Nick Adams, Original Project Manager, Lotus 23
(put picture of Nick's 23 here)
Nick Adams was the original project manager for Croft, the Federal Elise. He has an amazing way of communicating technical knowledge so car owners can understand. I have gathered some of his communications from various sources including personal interviews, forum posts, and magazine articles.
Wheel Sizes and Weights
| car | weight | width |
| Toyota engine Elise | 8.3 and 10.7 kg | |
| 16 spoke cast Exige | 8.8 and 9,4 kg | 1 inch wider at front |
| forged 14 spoke | 5.6 and 7.2 kg | 1 inch wider at front |
| 240R | 6.1 and 8.1kg | 16"x7J and 17"x8J (1.5 inch wider) |
On Limited Slip Differential
The Elise was always designed from the outset to work without an LSD. We have recently started to offer an LSD as an option on the Toyota engined cars, primarily in response to market demand from the Autocross enthusists in the USA, who need one to be competitive when accelerating away at full throttle from very slow, tight corners in first or second gear.
In this type of competition they do not tend to run high speed (100mph +) corners and therefore the increase in understeer on this type of corner which you get with an LSD is of little negative consequence to them and they therefore are better off with an LSD.
In our experience an Elise or Exige equipped with an LSD is at a disadvantage to one without an LSD on a typical European race track. On top of that the LSD bluntens the steering feel and repsonse of the car which we don't like.
If you want an LSD then by all means fit one, but please understand that there are negative as well as positive effects. In the instance you describe, instead of spinning the inside wheel as you accelerate away at full throttle (which can be easily fixed by modulating the throttle! Very Happy ) a car with a 2:1 LSD like the obne we supply will provide twice the torque to the outside wheel as it does to the inside one. This will increase the slip angle of the outside wheel and the car will tend to oversteer heavily on corner exit, requiring a reduction in throttle if you are not to spin.... It isn't much quicker, honest.
The optional diff we supply is a Torsen unit, the aftermarket unit supplied by Motorsport is a plate diff, with a similar 2:1 bias and no preload so in terms of action it mimics the Torsen closely.
Aerodynamics
Right, pin back your lugholes and settle down at the back, let's have a quick aerodynamics lesson. I learnt something today: the Lift co-efficients used by motor manufacturers to describe the lift or downforce generated by their cars are NOT related to the plan area of the car as I thought, but to the frontal area. This is because by its very nature, a larger plan area will generate more lift than a smaller one because of the inevitable difference in air velocity above and below the car.
Lift negation devices like wings, diffusers and clever body profiles are merely elements that affect the lift and are not effectively driven by the plan area, so it makes more sense to isolate them from the plan area when non-dimensionalising them.
Still with me? Good. To this end, by convention, Clf and Clr are therefore related to the vehicles frontal area, which has the effect of making the calculation of lift forces much easier, but at the expense of making comparisons between Cd and Cl co-efficients meaningless (not that I can think of any reason why you would need to compare them...)
So to get to the bottom line, the equation for force in Newtons is:
F = 1/2(pVVSC)
- p (the closest I can get to rho on this keyset) is the standard density of air, use 1.225kg/mmm for convenience .
- V is the velocity of the airstream in m/s
- S is the frontal area (1.6mm for the Elise)
- and C is the lift or drag coefficient.
Please note that for the above VV meand V squared, mm means metres squared and mmm metres cubed, again the fault of this useless keyset.
Assuming airstream velocity equals vehicle velocity and using 100km/h as the speed then the S2 generates about 150N of downforce at the front and 300N at the rear, there are 9.807N to 1 kg, so these figures equate to about 15kg of downforce at the front and 30 kg at the rear.
Now, lets see who's still awake. What is the S2 front and rear downforce at 200km/h? A clue, it's not 30kg and 60kg.....
(later post, same thread)
Sadly recovering old data from the archives here can be a slow and painful process, and despite several attempts I have still not been able to find known good data for the 111S and S160 configurations. What I do have is standard Elise data, which is for the base 118PS car at standard ride height 140/140 F/R, roof on.
Cd 0.408
CdA 0.653
Clf -0.30
Clr +0.53
which can be compared with the S2 figures (at 130/130 F/R roof on)of
Cd 0.407
CdA 0.651
Clf -0.02
Clr-0.04
As you can see we have reduced the front downforce and the rear lift on the S2 to achieve almost neutral balance which delivers consistant handling balance at all speeds, while the S1's high front downforce and rear lift contributed greatly to the cars tendancy to oversteer at high speed. It could be argued that reducing downforce is a retrograde step and that leaving the front in S1 configuration and then increasing the rear downforce further to achieve a Clr in the region of -0.3 would generate more lateral rip, but testing showed the drag penalties associated with doing so were unacceptable.
As I said earlier, I cannot be certain of the quality of the data, so don't hold me to it, but the figures I have uncovered suggest the 111S rear wing reduced the Clr by only 0.03 with a 0.05 increase in Cd, while the S160 wing apparently reduced the Clr to about + 0.35. I have no Cd figure for the S160 wing I'm afraid. The problem with the rear wing on the S1 is the poor quality of air fed onto it over the roof, which is why we lowered the roof on the S2.