And thanks, Ian, for providing the results of your tests. If in the test I propose the heavier bike goes faster on the way down, if it is not the weight that makes the difference, someone will have to investigate why. My intuition, and nothing more, suggests it may be something to do with centrifugal force, but I lack sufficient knowledge of physics and mathematics to elaborate.
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- Thread starter Ian
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I am struggling with this one flecc, I agree with the physics and math but at the same I too have noticed how I catch up lighter bikes on a long steep downhill run. Would it be something to do with the weight v air resistance relationship, to give an extreme analogy the falling of a feather and stone ?
Or is it all my imagination
Richard
Or is it all my imagination
Richard
As with Ian in his last post above, I still cannot agree Rooel.
Think about this. When you climb a tough hill your muscles work and disperse heat, this being the energy used in getting up the hill, and it's lost for ever. Likewise the motor expends heat and loses the energy generated for ever.
When an athlete runs 100 metres at high speed, he/she also expends energy through heat and does not store it in any way. If he/she did, they'd have kinetic energy stored trying to push them further with officials having to tether them, or have their muscles preloaded with unbearable tension.
Since all of the energy used in raising weight is lost in doing that, it simply cannot be available for reuse for any purpose at all.
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Think about this. When you climb a tough hill your muscles work and disperse heat, this being the energy used in getting up the hill, and it's lost for ever. Likewise the motor expends heat and loses the energy generated for ever.
When an athlete runs 100 metres at high speed, he/she also expends energy through heat and does not store it in any way. If he/she did, they'd have kinetic energy stored trying to push them further with officials having to tether them, or have their muscles preloaded with unbearable tension.
Since all of the energy used in raising weight is lost in doing that, it simply cannot be available for reuse for any purpose at all.
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Perhaps the real proof is that in races such as the Tour de France the riders don't add as much weight as possible in the form of water bottles and extra clothing before descents on the mountain stages. If there was even a tiny advantage to be gained they wouldn't miss that opportunity.
I think there are several components in this Richard and Rooel. Air resistance is one, though not affected by weight in terms of acceleration. Weight can also have an influence in breaking the "stiction bonds" of friction, rolling resistance, tyre compound drag and tread distortion drag, in these cases the weight exerting a pressure against the resistances. A greater weight exerts a greater pressure and therefore can be more successful in overcoming a resistance. Weight cannot increase the maximum rate of acceleration, but where a "stiction" is preventing the attainment of that rate, the weight can help in overcoming that "stiction" in some circumstances so that the normal rate of acceleration due to gravity can be attained. Since the gain due to this acceleration factor is cumulative, the impression of being very much faster can be given.
This is virtually impossible to measure since it's an infinitely repeating phenomenon, the weight opposing a resistance at each of an infinite number of static points summing as a distance travelled.
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In addition to my above answer, my 22.9 kilo T bike is much faster downhill than the 26 kilo Torq it was derived from, these being "on road" weights.
That's the opposite of what's being contended, but it's due to lower rolling resistance and much lower drag due to air resistance. The downhill acceleration rate is identical, but the ultimate terminal speed is faster.
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That's the opposite of what's being contended, but it's due to lower rolling resistance and much lower drag due to air resistance. The downhill acceleration rate is identical, but the ultimate terminal speed is faster.
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Another thought has just occurred to me, the cyclists I have gained on are just normal cyclists not racers so perhaps after reaching the top of the hill they just relax and coast down where as I tend to keep the throttle open until I have reached a speed where I know the motor is not contributing. that being the case I probably have a higher starting velocity than them and that would account for the gap closing, what do you think ?
Richard
Richard
That will add to the factors I gave above in my first reply to you Richard.
In that I've just added the fact that acceleration due to gravity is of course cumulative, so if there's a factor overcoming any resistance preventing attaining that rate, the increase gained becomes a permanent gain in speed, which can give the impression that the weight is creating faster acceleration than gravity permits.
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In that I've just added the fact that acceleration due to gravity is of course cumulative, so if there's a factor overcoming any resistance preventing attaining that rate, the increase gained becomes a permanent gain in speed, which can give the impression that the weight is creating faster acceleration than gravity permits.
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prState: they might be going as fast as they dare but I doubt they'd pass up an oportunity to accelerate faster between bends.
Richard. Its sounds feasible that you would begin a descent at a higher speed than a conventional cyclist. Most descents are preceded by an accent that would tire many cyclists, they would probably crest the hill almost at a standstill and then let gravity alone take over. By contrast you would perhaps crest the hill at 9 mph and quickly accelerate to 15mph before gravity took you to terminal velocity
Richard. Its sounds feasible that you would begin a descent at a higher speed than a conventional cyclist. Most descents are preceded by an accent that would tire many cyclists, they would probably crest the hill almost at a standstill and then let gravity alone take over. By contrast you would perhaps crest the hill at 9 mph and quickly accelerate to 15mph before gravity took you to terminal velocity
Couldn't you demonstrate this by rolling two different size metal balls down a magnetized plank? The smaller ball would be less able to overcome the pull right?
(I’m thinking small ball bearing versus grapefruit size ball and a fairly weak magnetic force)
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Rooel, it is difficult to explain and requires some sideways thinking to understand but the potential energy the bike has aquired going up the hill is required to get the bike down the hill, the bike may coast and apparently be using no energy but it is acctually using the energy it aquired on the way up.
I've given the ways in which weight can play a part in my post to you and Richard at 8.40 pm Rooel, and I think that may answer what you are getting at here. I'll expand on that shortly, just busy at this moment.
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Those observations are not objective, Rooel. You perceive and remember that effect as your observation only BECAUSE you have the preconceived notion that heavier riders descend faster.
As has been demonstrated, if you remove subjectivity and bias and do a purely objective observation, you find that weight is immaterial to downward acceleration.
It's just like when you are in a traffic queue on a motorway, you perceive that the other lanes are moving faster than your own. Most of the time, they are not (as you find out if you switch lanes) - but you can't escape the nagging doubt that it is true. It is just subjective conditioning at work.
In competitive downhill mountain biking the objective is to go as fast as possible using gravity as the power source. Downhill bikes do tend to be heavier than other cycles because of the great strength required, even so designers and riders go to extreme lengths to keep weight to a minimum in the sure knowledge that extra weight will not give any more speed.
OK, back again to explain what I mean and also give Rooel and Richard the explanation for what they observe.
First think of a 3 lb hammer driving a nail into wood, it will drive the nail in a given distance against the resistance of the wood. Now repeat that with a 6 lb hammer and the nail will go further in. Therefore the nail will be driven in faster with the heavier hammer.
Now to translate that into bikes. On a slope the maximum rate of vertical acceleration due to gravity will not be attained due to the rolling and other resistances which are akin to a kind of sticky bond with the road surface, the shallower the slope, the slower the acceleration rate.
So take Bike A with rider accelerating at a rate of 5' per second/per second, limited to that by the stiction resistance, at the end of 7 seconds it will have attained a speed of 137' per second.
Now take identical Bike B, but load it with extra weight. Like our heavier hammer it will be more successful in overcoming stiction resistance, so our Bike B can now accelerate at 7' per second/per second. At the end of the same 7 seconds it will be travelling at 166' per second.
Bike A reached the maximum acceleration rate due to gravity at roughly 6.4 seconds, but Bike B only took 4.6 seconds to reach it, so has gained overall.
The terminal speed reached for each bike is the accumulation of the distance travelled in each second. For Bike A in those 7 seconds, the accumulation is of 5, 10, 15, 20, 25, 30 and 32', each increasing by the 5' rate until the maximum and totalling that 137'.
For Bike B in those 7 seconds, the accumulation is of 7, 14, 21, 28, 32, 32, and 32', totalling that 166'. You can see how by reaching the maximum rate of 32' per second/per second earlier, Bike B got three of those 32' in the last three seconds, Bike A only managing one.
Hope that clears up the mystery.
P.S. I see that what I've posted clashes with the posts by Ian and DBCohen. Sorry guys, but I think you'll see where I'm coming from on this and where weight can have a small initial influence and place one bike at a faster speed when the maximum rate of acceleration is attained. This will account for the constant gain on another bike which has been observed, but does not conflict with gravitational law.
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First think of a 3 lb hammer driving a nail into wood, it will drive the nail in a given distance against the resistance of the wood. Now repeat that with a 6 lb hammer and the nail will go further in. Therefore the nail will be driven in faster with the heavier hammer.
Now to translate that into bikes. On a slope the maximum rate of vertical acceleration due to gravity will not be attained due to the rolling and other resistances which are akin to a kind of sticky bond with the road surface, the shallower the slope, the slower the acceleration rate.
So take Bike A with rider accelerating at a rate of 5' per second/per second, limited to that by the stiction resistance, at the end of 7 seconds it will have attained a speed of 137' per second.
Now take identical Bike B, but load it with extra weight. Like our heavier hammer it will be more successful in overcoming stiction resistance, so our Bike B can now accelerate at 7' per second/per second. At the end of the same 7 seconds it will be travelling at 166' per second.
Bike A reached the maximum acceleration rate due to gravity at roughly 6.4 seconds, but Bike B only took 4.6 seconds to reach it, so has gained overall.
The terminal speed reached for each bike is the accumulation of the distance travelled in each second. For Bike A in those 7 seconds, the accumulation is of 5, 10, 15, 20, 25, 30 and 32', each increasing by the 5' rate until the maximum and totalling that 137'.
For Bike B in those 7 seconds, the accumulation is of 7, 14, 21, 28, 32, 32, and 32', totalling that 166'. You can see how by reaching the maximum rate of 32' per second/per second earlier, Bike B got three of those 32' in the last three seconds, Bike A only managing one.
Hope that clears up the mystery.
P.S. I see that what I've posted clashes with the posts by Ian and DBCohen. Sorry guys, but I think you'll see where I'm coming from on this and where weight can have a small initial influence and place one bike at a faster speed when the maximum rate of acceleration is attained. This will account for the constant gain on another bike which has been observed, but does not conflict with gravitational law.
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Yes, flecc, that sounds a reasonable explanation, and does confirm that heavier bicycles go faster downhill, thanks not to gravity hauling them all the way down, but to their greater weight overcoming stiction resistance more effectively near the start of the descent.
At the end of the day most electric bikes are not that much heavier than conventional cycles, myself plus Torq weigh about 100kg, there are plenty of 6'+ athletes who's weight combined with that of a lightweight bike is about the same, the athletes probably have a short term power output of 500W, I probably can manage 200W for a short time, but add 500W from the motor and its easy to see how I catch them on the hills, I'd catch 'em on the level too if it wasn't for the speed restrictor.
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