No, what I claimed was that at constant speed, acceleration = 0 which isdrumroll....
correct!
Acceleration is defined by the rate of change of velocity, not the rate of change of speed. As velocity is a vector quantity it has both magnitude and direction. Speed is a scaler quantity and therefore only has magnitude. It is perfecty possible to accelerate without changing speed.
We could argue all day but it is pointless and we're at 4 pages nearly.
What we need is a controlled experiement repeated multiple times and then statistically analysed.
Anybody actually want to do this? (It has been done countless times)
Maybe get some professionals with equipment and sponsership?
Volunteers?
glenh - Member
Exellent plane on a conveyer style troll Jon
I've got these empty honey jars, if could just somehow attach them to the hubs and rims of 2 identical sets of wheels I'm sure we could carry out some kind of meaningful experiment.
Hi Guys,
Interesting article and interesting to hear the arguments. I run a blog as part of my Research group and thought this would be a good subject for a topic (I'm a sports Engineer) so have done some quick calcs with some simple assumptions. I'll write it up this week and get it on ASAP (there's a good article waiting to go up so can't push in front) but to get a sneak peek on the numbers.
Assuming a rider + gear weight of 80kg, total bike weight of 12.7 kg and wheels weight (in tyres etc. of 3.12 kg) by decreasing the rim mass by 100 g (and putting the mass elsewhere on the bike) you get a 4% increase in acceleration for a zero change in the bike mass.
Interesting stuff, I'll write it up properly and put in some real examples (i.e. how does it relate to time, given an equal torque at the wheel)
Also, realise this is a very simple assumed system, accleration is exactly that, accln of a point mass with two rotating bodies depending on a torque applied at on a rotating mass.
Hope that's of some help to the OP. I can't answer whether that 4% is noticeable though!
Si C
Some numbers! At last
Good stuff weirdchops
The OP was something to do with the hub going at the same speed as the rim. Which part of the rim?
The hub goes at the same speed as the bike.
However the rim (and tyre) at the ground is going at the same speed as the ground (its either stationary, or it is skiding).
The top of the wheel is travelling at twice the speed of the bike.
If you try to trace out the path off a single point on the rim as the bike moves forward. Something that you can try to do with your finger it is a curious pattern.
For a point on the rim to accelerate from zero to twice the speed of the bike and back to zero each revolution is fairly rapid acceleration.
Incidentaly one of the limiting factors for a 100m sprinter (Usain Bolts for example) is the ability to take the foot from zero (mph) on the track surface to twice the speed of the rest of the runner (approx 60mph).
Apologies guys, let me correct that, everything's the same other than the answer. A classic case of not carrying the 10. Let me make that .4% ! Error in the typing, my mistake.
Further analysis:
100 g from bike (non-rotating) = 1.5 % increase in accln
100 g from bike (rotating) = 1.9 % increase in accln
These assume weight of bike reduces.
I stand by original claim. I can't say whether the .4 % is noticable!
Si C
Wiredchops.
I only get a 0.1% change by slicing off 100g from the rim based on your data.
If you manage to shave off 1kg then I calculate you'll accelerate 1% quicker.
Look forward to your calcs.
I think all this is overrated. We don't get the massive advantage seen by engines due to the fact that the tip of our cycle wheels are travelling at the same speed as the bike itself whereas a flywheel will rotate much faster than the transverse speed of a vehicle. In low gears the lightening of a flywheel offers a much better advantage due to the relative speed difference between the transverse speed and the flywheel rotational speed.
Perhaps we should investigate trimming down the teeth on our outer driving cog!!!!
The above calcs assume that weight is put back on the bike, I suppose this is unreasonable so double the % figure to get some idea of the advantage of losing the weight entirely.
asol,
Again, rushing through some calculations to try and get them on a forum is proving costly to my kudos. Didn't include the mass of the rider in my spreadsheet which explains the discrepancy. I did think the improvement was quite large.
We're talking a .06% difference by removing 100 g from both wheels.
I think we're getting in to 'it's neglgible' territory 
Especially if I've got the chuffin' calculations right this time!
It might not be the massive gain some expected, but a gain is a gain, and its a gain that feels better to ride (unlike say loosing 100g from the bars/stem).
It might not be the massive gain some expected, but a gain is a gain, and its a gain that feels better to ride
I only agree with half of that statement. I think to claim it feels better to ride is pretty spurious and opens up a big can of worms. However, this place feeds off of worms, so dig in.
This is like another Kaesae thread, only not as funny.
Bring back Kaesae!!
I think we worry too much about weight. All the bike companies and magazines go on about it all the time because it's another way for them to sell their stuff. I recently put heavier forks on my bike and was worried about the weight penalty because everyone seems to think it's so important to shave off grams here and there. This fork upgrade has made my bike much faster uphill and down. A heavier bike may affect handling a bit but I rate strength and functionality more highly.
Excellent point
Good point re weight - it's not the be all and end all by any means, but light is good (assuming all other factors are good enough)
aracer - Member
On the contrary, it's impossible to feel a 0.06% difference - we're into placebo effect here.
How on earth can you prove that?
cynic-al. It's very difficult. especially as high levels of acceleration (when small percentage changes can make a real difference) may be relatively sparse.
Let's assume for a second that a ride can accelerate constantly at 0.5 g (4.905 m/s/s). He will ride for 100 m, he finishes in 6.386 seconds.
He switches to a bike which gives a 0.06 % increase in acceleration
He finishes in 6.3836 seconds
Less than 3 milliseconds difference in 100 m of constant acceleration.
I ain't saying you can't notice, but you'd have a very finely tuned sense of judgement if you could.
Kinetic energy of a non rotating moving object = 1/2 x mass x velocity squared.
Total kinetic energy of a rotating hoop (ie a wheel) roughly = mass x velocity squared.
So the total kinetic energy of the rotating wheels of a moving bicycle roughly equates weight for weight for twice as much kinetic energy as the non rotating parts of the bicycle. Hence the reason for the saying that 50g off the wheels is worth 100g of the frame. When you accelerate, you are adding kinetic energy.
That's the maths, as too how much effect it has is arguable - see all above threads.... 
That's my point chops.
IMO/E you develop a feel for your bike and can detect minor changes. I'm not saying you could detect 100gm of both wheels, but no-one should say you couldn't without some proof!
If anyone wants to do real numbers I'd be most interested in seeing the difference between a 2kg Alu wheelset vs 1.5kg carbon wheelset.
I presume if I said gravity made things fall down you'd want me to prove it?
Assuming about 100kg for rider plus bike, then swapping out 2kg wheels for 1.5kg wheels would see about 0.5% reduction in potential energy input for hill climbs plus 0.5% reduction in kinetic energy input for acceleration.
For those that argue that 0.5% is naff all, it's worth remembering that it's not a linear scale - ie if I put a 100kg weight on the back of my bike I'd take more than twice as long to get up some of the hills round here! Also,0.5% is half a mile over a 100 mile course.
(not saying you;ll get that much gain because of other factors - but just demonstrating that 0.5% gain is not to be sniffed at)
All the calculations are based on saving time...do those % savings directly translate into energy saved? Ie not increasing acceleration but decreasing effort required for the same acceleration.
stuart - all my calcs are based on reduction of energy input 
stuart - all my calcs are based on reduction of energy input
0.6% = woopiedooo!! - I'll have me some of that! I had some weight weenie wheels on my full suss, ended up with one rim dinged and the other tacoed - needless to say my latest wheels have erred on the side of robustness I'm now of the opinion - if it weighs less than 30lb, OK, but sub 25lb prob isn't necessary. will you be able to feel that improvement of 0.6% - I think you're kidding yourself if you think you can.
I can't imagine it makes any difference at all.
I mean, when you watch MotoGP they can pick a wheel/tyre etc up easily in one hand, whereas on a road bike the wheel is heavy enough, without the tyre/disc etc. But thats' just fashion of course.
I can certainly tell the difference; we once went to Afan after a day uplifting in Cwmcarn - heavy wheels, thick tubes, 3-ply tyres and 203 discs - jesus was it difficult, and unenjoyable.
aracer - Member
I presume if I said gravity made things fall down you'd want me to prove it?
This topic has been closed to new replies.
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