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Gaining easy MPH


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It was Mr Chapmans lifelong theory of 'lightness=speed' (or something thereabouts!)

So, it would seem that the easiest way to gain speed would be to keep weight in the vehicle reduced to a minimum.....bear with me here.

If any eggheads on here can work out an algebraic equation.......how much weight ( in pounds) would it take for the driver to gain/lose, to increase/decrease the power of the actual car? Working on the principal the average driver is 200 pounds.

Just wondered if I need to go 'anorexic' to speed up a little, or just throw the wife out of the car!

Deja Moo: The feeling that you've heard this bull before.

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Also consider trains - massive weight but top speed is unaffected even when empty or loaded.

If you have x amount of torque at the wheels then there will come a point when drag and mechanical resistance equals this and then acceleration will be zero.

Weight just influences how long it takes to get there.

But reducing weight will make you faster on track as you very rarely attain vmax.

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Agree - reducing weight speeds up acceleration but has no effect on top speed.

Half the weight, keeping the same power and the acceleration is approx twice as fast (ignoring drag, mechanical friction etc).

From memory the S4s weighs 1320 kg so if we eat a few less pies and loose 10kg you will speed up your 0 to 60mph by 0.03 seconds. Seems hardly worth it?

A better option might be to fit one of the light weight (ie no silencers) exhausts from the LEF store. This saves weight, adds performance and sounds great all without giving up on the pies.

Alan

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Light weight also means stopping and cornering better :detective: Vmax is unaffected unless you change the aero or mechanical drag on the car.

Lotus Esprit S4 - Work in progress

Porsche 924 Turbo - Parts chaser

Smart Roadster Coupe - Hers

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Weight does affect top end speed, its Newtons second law (still think it's amazing the fella thought all this shit up donkey's ago and it's STILL 100% applicable today...amazing....anyways)

Newtons 2nd Law - A body of mass (car) subject to a force (engine, powering the wheels) undergoes an acceleration that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass.

We did a practical on it using a glass plate a pingpong ball vs a steel bearing, same size, same rolling surface but the steel ball required like 15 times the energy to move and so on to accelerate.

Given the engine's power is finite ie 265hp, the heavier your car the more energy is taken up accelerating it.

With the car the limits are the gearbox etc - it's top speed is govered by the rpm of the engine and the ratio of the highest gear, minus the power and the affect of drag and mass.

If your car was lighter and the gear ratio could be much longer, the same HP of the engine would be sufficient to take it faster.

Inversley, if the Esprit was 4 tonnes with the same engine, do you think it could manage 165 mph ?

Thats no btw :D

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Inversley, if the Esprit was 4 tonnes with the same engine, do you think it could manage 165 mph ?

Yes. Providing everything else was equal (aero, wheel bearing drag etc). It would obviously take much longer, but it would do it. Your ping pong ball experiment only proved weight affects acceleration.

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Not quite, Neutons first law only concerns acceleration not speed.

The steel ball bearing required more energy to accelerate it at the same rate as the ping pong ball.

Once they are both moving then the only thing to consider is the drag affecting them.

If the acceleration force (in our case engine torque) is greater than drag (rolling resistance adn aero) you accelerate. If it's lower then you declerate.

Drag is affected by mass a bit (the tyre contact patch changes so your rolling drag changes) but not much really as over around 100 mph your aero drag is much more significant.

A heavy car accelerates more slowly but only when drag=thrust do you stop accelerating and you reach v-max.

Lotus Esprit S4 - Work in progress

Porsche 924 Turbo - Parts chaser

Smart Roadster Coupe - Hers

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What about when it gets closer to the speed of light? :huh:

88 Esprit NA, 89 Esprit Turbo SE, Evora, Evora S, Evora IPS, Evora S IPS, Evora S IPS SR, Evora 400, Elise S1, Elise S1 111s, Evora GT410 Sport

Evora NA

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Well I've just gone from 178 lbs to 143, so when I've finished fitting the re-entry shield I'll report back.

Life is like a sewer, what you get out of it, depends on what you put into it. (Tom Leahrer)

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Don't forget weight also greatly affects stopping distance. (or , the lighter car can get away with smaller brakes or stop a lot sooner with the same brakes -something I think Lotus counted on too much or didn't

factor in added cooling too for multiple stops like at the track).

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Exactly - the steel ball requires MORE energy to accelerate, so to get it to the same speed as the plastic one you need more power - a finite power will result in the thing not being able to accelerate any more.

What is top speed, a product of acceleration, if I run out of acceleration I run low of top speed.

What is the maths ? Something like to double the speed you need 4x the energy - so to move an object from 1-2 mph might take 5hp, to move it form 2-4 mph will take 20hp, 4-8mph = 100hp and so on.

Take a lighter object which 1-2mph takes 1hp to move, 2-4mph takes 4hp, 4-8mph takes 16hp - if the finite energy was 100hp the heavier object would puff out at 8mph, the lighter one would accelerate onto a higher speed (approx 20mph)

Acceleration is very important to max speed.

Rifles have the same issue - bullets shots have the same shape and size and propellant but can be different grains (or masses, eg snipers have heavy grains to make the round more stable) - for all of that the muzzle velocities are different for different grains becuase of the power needed to accelerate them. OK the barrel plays a lot of a part in that but even if you match the round with the barrel a heavier shot's max speed is slower for the same charge.

Edited by Jonathan

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Whoa there....

In our example force is a function of torque and gearing which provides a thrust force to accelerate a vehicle. Torque is force x distance, distance in this case is to do with gearing = shorter gear = more force available.

The vehicle will accelerate according to Force equals mass times acceleration (or a=F/m) so a greater mass produces a smaller acceleration for the same force.

The vehicle will continue to accelerate until the drag equals the thrust: Newtons laws of motion. (Note there are no speeds mentioned in newtons laws at all)

Drag is a function of aerodynamics and rolling resistance. (Note: A mass change might change the rolling resistance a bit)

The mass of a vehicle does not affect the v-max directly, is affects the acceleration. It MAY have an effect on the rolling resistance which will affect v-max a bit but not significantly.

Edited by Rich H

Lotus Esprit S4 - Work in progress

Porsche 924 Turbo - Parts chaser

Smart Roadster Coupe - Hers

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Will it look faster to a stationary observer?

88 Esprit NA, 89 Esprit Turbo SE, Evora, Evora S, Evora IPS, Evora S IPS, Evora S IPS SR, Evora 400, Elise S1, Elise S1 111s, Evora GT410 Sport

Evora NA

For forum issues, please contact the Moderators. I will aim to respond to emails/PM's Mon-Fri 9-6 GMT. 

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On a road car you could not loose enough weight i.e the driver to make hardly any difference unless you were morbdly obese and lost half your body weight.

if you were talking about the car then you could go the NSXR, Superleggera, GT3 etc route and fit thiner glass, perspex rear, stick on badges, lighter seats, Titanium wheel bolts etc and then move on to lighter piston rods so there is less mass to move in the engine, but all in all it will make no difference when sitting on the M25 in a tailback

And as you say the top speed is affected by the pure power available due to wind drag and fluid dynamics, i.e the mass of the air hitting the car increases at speed. therefore you need more power to punch through it. Power is the key to going fast.

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Professionals built the Titanic

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A car reaches VMax when the power at the wheels equals the rolling resistance plus the aerodynamic drag.

Rolling resistance is proportional to the vehicle mass times the velocity

Aero drag is proportional to the velocity squared

As speed increases the exponential aero drag takes over which is independent of the vehicle mass.

[i've left out the drivetrain losses which are also affected by the vehicle mass (e.g. a heavier car require a bigger power sapping gearbox) but again this becomes less significant compared to the aero drag.]

Edited by neal

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May: Cause it's the tool of a pikey.

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