Sorry about the rambling nature of this post - it's arisen from a conversation with a friend who drives but knows nothing at all about car engines and transmissions, but who is also used to riding a multi-geared pushbike.
Thinking about things, I seemed to have tied my own brain in knots. Perhaps Number Cruncher will come along and define it all in black and white!
A car will accelerate in top gear if it is already doing say 30 mph.
A cyclist can accelerate in top gear if he's already doing say 10mph.
But a cyclist will struggle to set off from rest in top gear, even if the pedal crank is at its most favourable angle (horizontal).
I can understand a petrol/diesel car will struggle to set off in top gear because of insufficient torque developed at the low revs that would be forced upon it by the gearing. But given a sufficiently thick clutch plate or sufficiently-cooled fluid clutch, would it set off provided the engine revs can be held at the speed of maximum torque?
It's all about starting torque, because typical electric cars will set off from rest with no gearing at all.
Does the human leg also develop less force when pushing against a static pedal than when it is already rotating?
The mass of the vehicle is the same, so if the force/torque is the same, in the same gear would a car or bicycle not add 10mph to its speed in the same amount of time starting from 30 mph as from zero mph?
F = m a suggests it would, but subjectively it doesn't feel as though it would.
Is this purely due to the outside factors like torque not being able to be developed at low speeds / slippage in the clutch etc? Or is starting from rest a special case?
I presume that a (theoretical) motor with utterly constant torque from zero (no clutch) to 6000 rpm would give a vehicle a constant rate of acceleration (assuming there was no wheelspin) until wind resistance became an ever-increasing restriction?
Momentum actually plays no part?
So is it that the bottom line is indeed that a cyclist's leg cannot develop as much force when pushing against a static pedal as when everything is in motion?
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The cyclist can push on the pedal with the same force whether the bike is moving or static. Whether that force is enough to overcome the combined intertia of bike & rider depends on the mechanical advantage (i.e. the gearing).
Likewise, a car will set off in a high gear if the engine is producing sufficient torque to get mechanical advantage through the drivetrain, and overcome the vehicle's interia without the engine stalling.
It will do this either through clutch / transmission slip, or tyre slip. In fact, when drag racing motorbikes, the preferred technique is to dump the clutch as quickly as possible and let the tyre spin for the first few feet.
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I would put money on my car with a diesel engine and 360Nm of torque being able to damage the clutch if I start in 6th gear. In first the max torque is limited.
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But Craig,
Surely the INERTIA of the vehicle is the same when it is at rest as it is when it is moving at a constant speed?
This was the nub of my friend's niggle:
on his pushbike, why can he accelerate faster, or alternatively not feel to have to apply as much effort, when he starts from a situation of doing 20mph rather than doing zero mph?
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A lot of different factors here tc.
A friend had when at university a 1948 Buick Special that had a straight-eight engine. I can't swear to it at this distance in time - it was 1959 or so - but I think it was side-valve too. Anyway it would start from rest quite easily in top gear, as we discovered when the column-change linkage broke at the root of the lever and the spare part took a while to arrive. You got someone to depress the clutch pedal, opened the bonnet and selected the gear by hand by pushing and pulling the rods. Not really recommended though for a lot of reasons, safety and clutch abuse heading the list.
Electric motors develop maximum torque from rest and run to high rpm, unlike piston engines. This characteristic gives electric vehicles good acceleration from rest, making them quite well adapted to town work. Adapting them for high cruising speeds involves greater complexity though. We should expect perhaps to see a burst of development in transmissions for electrically powered vehicles. In the days when they existed, trolleybuses used to have twice the acceleration of ordinary diesel ones. You had to hold tight until you were sitting down, or you could be thrown around when the driver floored it away from a bus stop. But they couldn't go all that fast.
One would think a cyclist's leg would be the same, although maximum torque can only be applied in pulses, two per revolution, when the leading pedal is in a shortish arc around horizontal. On reflection the mechanics of that are quite complicated, with many variables. No doubt competitive cyclists have proper scientific diagrams.
Edited by Lud on 19/06/2009 at 18:37
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If you take as your premise the assumption that the cyclist produces the same torque at all revs, the bit you're missing is the work done. Double the revs and he's doing double the work. At almost no revs, almost no work, so almost no acceleration. QED.
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I remember someone describing a 'Party trick' with a 5.3 XJS manual. Put it in 4th give it a bootful of revs and drop the clutch and watch it disappear up the road. Don't know if it's true, but I suppose it's likely. Mid life XJS team where are you??
Best reg's.......Martin.
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Speaking of party tricks.. A friend used to have a BMW CSi coupe. Think it was a 3.5 or a big engine anyway. He used to like using the "manual" function on the autoshift. Holding it in gears and slotting it when he chose. One time, during a traffic light Grand Prix he diod that and at about 50 mph managed to inadvertantly select reverse....
Ever been on a waltzer...?
We were much younger then of course...
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Or is starting from rest a special case?
It is in one respect. You need to provide more tractive force to "breakout". This is because the static friction is generally higher than dynamic friction.
So, the cyclist needs to apply extra force to simultaneously overcome static friction in the pedals, the crank, the active rollers in the chain, the front and rear wheel hubs, and the tyre deformation. In a situation where the acceleration from rest is very slow, it's easy for the cyclist's leg to move to an unfavourable angle, and for static friction to begin to regain its grip.
Two ideas which, via sounding wrong at first, might help;
Once in a particular gear, and ignoring the variation of drag with speed, the acceleration of a car with a standard manual gearbox is proportional to the engine's torque, and hence, best acceleration in any particular gear is obtained at the engine speed for maximum torque.
If you have a car with a perfect CVT, again ignoring the variation of drag with speed, to obtain the fastest acceleration change the gear ratios to keep the engine always running at the speed for maximum power.
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I can vouch for the veracity of Lud's tale. My GMC pick-up will pull away from a stand in top, on a level road and unladen.
Many older American vehicles will do this trick, to a greater or lesser degree. Large-capacity vee engines have the characteristics of high torque at low revs.
Edited by Harleyman on 20/06/2009 at 01:54
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