Rotational mass

johngti

Just spotted this referred to in road beginners and thought I'd share something that occurred to me the other day.

Minimise your rotational mass (ie wheel rim weight) and you'll go better uphill etc. Light = GOOD!!!!

Argument against - that's fine but its only of use when you're accelerating so don't get caught up in the hype.

All good and all fine but the thing that suddenly popped into my head was this. Unless you're going downhill, you're having to put some effort into maintaining speed, even on the flats. If you weren't, you'd be slowing down to a stop. In order to overcome the force due to friction and air resistance, you have to produce a force in the opposite direction to the resistive forces. In order to produce this force, you have to accelerate the system (of which you are the driving force part). So even if you're maintaining a constant speed on a flat road, there is an acceleration in the system that will be easier to produce if your wheel rims are as light and aerodynamic as possible.

Just an example of the daft stuff that I think about while commuting.

drlodge

Some truth there but I wouldn't overplay it. All the time while riding were are accelerating/decelerating on each pedal stroke, its more pronounced when going slowly uphill. The lighter you are, the easier it is to accelerate and losing rotational mass (especially at the rim/tyre) is more beneficial than losing static mass.

Even if your wheels weighed nothing, you'd still have the mass of the rest of your bike and your body :-)

imposter2.0

A light thing accelerates quicker and also decelerates quicker. A heavy thing accelerates slower and also decelerates slower, so the net effect is about the same, I believe.

I'm not a physicist though...

briantrumpet

Vague recollection of A-level physics is it's all about inertia (using the word in the technical sense, not the broader sense in general use).

NB. I got an E, in 1981, so my initial understanding, or my memory, might be faulty. So I too probably ought to add "I'm not a physicist though".

drlodge

Imposter wrote:

A light thing accelerates quicker and also decelerates quicker. A heavy thing accelerates slower and also decelerates slower, so the net effect is about the same, I believe.

I'm not a physicist though...

For a given force yes. But acceleration under gravity is constant...9.8 m/s/s (A level Physics grade A 1985)

briantrumpet

drlodge wrote:

(A level Physics grade A 1985)

Swot.

darkhairedlord

ugo.santalucia

These threads always fill me with joy. Plenty of folks defending the heavyweight and the inertia when they'd be horrified to ride anything that weighs over 8 Kg.

If you really believe the Halfords 11 kg racer is just as good as the Venge, then you should buy it :-)

drlodge

briantrumpet wrote:

drlodge wrote:

(A level Physics grade A 1985)

Swot.

Ah. Mistake. O-level was grade A, A level was Grade C together with 2 other grade Es and an F :-(

drlodge

ugo.santalucia wrote:

These threads always fill me with joy. Plenty of folks defending the heavyweight and the inertia when they'd be horrified to ride anything that weighs over 8 Kg.

If you really believe the Halfords 11 kg racer is just as good as the Venge, then you should buy it :-)

What's the saying - train heavy, race light. That Halfords 11kg racer is the perfect training machine. Which reminds me, I must get out on my 12.5 kg CX bike for a good work out soon...

johngti

drlodge wrote:

Even if your wheels weighed nothing, you'd still have the CONSIDERABLE mass of the rest of your bike and your body :-)

FTFY!

Of course, the obvious argument is that (some of us?) we like spending money on nice, shiny, light things!

johngti

briantrumpet wrote:

Vague recollection of A-level physics is it's all about inertia (using the word in the technical sense, not the broader sense in general use).

NB. I got an E, in 1981, so my initial understanding, or my memory, might be faulty. So I too probably ought to add "I'm not a physicist though".

Inertia and work done against the forces dragging you backwards. In which case, getting really really nice hubs would also be a good idea due to friction reducing design?

Just thought it was interesting (degree in maths with a ton of theoretical physics!)

briantrumpet

drlodge wrote:

briantrumpet wrote:

drlodge wrote:

(A level Physics grade A 1985)

Swot.

Ah. Mistake. O-level was grade A, A level was Grade C together with 2 other grade Es and an F :-(

My claim to fame was an A at O-level maths, and an O at A-level.

drlodge

The friction in hubs is negligible. The two main things holding you back are air resistance (at speed) and overall mass working against gravity (going uphill).

drlodge

briantrumpet wrote:

drlodge wrote:

briantrumpet wrote:

drlodge wrote:

(A level Physics grade A 1985)

Swot.

Ah. Mistake. O-level was grade A, A level was Grade C together with 2 other grade Es and an F :-(

My claim to fame was an A at O-level maths, and an O at A-level.

I'll see your O at A level, and raise you a GCSE grade 5 in French.

briantrumpet

Maybe I'll just gen up on this light reading... http://www.feynmanlectures.caltech.edu/I_19.html

bendertherobot

I have an A in music at A level. Just saying.

briantrumpet

drlodge wrote:

I'll see your O at A level, and raise you a GCSE grade 5 in French.

The annoying thing was that I actually did a fair amount of revision for maths A-level. Music - literally not one minute, and I got an A. It's probably just as well I took the examiners' hint and didn't try to become a particle physicist, or an engineer.

Even so, Richard Feynman is a bit of a hero. I was trying to find out the stuff he came up with about very large objects spinning fast (e.g. planets) and their weight (or mass, I can't remember), but it eludes me. Thoroughly irrelevant to a bike wheel, but interesting, nonetheless.

johngti

briantrumpet wrote:

Maybe I'll just gen up on this light reading... http://www.feynmanlectures.caltech.edu/I_19.html

Feynman lectures make awesome reading! Perfect for a quiet weekend at home

briantrumpet

bendertherobot wrote:

I have an A in music at A level. Just saying.

Ditto. The slightly annoying thing was that in my youthful arrogance I deliberately did no revision for it, but spent hours on the maths, which I failed. I guess that rather suggested where my strengths lie...

drlodge

Music, interesting. I did O level music a year early and got a B. Then spent 2 years doing A level Music and got an F despite having passed 2 of the 4 exams due to having grade 6 guitar and grade 8 piano. Missed out on Uni, went to a Poly instead!

briantrumpet

johngti wrote:

briantrumpet wrote:

Maybe I'll just gen up on this light reading... http://www.feynmanlectures.caltech.edu/I_19.html

Feynman lectures make awesome reading! Perfect for a quiet weekend at home

I'm not one to use the word 'genius' very often, but Feynman was undoubtedly one. Even for me, a failed physicist, his stuff is inspiring, especially "Surely You're Joking, Mr Feynman", and the biography Genius: Richard Feynman and Modern Physics.

imatfaal

johngti wrote:

...

All good and all fine but the thing that suddenly popped into my head was this. Unless you're going downhill, you're having to put some effort into maintaining speed, even on the flats. If you weren't, you'd be slowing down to a stop. In order to overcome the force due to friction and air resistance, you have to produce a force in the opposite direction to the resistive forces. In order to produce this force, you have to accelerate the system (of which you are the driving force part). So even if you're maintaining a constant speed on a flat road, there is an acceleration in the system that will be easier to produce if your wheel rims are as light and aerodynamic as possible.

Just an example of the daft stuff that I think about while commuting.

At a constant speed there is no acceleration (linear nor rotational) and no net force - you are merely balancing forces (you vs rolling friction and air resistance). If there is a net force (ie an imbalance) then you either get faster or slower and it is with the net force that we do the calcs with force equalling mass times acceleration. With a heavy setup you will get faster at a slower rate for any given force and get slower at a slower rate if you freewheel.

On the rotational mass thing - it would be better to have the mass nearer the hub and a lighter rim. Rotational inertia and the torque required to overcome it is determined by the moment of inertia which related to the mass and the distance to the axle squared - thus a given mass on the rim is harder to get turning (and to stop turning) than the same mass on the hub.

Alex99

drlodge wrote:

Some truth there but I wouldn't overplay it. All the time while riding were are accelerating/decelerating on each pedal stroke, its more pronounced when going slowly uphill. The lighter you are, the easier it is to accelerate and losing rotational mass (especially at the rim/tyre) is more beneficial than losing static mass.

Even if your wheels weighed nothing, you'd still have the mass of the rest of your bike and your body :-)

Yep, riding at a steady rate on the flat, wheel weight doesn't really come into it (despite the micro accelerations described). Where there are repeated accelerations e.g. criterium racing it might be a factor. Climbing at a steady rate, 1 kg on the wheels has the same effect as 1 kg anywhere else.

Alex99

drlodge wrote:

ugo.santalucia wrote:

These threads always fill me with joy. Plenty of folks defending the heavyweight and the inertia when they'd be horrified to ride anything that weighs over 8 Kg.

If you really believe the Halfords 11 kg racer is just as good as the Venge, then you should buy it :-)

What's the saying - train heavy, race light. That Halfords 11kg racer is the perfect training machine. Which reminds me, I must get out on my 12.5 kg CX bike for a good work out soon...

Nah, just go faster if you need to work harder.

johngti

briantrumpet wrote:

johngti wrote:

briantrumpet wrote:

Maybe I'll just gen up on this light reading... http://www.feynmanlectures.caltech.edu/I_19.html

Feynman lectures make awesome reading! Perfect for a quiet weekend at home

I'm not one to use the word 'genius' very often, but Feynman was undoubtedly one. Even for me, a failed physicist, his stuff is inspiring, especially "Surely You're Joking, Mr Feynman", and the biography Genius: Richard Feynman and Modern Physics.

I stole an idea of his for my final year project at uni - whole thing on rolling/spinning disks using classical mechanics and then analysing the problem using the calculus of variations and stuff. It was, undoubtedly, nothing more than a very poor shadow of an imitation of his very first thoughts on it but I enjoyed it!

It was inspired by his story of watching a plate spinning while he was at Princeton. Think that one's in Surely You're Joking...

fenix

I always think this is an interesting experiment.

http://www.training4cyclists.com/how-mu ... lpe-dhuez/

ugo.santalucia

Middle aged men bragging about A level results... Jesus Christ... :-(

briantrumpet

ugo.santalucia wrote:

Middle aged men bragging about A level results... Jesus Christ... :-(

Didn't do RE.

veronese68

ugo.santalucia wrote:

Middle aged men bragging about A level results... Jesus Christ... :-(

Some of us have nothing to brag about. Besides, I'm still on my year off.
Also, lucky for Brian he was good at music as Brianmathematician or Brianphysicist don't have quite the same ring to them.

Bobbinogs

ugo.santalucia wrote:

Middle aged men bragging about A level results... Jesus Christ... :-(

Yeah, as if we don't have enough of guys blowing their own trumpet, oh, hang on...