bigger M is the bigger F (force) must be. The 'a' (acceleration) will go up or down, depending on whether you are speeding up or slowing down, but it is always a positive value. And that 'F' (force) comes from your energy. It is probably faster to check Merriwether's citation, above, then to wait for me to unpack my boxes of books. Whoever wrote that book must have quoted the science behind it.

just further on the ankle weights example. The weights on each ankle would cancel each other out, as the one on the down stroke would balance the one on the up. The only way I can see it making a difference is if you spin fast, then the force pushing them outwards would pull on your ankles, and you would feel that. The same for the wheels with the weight on the string example. The force holding the weight in (like the string) would pull through the spokes to the other side of the wheel, which would be pulling the other way and cancelling out the force.

the only effect of lighter wheels I can see is the 'flywheel' effect. Feel free to prove me wrong, I need to justify lighter wheels :D

FWIW my 14kg FS feels easier to throw around than my 12kg HT or my 10 kg roadbike. Go figure.

I think i recall reading that at least 1/3 of the cyclist's energy is used just to overcome wind resistance. Are there any bike fairings being made these days?

Ankle weights would not just go up and down. They would be rotating and would be the same as the weight going round with your wheels/tires. Only difference is the radial distance is considerably shorter and I don't recall right now exactly how this figures into the formula (linear or by the square, etc.). But it is basically the same. Acceleration must be there to make the mass rotate. And it is applied through through the force of your legs (your energy). If you example were to be accurate than there would be no energy required to make a wheel rotate, either. As the one side (front) of the wheel is going down while the back side is going up, cancelling each other out. Unfortunately, as your legs will atest after a strenuous ride, this is not so.

But, wind resistance is a big deal. And I do recall that it is not linear. Believe it goes as the square of the speed. Twice as much energy (force) to overcome at 20 mph as at 10 mph, etc. Should have my phyics tome unpacked in a few weeks to provide some details. Or if I get a chance to get to the library.

Sdksl- the ankle weights cancel ought only as long as you turn the cranks at constant speed.
You have to add extra energy to accelerate the ankle weights to a rotational speed.

Kinetic Energy =.5 X mass X velocity X velocity.

The rotational velocity here is the arcate path the pedals take.
Velocity = 2 X Pi X (crank arm length in meters) X 60 /(cadence per minute)
To calculate amount of energy changed, calculate the energy at speed 1 vs. speed 2.

That is the extra rotational energy you need to add to change your pedal speed.

Substitute the rim radius for the crank raduis and the tire mass for the ankle weight if determining rotational energy on the tire.


Roadbuzz – momentum is mass times velocity.

The bigger factor is the wind resistance of the rider. The wind resistance of the spokes is however one of the easier resistive components to change.



ParamountScapin:

Not only is the radius smaller on the cranks than the rim, the rpm’s are slower (assuming the chainring is bigger than the sprocket)- making the pedal even less significant than the wheels.

And the effect is based upon the square of the velocity- as seen above.

Ebro38- much higher than 1/3 of the energy is in wind resistance unless your traveling around 5-10 mph.

At about 20 mph, 70-80% of the energy is used to overcome wind resistance.
The rolling resistance goes up linearly with velocity, the wind resistance goes up with the square of velocity.

Fairings are being made and used for recumbents.




That 350g saving on a 26” tire, means you saved in a single 0-20 mph acceleration: 3.6 Joules (.00467 hp-secs) in rotational energy. As Merriweather pointed out, there would be an equal amount of energy saved due the linear velocity of that tire down the road in accels.

The 350g you saved should have a bigger effect on your rolling resistance.


The rotational energy is much more significant in motorspors due the high rpm’s found since the energy goes up with the velocity squared.

Motorsports uses a engin,we use our legs.Of course wind in bad but the thread asked about tyres rotational weight.You guys use heavy ass rims and tires and cranks,i wont.I cant belive people dont think rotational weight doesnt make a difference.

Those that don't believe that rotational weight makes a difference obviously do not believe in the laws of physics.

Lighter is better.

Heavier wheels take longer to accelerate and longer to slow down.

How come the gyroscopic effects of rotation havent been mentioned?