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Old 12th September 2009, 01:37   #1
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Default Should we consider Acceleration to Braking ratio seriously?

Im not talking about 0-100-0 here.

This test has more to do with finding out if a cars braking distance is relative to its acc.

For EG : If we take 400m as a benchmark distance, let each car acc to whatever speed it can till it touches the 400m mark and see how quickly it can come to a halt from there.

This makes more sense in the real world bcoz a guy driving a Veyron can prob reach 2.5 times the speed of a family sedan on a 400m stretch of road but can he actually stop in the same (or lesser) distance from that speed?

I think this would be an interesting test to try out even on Indian cars coz we keep seeing companies launching cars that can go a like a bullet but have braking times similar to or slightly better than cars that are much slower in acc.

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Last edited by Shan2nu : 12th September 2009 at 01:43.
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Old 12th September 2009, 08:28   #2
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Quote:
Originally Posted by Shan2nu View Post
This test has more to do with finding out if a cars braking distance is relative to its acc. For EG : If we take 400m as a benchmark distance, let each car acc to whatever speed it can till it touches the 400m mark and see how quickly it can come to a halt from there.
Nice, but don't we already have tests for braking distances ? The 400m acceleration test has quite a few factors affecting it, apart from BHP & torque through the rev range - like gear ratios, final drive, & aero-drag.

If you have the braking distance for a car, say from 100Kmph to a full stop it should be possible to estimate quite fairly the braking distance at the end of the 400m sprint since energy is squared with speed. Of course this way does not include brake fade.

What is proposed, i.e. a 0-to-quarter-mile-to-0 run should throw up some interesting statistics nevertheless
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Old 12th September 2009, 09:10   #3
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It is a good thought but it may not be a straightforward task measuring the braking effectiveness. There is only one way to accelerate but you can brake in different ways using foot brake, hand brake, engine brake or a combination of these. Some drag cars even use aerodynamic brakes to stop.

The braking distance would also depends on the surface conditions, tyre conditions, temperature of the brake pads etc.
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Old 12th September 2009, 12:45   #4
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Nice, but don't we already have tests for braking distances ? The 400m acceleration test has quite a few factors affecting it, apart from BHP & torque through the rev range - like gear ratios, final drive, & aero-drag.

If you have the braking distance for a car, say from 100Kmph to a full stop it should be possible to estimate quite fairly the braking distance at the end of the 400m sprint since energy is squared with speed. Of course this way does not include brake fade.

What is proposed, i.e. a 0-to-quarter-mile-to-0 run should throw up some interesting statistics nevertheless
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It is a good thought but it may not be a straightforward task measuring the braking effectiveness. There is only one way to accelerate but you can brake in different ways using foot brake, hand brake, engine brake or a combination of these. Some drag cars even use aerodynamic brakes to stop.

The braking distance would also depends on the surface conditions, tyre conditions, temperature of the brake pads etc.

See, this is not just about braking. Its the ratio between acc and braking, if a family sedan can do 0-400m and then come to a halt in say 60mtrs (from whatever trap speed it can achieve), that would mean that it only needs 15% of the distance to stop from a 1/4 mile acc.

Now compare this to other performance cars and see what % of braking distance they need from the speeds they can achieve in a 1/4 mile.

As cars get faster n faster, its also important for manufacturers to make sure that the braking power is also increased in relation to the acc capabilities.

If a car is 4 times quicker than a normal sedan but only has 2 times the braking power, the sedan actually ends up having a more efficiant braking system, compared to its rate of acc.

Cars can use whatever braking tech they want, this test is only to see if the manufacturer has improved braking performance as well as they have done with the acc.

This is something like power to weight ratio, having only power or having only lightness doesn't count, you need a combination of both to make it work.

Shan2nu

Last edited by Shan2nu : 12th September 2009 at 12:48.
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Old 12th September 2009, 14:17   #5
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Won't the results be different for the same car with stock tyres and upgraded tyres? Even within the same make there could be lot of variance in the results. Not to forget there are ABS and non-ABS variants too.

Another thing to consider is the vehicle weight. There are hatchbacks, sedans, SUVs, pickups, buses and trucks. Bringing an unladen truck to a stop is a lot different than when it is loaded to capacity.
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Old 12th September 2009, 14:30   #6
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Originally Posted by Shan2nu View Post
Cars can use whatever braking tech they want, this test is only to see if the manufacturer has improved braking performance as well as they have done with the acc.
Hmmm... interesting query, to say the least.

My take on this:

Going by simple physics, beyond a point, the braking distance of a car cannot improve. The braking performance is directly related to the grip of the tyres with the road surface, and to an extent, the dynamics of the vehicle. Wider tyres = better braking. Equal distribution of weight between front & rear = better braking. Heavier car = better (more stable) braking.

But...

Lighter car = better acceleration. More weight on the driven wheels = better acceleration. Too wide tyres = Lesser acceleration.

So we find that those very features which improve braking, hamper acceleration.

Therefore, should we be forcing car manufacturers to strike a balance (or a fixed ratio) between acceleration and braking efficiency? Or, should we trust in the driver's skills and judgmental abilities to drive safely?

The answer is a foregone conclusion IMO...
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Old 12th September 2009, 15:43   #7
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Therefore, should we be forcing car manufacturers to strike a balance (or a fixed ratio) between acceleration and braking efficiency? Or, should we trust in the driver's skills and judgmental abilities to drive safely?
Well, its better to have a balance between acc and braking to start with. As a car gets quicker it should also have improved braking. Otherwise we will just end up with cars that can go like a bullet but have poor braking wrt to its acc.

Quote:
Won't the results be different for the same car with stock tyres and upgraded tyres? Even within the same make there could be lot of variance in the results. Not to forget there are ABS and non-ABS variants too.

Another thing to consider is the vehicle weight. There are hatchbacks, sedans, SUVs, pickups, buses and trucks. Bringing an unladen truck to a stop is a lot different than when it is loaded to capacity.
Thats what makes this test interesting. All we have seen so far is braking from a fixed speed (irrespective of how long it takes that car to reach it).

Shan2nu
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Old 12th September 2009, 16:04   #8
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Originally Posted by Shan2nu View Post
let each car acc to whatever speed it can till it touches the 400m mark and see how quickly it can come to a halt from there.

This makes more sense in the real world bcoz a guy driving a Veyron can prob reach 2.5 times the speed of a family sedan on a 400m stretch of road but can he actually stop in the same (or lesser) distance from that speed?
But, in real world do all people try to achieve top speeds ?

IMO if a certain top speed is mentioned as the limit say 140kph then braking from that speed would give you some conclusive data.

Say one car reaches 190kph in 400m and another car reaches only 120kph in 400m, its obvious that the car doing 120kph would stop before the car doing 190kph. ( I'm assuming, correct me if i'm wrong)

Last edited by kpzen : 12th September 2009 at 16:07.
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Old 12th September 2009, 16:44   #9
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But, in real world do all people try to achieve top speeds ? IMO if a certain top speed is mentioned as the limit say 140kph then braking from that speed would give you some conclusive data.
Im not talking about top speed, this is speed in relation to rate of acc.

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Say one car reaches 190kph in 400m and another car reaches only 120kph in 400m, its obvious that the car doing 120kph would stop before the car doing 190kph. ( I'm assuming, correct me if i'm wrong)
Yup, thats what im talking about. Im not talking about braking from a cruising speed here.

When you drive for performance a quicker car can reach much greater speed in a given distance, so maybe it makes sense to see how that car performs when asked to brake from that speed.

Like when i drive the Innova, i reach good speeds very quickly, but the braking is really pathetic for its rate of acc.

End of the day, no matter what tests you perform, you cannot replicate real world situations. But it would be something diff to look forward to in addition to the usualy 80-0 or 100-0 tests.

Shan2nu

Last edited by Shan2nu : 12th September 2009 at 16:48.
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Old 12th September 2009, 17:12   #10
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Originally Posted by kpzen View Post
...its obvious that the car doing 120kph would stop before the car doing 190kph. ( I'm assuming, correct me if i'm wrong)
KP you're absolutely right - the car that achieves 190 km/h will take more distance to stop than one doing 120 km/h.
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Originally Posted by Shan2nu View Post
...a quicker car can reach much greater speed in a given distance, so maybe it makes sense to see how that car performs when asked to brake from that speed.
There are a lot more variables to tweak while accelerating - fuel, engine size, engine type etc. - than when decelerating - just tyres and road (where in most cases you don't have control over the road surface). Therefore, if the test you propose is a determinant of safety, the safest vehicle would be an Omni wearing 195/20-12 Z-rated radials with 250 kg of sandbags in the boot.

Er... and the most unsafe would be KP's rocket-powered rollerskates riding on their original tyres!
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