Facebook discussion of light vs heavy rims

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Velocomp CEO
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Facebook discussion of light vs heavy rims

Post by Velocomp »

We've had a lively discussion on Facebook regarding light vs heavy rims. It has become too cumbersome to handle within the confines of Facebook, but we thought it would be of interest to forum members, so we've created a discussion here.

A post appeared on Facebook. The post stipulated the following conditions:

1) Bike A has a total weight of frame and wheels of x (let's say, 15 pounds).

2) Bike B also has the same total weight of frame and wheels of 15 pounds, but the wheel set of bike B has lighter rims, say ½ pound lighter.

Both bikes are being pedaled up a hill. The post claimed that the lighter wheel is easier to pedal up hill. How does the iBike account for that?

The answer is: lighter rims can be beneficial whenever the bike's speed is changing (i.e. the bike is accelerating). However, a constant bike speed, whether on the hills or on the flats, bike A and bike B perform identically.

Power is defined as Force x Speed. From Newton's law, F = m*a, where m is the total mass of bike, rider, wheels, etc, and a is the total acceleration of the bike (includes bike changing its speed, and gravitational acceleration on hills).

The instantaneous power required to linearly accelerate a mass m is p = m*a*v, where "a" is total acceleration (this would be both bike acceleration and acceleration due to hill slope), m is the total mass of the bike (including the wheels), and v is the speed of the bike in the direction of travel.

However, on bikes there is also rotational acceleration of the wheels that enters into things: it also takes power to change the speed (accelerate) the bike's wheels. How much? It depends on starting bike (linear) speed, bike acceleration rate, rim weight, rim size, and more.

Suffice it to say, it's complicated. But whenever the bike is not accelerating, rotational acceleration is zero, so rim weight makes no difference.

In situations where the rider is accelerating rapidly (say, in a crit), then slamming on the brakes to slow down to avoid crashing into the rider ahead, there's no doubt but that lighter rims are better. But on hills? The answer is not obvious...In fact, in an article written by Lennard Zinn in VeloNews, he has this to say about heavy rims on hills:

"The bike always has to accelerate at least once to get up to speed, and that will take more energy to do if the added mass is at the rim than if it has instead been added to the frame. One question is whether the extra energy required for this initial acceleration is trivial and can be ignored or not. After that, even if the rider speeds up and slows down the same way on each bike without using the brakes, it will not matter where the extra weight is located, at least in the “ideal, frictionless universe” used in elementary physics calculations of motion. If the rider stops pedaling, even on a climb, he will be carried further up the hill by the flywheel effect of the heavier rims than he will be on the bike with weight added to the frame. Then when he starts pedaling again, he will end up at the same point in the same amount of time on either bike. This is the principle that Ondrej Sosenka depended upon when he set the hour record with heavy rims; he reasoned that the heavy rims would carry him along and keep the speed more constant as he went through periods of weakness and strength. It seemed to work for him; I’m not going to argue with that result." (Italics added)

Read more at http://velonews.competitor.com/2012/06/ ... UVRB5U8.99
John Hamann
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Re: Facebook discussion of light vs heavy rims

Post by ronpei »

I think the key thought was that of inertia. That is why they put so much mass in a fly wheel, it will want to keep rolling, so once a heavy wheel gets up to speed it will want to stay there more so than a light wheel but conversely the lighter wheel will be more willing to change speed so it is an short time force in the case of acceleration so really unless you are really interested in maximal power it is not that important over a full ride. So I would assume if the only difference is the rim weight then the heavier rider will maybe not be able to accelerate as fast as a light rimmed rider (this depends totally on power to weight ratio of the rider but it will still be harder to accelerate the heavier heels) but on the other hand when they go down hill the heavier rim will impact differently on the braking (it ill take more force to reduce the speed) and as well will have a different gyroscopic force and so would not want to lean or change its line as easily. I would consider that more important to overall riding than the effect on acceleration. It has been a long time since looking at this area of physics but Travis should have these principles front of mind, I would be interested in his opinion, or maybe a good physics prof? As for does an iBike take this inertial mass into account for acceleration and deceleration? I think maybe it does implicitly if you do a coast down calibration?
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Re: Facebook discussion of light vs heavy rims

Post by Russ »

Heck, I will throw in my 2 cents worth :-)

My twist be after a couple of points will be trainer oriented but first. I have read that the sprinter and the person, in a race, wanting to bridge up or to get away might enjoy the most gain from having light rims as the rapid accelerations are most impeded by heavier rotational mass than total mass.

On to the trainer.... with the simple function of power from speed used in iBike and most other non direct force power meter approaches for trainer power use.... Accelerations are not measured at all. So you can burn all the rubber you want and not get any credit for except as you actually increase the speed.

I see no reason why an enterprising physicist/programmer might not add coast downs on a trainer and measure accelerations as an added function to the speed to power conversion. I think some of the 'virtual' trainer power applications are tinkering with this approach.

Meanwhile on the trainer, I don't 'waste' my watts attempting a rapid acceleration but make sure I apply steady enough power to not hear the squeal of 'burning' rubber. In fact, on the trainer I keep my old cheaper road bike with heavier everything, including peddles to increase the coasting since my Road Machine trainer is not the pro with heavier fly wheel option. I can still quickly get my power high enough above CP (see other posts) to use up all of my W prime power reserve, should I have the gumption. I sometimes contemplate adding fishing weights to my spokes :-)

On that point I would love to see iBike Newton add CP into the firmware with a remaining W prime display choice. See GoldenCheetah and or PhysFarm.com for more details on that.

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Re: Facebook discussion of light vs heavy rims

Post by EHB »

Will it not depend on the riding style, including cadence?

It is no secret that secret that different amounts of rotational force are applied to the pedals at different parts of the pedal stroke. Powerstroke shows this beautifully.

As part of the energy/momentum of the cyclist is carried by the wheels would not a heavier wheel serve to reduce fluctuations in speed? I would guess that an untrained cyclist with slow cadence, "stomping" style & relatively untrained hamstrings/tibialis anterior would find the heavier wheel to be of lesser physiological cost, as there would be less accelerating and decelerating. Or indeed someone with flat pedals.

As it happens, I was just thinking about this when reading about the Watts needed for different tyres in a post above. As well as not calculating wind resistance, which they could presumably have tried to calculate by spinning the wheel in the air, they used an SL wheel! Why? It's nuts. At least try to minimise wind resistance by using a disc wheel.
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Re: Facebook discussion of light vs heavy rims

Post by eight10man »

This can't be true!

F = m * a.

When riding up hill there is a component of gravity "sin(slope%)" which constantly tries to decelerate the mass.

So a greater mass trying to maintain the same speed going uphill requires a constantly greater force to avoid deceleration due to the gravity component.

So a lighter mass will always requires less power to climb a hill at the same speeds!

Reductio ad absurdum: Nobody would ever argue that cycling uphill using big massive truck tyres on your bike would be just as easy as having light carbon wheels, right?!
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