Effect of reducing rotational mass

[prState]

I've never really researched bike tires & rims and their weights, though I know on cars if you can get low unsprung weight whether it's on a tire or just lighter pulleys or other parts of the moving mechanism, you get a better tradeoff than just removing sprung mass.

For instance, if I could reduce the rotational mass of my Twist rear tire & wheel by 30 or 40%, I'm wondering if I would get a noticable improvement? (the rear specifically, because that's the drive train part)

Or is it different with bicycles when you often don't even have a suspension?

{Btw, speaking of low mass, anyone notice how fast that front wheel crumpled on the tour de france's rider's wheel hitting that dog? In the slow motion playback, it almost looked like it melted.}

 

[BigBob]

I've never really researched bike tires & rims and their weights, though I know on cars if you can get low unsprung weight whether it's on a tire or just lighter pulleys or other parts of the moving mechanism, you get a better tradeoff than just removing sprung mass.

For instance, if I could reduce the rotational mass of my Twist rear tire & wheel by 30 or 40%, I'm wondering if I would get a noticable improvement? (the rear specifically, because that's the drive train part)

Or is it different with bicycles when you often don't even have a suspension?

{Btw, speaking of low mass, anyone notice how fast that front wheel crumpled on the tour de france's rider's wheel hitting that dog? In the slow motion playback, it almost looked like it melted.}

I'm no expert on bikes but I have an engineering background. I wouldn't like to comment on the effects on the handling on a bike of lowering the rotating mass.

Two other things spring to mind though - and I stand to be proved wrong on both of them, as I say I'm no cycle expert - one is the flywheel effect a heavier wheel will have and the second is the change in gyroscopic effect.


.............. and yes, I did see the way that wheel collapsed - frightening




BB

 

[coops]

The adage that "a pound off the wheels equals two off the frame" appears to be true in that the kinetic energy (which is proportional to mass aswell as square of velocity) of each pound of rotating bike wheel is twice that of each pound of frame material.

The energy required to accelerate a wheel is also proportional to its weight, so on both counts a lighter wheel means less energy required, aswell as presumably lighter handling.

How noticeable the improvement would be I couldn't say though, and by far the biggest improvement most of us can make (me included) is to lose weight ourselves! (no offence intended! )

The Tour de France crumpling tyre was, I believe, designed to do that? Like vehicle crumple zones or F1 safety features I guess, to reduce the deceleration and hence forces involved in any impact to minimise injury chances to the rider? They can just replace wheels if necessary: most of us don't carry spares though .

Stuart.

 

[flecc]

Ditto, I saw that collapse! Those wheels are just strong enough for the job, but no extra for extras!

In practice reducing the rotational mass makes very little difference on bikes. The large diameter of the wheels and relatively low road speeds produce quite small gyroscopic forces compared with other vehicles.

There is a benefit in reducing the mass when running on poor surfaces which convert forward energy into vertical, the higher the mass, the higher the energy wasted in this way, but it's fairly low ranking in normal cycle efficiency terms, but more important when a hub motor is in the wheel. Again though this tends to be hidden in practice to a fair extent, since the heavier motors are usually the most powerful.

I don't think any change possible in your Twist's rear wheel would make a worthwhile difference. What were you thinking of by the way, a light race style thin tyred wheel with derailleur gears in place of the SRAM P5? That would be a bit uncomfortable and there could be problems with having spring loaded chain idlers at both ends.
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[Beeping-Sleauty]

The adage that "a pound off the wheels equals two off the frame" appears to be true in that the kinetic energy (which is proportional to mass aswell as square of velocity) of each pound of rotating bike wheel is twice that of each pound of frame material.

The energy required to accelerate a wheel is also proportional to its weight, so on both counts a lighter wheel means less energy required, aswell as presumably lighter handling.
Stuart.

That makes sense, i replaced my lightweight wheels on a MTB for some solid cast Grimeca alloy wheels,

Grimeca - components

( i used to buckle a set of spoked wheels almost every year riding in London) these are really heavy and made a noticeable difference in terms of effort to get moving, but once the bike was rolling the increase in mass was considerable, both in steering and the 'free-wheel' distance. always thought of it as flywheel effect.

beeps

 

[coops]

Yes, I forget exactly how it works: I meant to say before that the extra "chunk" (technical terms here! ) of kinetic energy (half its total) probably comes from the forces involved in that rotational flywheel/gyroscopic effect BigBob said, but please correct me anyone if its wrong.

Essentially the total k.e. of the wheel has two equal parts: it shares a component with the bike frame due to the bike's velocity, but has an equal-sized second component due to its rotation, and both parts are proportional to its mass.

I think you'd have to add/remove a lot of weight to/from a wheel for it to make any noticeable difference though - beeps's case showing the effect of very heavy wheels: weight distribution makes a difference too I think - the forces increase as the weight is further from the spin axis??

Stuart.

 

[flecc]

Yes, adding a lot of weight to a wheel is much more noticeable than trying to pare the weight of a normal cycle wheel. And of course with some of our hub motors in the front wheels, that added 3 kilos is very noticeable.

But I don't see paring a few grams off a normal cycle rear wheel like that in prState's Twist as making any meaningful difference to a rider.
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[prState]

I don't think any change possible in your Twist's rear wheel would make a worthwhile difference. What were you thinking of by the way, a light race style thin tyred wheel with derailleur gears in place of the SRAM P5? That would be a bit uncomfortable and there could be problems with having spring loaded chain idlers at both ends.
.

I think I've been effectively discouraged by the several posters 'weighing in' (no pun intended, well maybe a little)

 

[coops]

And of course with some of our hub motors in the front wheels, that added 3 kilos is very noticeable.

I was going to mention that, but hesitated .
In a way, I'm surprised I don't notice its weight more, perhaps its because its so visible (i.e. not a "hidden" effect so much, like gyroscopic heavy rims ) and, well, it has to be there (in the wheel, not necessarily the front one) I think I've adjusted to it: the gyroscopic effect is minimized due to its small diameter in a Torq, but would a larger diameter & maybe heavier (e.g. f-series) motor be much more noticeable do you think?

The rear-motor T-bike must handle very differently then flecc?!

Stuart.

 

[flecc]

I wouldn't worry too much prState.

Your's is one of the lightest electric bikes ever produced, and as one of the very few e-bike producers who are actually bicycle manufacturers, and heavily involved in racing, Giant know how to design for efficiency. Your Twist and mine are excellent in this respect.

If you ever get a chance to pedal a few hub motor equipped bikes with the power switched off, you'll certainly see what I mean and be left in no doubt.
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[flecc]

The rear-motor T-bike must handle very differently then flecc?!

Stuart.

I meant more in terms of losses through conversion of forward energy into vertical Stuart. The steering does feel both faster and more precise now, and I've greater confidence in steeper banking on corners, but much of those will be due to the change to narrow profile better tyres.

The real gain from shifting the motor is that I can have a very narrow front tyre at 80 lbs gaining a lot in rolling efficiency while also gaining a small degree of comfort. Of course I could have kept a fat tyre for maximum comfort, but the T bike is specified as an out and out performance bike. (he says, grinning after getting off it a few minutes ago having done a fast dash for a 28 mph loaf of bread!)
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[coops]

(he says, grinning after getting off it a few minutes ago having done a fast dash for a 28 mph loaf of bread!)

Express delivery?...

the T bike is specified as an out and out performance bike.

Yes, I've noticed since you "reformed" & enhanced your Torq into the T bike there's been no more talk of slowing it down! Sorry to take this off topic prState, but I think its remarkable how both the Q & T bikes "bring out" the unfulfilled potential of each bike's strengths: it strikes me that even those who don't follow what's been done, or how it was achieved, can at least see the bikes' potential all the more clearly once they're enhanced in that way.

Stuart.

 

[flecc]

Yes, I'm really enjoying these bikes. Crusty bread is really a Q bike job, but it's been hard at work all morning on trailer loads of recycling stuff and was on charge, so the T was pressed into service with a pannier as I also wanted to run the radical battery a bit lower before charging. On charge, and then clunk, all mains lost as two postcodes lost all power. I thought I'd blown the sub station.

Back on now though.
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