Discussion
domster said:
Interestingly, any car pulls 1g for a split second when it starts rolling.
It's a little bit early for an April fool isn't it?
Even for a fraction of a second, force = mass x acceleration.
Mass is fixed and force is finite, so acceleration can't be 1g if the engine hasn't got the oomph to do it.
I think.....
Actually I would have thought that the instantaneous accelerative force from standstill is fairly meaningless since it tends towards infinity anyway.
(Force is proportional to rate of change of momentum and since momentum is zero at standstill and greater than zero instantaneously, force and therefore acceleration is theoretically infinite immediately upon moving.) Anyone still awake?
Edited to say: as per sprior's neat little calculator, sustained 1g acceleration would have to be pretty mind blowing.
>> Edited by Harris_I on Sunday 7th March 11:40
(Force is proportional to rate of change of momentum and since momentum is zero at standstill and greater than zero instantaneously, force and therefore acceleration is theoretically infinite immediately upon moving.) Anyone still awake?
Edited to say: as per sprior's neat little calculator, sustained 1g acceleration would have to be pretty mind blowing.
>> Edited by Harris_I on Sunday 7th March 11:40
jeremyc said:
Autocar's 0-100-0 test measured decellerative G.
The Noble managed 1.44g
The stoppping distances in the highway code assume 0.67g as the max decelaration for cars, and even then it qualifies it with 'braking system and tires in good order, dry conditions, etc' (or something like that).
1.44g just goes to show how out of date that is - that's less than half the stopping distance that the highway code assumes. Now, maybe, given that speed limits were set when the brakes on cars were little better than pushing your feet onto the tarmac through a hole in the floor, it might be time for a rethink on speed limits?
Assuming that the reaction time of 0.68sec used in the highway code table stays the same, at 1.44g braking, the 30mph stopping distance can be achieved from 44 mph, 50 mph is equivalent to 73 mph, 60mph = 88mph and 70mph = 103mph.
I might try this argument next time I get pulled over....
>> Edited by sprior on Monday 8th March 17:44
Harris_I said:
Actually I would have thought that the instantaneous accelerative force from standstill is fairly meaningless since it tends towards infinity anyway.
(Force is proportional to rate of change of momentum and since momentum is zero at standstill and greater than zero instantaneously, force and therefore acceleration is theoretically infinite immediately upon moving.) Anyone still awake?
Edited to say: as per sprior's neat little calculator, sustained 1g acceleration would have to be pretty mind blowing.
>> Edited by Harris_I on Sunday 7th March 11:40
I think I meant infinite. Sommat like that. All based on long forgotten hearsay not scientific fact, so you guys are undoubtedly correct. I was fairly sure 1g acceleration was pretty impressive for a road car tho'. Apparently a Radical SR3 can do 2g when cornering...
domster said:I still think the zero or infinite accel from rest is not true. Accel is the speed difference / time, so there is no difference accelerating from rest than from any other speed (unless you have a very fast car that approaches the speed of light, when relativistic mechanics come into play, or have a really small car of subatomic proportions, when quantum mechanics rule.
All based on long forgotten hearsay not scientific fact
sprior said:
I still think the zero or infinite accel from rest is not true.
It isn't. The technical term for that is "b*ll*cks"
On the original question, a RWD car with 60% rear weight bias, tyre friction coefficient of 1.0 and a c of g height of 450mm (probably in the right ballpark for a 996) can initially generate 0.6g (corresponding to weight on the rear wheels), then as the accel causes weight transfer to the rear can accel harder until it reaches a max sustained accel value of 0.75g. With some sticky race tyres on (say a friction coeff of 1.2) that limit would go up to 0.94g. That's a simplified view as the effect of tyre wind-up at launch is ignored, but it gives the right idea.
A 996TT might get above those figures if the fronts could be persuaded to contribute a reasonable amount of traction, but as the slip ratio at the rear needs to be fairly high before the viscous coupling directs much torque to the front it is unlikely to improve more than a little. In any case, the TT only (!) has enough torque to manage those kind of accels in 1st gear, in 2nd the max possible would be about 0.7g.
Actually, we had a car that ran a similar power-to-weight ratio as a 993TT that would pull 1.0g from a standing start, which is achieved when you travel the first 64ft in under 2 seconds. The car involved was a 925kg 2.7RS Replica with a 3.5 engine at 260bhp on P Zero C road tyres. In 2nd gear the g drops to 0.7g, 3rd 0.6 and 4th 0.5g. Same car around bends hits 1.35g and 1.2g under braking, our 2-axis G-meter logged the results.
My 993 RS CS pulls only 0.92g in 1st but has recorded
1.45g around bends and 1.5g under braking, thus justifying 25 years of development...
>> Edited by ninemeister on Thursday 11th March 16:25
My 993 RS CS pulls only 0.92g in 1st but has recorded
1.45g around bends and 1.5g under braking, thus justifying 25 years of development...
>> Edited by ninemeister on Thursday 11th March 16:25
domster said:
Shows I know sod all about physics
I think you've been put upon here.
After a bit more thought, I think there's something in this...
When the accelerating force is applied, like when taking off from stopped, the force is applied suddenly.
Now, F=ma, so as the rate of change of force is a big number, then the rate of change of acceleration is a big number too (and tends to infinity, the quicker the force is applied).
This may well be where the infinite accel thing came from (even though that is Bowwwguuuus, as 'Tom and Ray' would say).
BTW, the official term for rate of change of acceleration is the 'jerk'.
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