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Lotus in the Peak
28th - 30th June 2024

Elise S1.... Not 50/50 Weight Distribution !


John FISH Curtis

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Decided this afternoon to put the Elise on axle stands as she's SORN'd till April and don't want the tyres to flat spot, plus I want to do some work over winter.

 

Jack up, axle stands in correct place at the front, then jack up the rear and put axle stands at the cross member section. Removed the front wheels (nuts already loosened) and the front of the car lifted.

So much lift that the car is 4" off the front axle stands, the rear wheels are not touching the ground.... the car is balanced like a seesaw. Very slight pressure pushes the front back to the axle stands.

 

Front wheels are now back on and resting on the stands.

 

Tomorrow night i'll try to get the axle stands at the rear further back.

 

Quite funny to see :)

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There is a myth around that 50/50 is good, spread by marketing departments rather than engineers. Of course, the weight moves about a lot as you speed up and slow down, or going round corners. What is best really depends on what you want. Safe, dependable, understeer if pushed - more weight on the front. For traction, more neutral steering but oversteer if pushed on and trickier to drive - more weight at the back.

 

That is is why the Elise is not 50/50, it is not what you want for an edgy sports car and, Lotus are limited by the parts bin of heavy parts like the engine and gearbox.

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OK here goes with the "F" word - it is all about Fisics.

 

The coefficient of friction between rubber and road is just about 1. That means that the force that the tyres can transmit to the road is the same as the force acting downwards - ie the weight on the wheels.

 

If you have 50:50 weight distribution and no aerodynamic down-force, you have a force of 0.5 times the weight of the car acting on the driven wheels ie half a "g" that means that the maximum force that the tyres can transmit without slipping is half the weight of the car and this means max acceleration is half a "g"

 

If you accelerate at a constant 0.5g it will take you 5.47s to reach 60mph

 

If you have 70% on the driven wheels you can hit 60mph in 3.9s

 

If you are on a motor bike with the front wheel just off the ground all your weight is on the driven wheel and 60mph comes up in 2.7s (if you have enough power)

 

Providing more power will not change these times (unless Newton was wrong) - you will just spin the wheels

 

To accelerate faster you must increase the force on the driven wheels and this is where the aerodynamics come in with wings etc

or

use stickier tyres with a higher coefficient of friction

 

F1 cars for instance, use both these, at high speed they generate >3g in downforce - so 3g acceleration and cornering is possible

 

Of course it is not as simple as this because as you accelerate weight transfers to the rear wheels meaning more weight goes through these wheels - if the driven wheels are at the back it has the same effect as moving the weight distribution back - more grip and more acceleration. If the driven wheels are at the front, the opposite happens and traction is reduced.

 

In a corner the forces work through the car centre of gravity - which in a Lotus is towards the back and so to balance the grip at the front (lighter) end of the car we use smaller tyres.

 

When a BMW or MX5 owner starts extolling the benefits of 50:50 - ask why they need smaller wheels at the front if it is so good!

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The frictional force is not linear against normal force. Grip lost as weight is transferred is not made up by grip gained on the other side/end.

 

You don't want weight transfer when you brake, and having the engine further back reduces the weight transfer.

 

It also exaggerates the weight transfer when you hit the gas, which is a good thing.

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