Thought we might kick this around for a bit, have a look at the pic below.



You will notice that the lower cams in the 4V and 2V have the same height, however that is where the similarity ends, the top of the cam box on the 2V is the same height as the intake ports on the 4V.

I weighed my 2V components and these are slightly lighter than standard parts but essentially similar to stock in terms of what is shown. The intake will hopefully be carbon so the entire intake should weigh no more than 5 kgs or 10 pounds, the bulk of that will be the throttle bodies near head, again keeping the COG low.

So the purpose of the thread is what is thought of having 20 kgs less at the top end of the engine? That is just the difference in the major components which the 2V 928ers have, this is not counting the intake system. The difference with the intakes would be another 10 kgs bring the total to 30 kgs or 66 pounds at the highest point of the engine.

What are the thoughts on the extra weight and higher COG effect and different polar moment on the car?

I believe we need to consider weight balance fore and aft and its effects on braking and acceleration and change of direction, that will be the big one if I am right. Please consider both the 20 and 30 kgs difference, here that would be a bag and half of cement on top of the engine.

Now we are a talking a circuit car here, not drag racing or open road racing, all other parts of the car are equal. I suppose we can throw in another question, how much extra power does the 4V car need to equal the 2V? Any other interesting questions?

 

Hi Greg. The only place you will see anything is in straight line acceleration from decreased weight, which will be very slight. 100 lbs is 0.1 second in ET in drag racing... You won't notice (and probably couldn't measure) any change in dynamic handling.

 

Concur with John. The vertical polar moment of inertia difference is on the order of a few hundred grams/CM.

 

Really surprised by your responses, my belief is the straight line is where it is least noticeable. I will try to find an article I read years ago about the study Renault F1 did. They had two very different engines, like ours, one with high COG and the other with low COG. One was 72 degree and the other was 111 degree. Given how light these engines are anyway they noticed big differences and studied the effects. Now there was no difference in the weight just COG.

It of course is not just F1, Ferrari insisted on a plastic rear window in the convertable and Porsche just fitted the Panamera with magnesium window frames. On Top Gear when Usain Bolt did his lap the audience was told that if he was lighter he would have been right up the top of the list, or another 1 second or so faster.

 

The height of the CofG affects performance in various ways.

The most important one is increasing the weight transfer side to side. This, in turn, make the tire temp less stable, getting too hot in the outside or to cold in the inside (or both). Distributing this weight transfer front to rear (via anti roll bars) is the easiest way to balance the car.

The handling difference will be neglectable, but the average lateral acceleration will be a touch smaller with higher CofG, and laptimes will suffer a bit.

About straight line, figures are easy, 30kgs are like 4% of the total weight, so 4% of power increase (12-15cv over stock figures) will compensate. In fact it will be better, due to increased top speed, with similar acceleration.

At the track, heavier vehicle will need more rubber to keep temps in the window. Higher CofG will decrease average lateral acceleration, and there is no way to compensate that.

So far the theory. In real life, this is true only when you have everything perfectly under control. For instance, if you are running too big of a tire or too hard a compound, or to weak rear brakes, increasing the CofG height will in fact improve laptimes, because it will help to put the tire inside temperature window.

To sum it up, I would say (personal feeling) that extra 4% weight AND higher cofg height will be compensated with extra 4% power, unless you're never full throttle (rallying or slow tracks) or your tires or brakes are already fading badly after second lap.

 

From Wikapedia, regarding the BMW CSL, I thought it was interesting and relevant.

This from before BMW developed the M3 CSL, it was the prototype back in 2001

I do some more research when I get the time.

 

My .02 is that COG difference (weight neutral) will have no effect on acceleration, slight effect on braking because of dive, but most effect on handling - taking turns, body roll. Same reason why a top heavy 4X4 will roll-over in safety tests and a go kart won't.

Experiment - hold a baseball bat upright with a ten lb. plate on it at the hand grip area and move it fast left and right, now move the 10 lb. weight to the top of the bat and then repeat movement, going to be a lot harder to control.

Width of wheelbase, width of tires, and lower COG all help cornering and braking stability.


but since you asked for questions - will this car be completed before the little fella in your avatar gets his drivers license?

 

30lbs off the top of the engine that sits relatively low in the car will have negligible affects in roll. Also, this isn't an Audi with the engine front of the front axle so it's not going to have a big affect on the polar moment either.

Yes, in F1 30 pounds off the engine would be huge.

 

shaving hairs, in a BIG WAY! no difference to notice in reality. now, if you moved the engine rearward by a foot, that would be substantial to feel.

 

Engine in front of front axle, bad.

Engine behind front axle, not bad.

Saving some weight is always great (for racing), but saving weight there would likely be minimal in gains. If you can lose unsprung weight, or weight before the front axle or after the rear axle, you have more immediate benefits.

 

Just for the record, it is 30 kgs = 66 pounds not 30 pounds quoted. The F1 car in the study does not loose 30 pounds, I will look for the study today and post the numbers and one must remember that in an F1 car weight is not saved it is just rearranged.

Weight optimization is a big thing in race cars, Paddy Lowe said when asked by Max Mosely in an FIA hearing into the Ferrari spy scandal said that a great amount of lap time is found by weight optimization for any particular chassis. Current weight balance in F1 cars just for the sake of interest is close to 48 front 52 rear. This tends to change year to rear with the change in tyre sizing. That part is relevant to us.

By 123 Quatro

 

IMO the 32V cars wash more in really tight AX course turns. I think RKD in OKC just did a little more rear bar to make his GTS handle the way he wanted. Obviously, if you are running as much tire as MK, this is not a problem, but there is a difference in handling that you can feel.

BTW, this is my subjective opinion, and it is a stand alone observation. I am not debating the point. There is a reason people go to bigger tires, etc. You wouldn't run a 225 on a new Viper or Vette.

 

I also have the bigger front DEVEK anti sway bar.

 

what I said above in eggheadese - from wiki

Center of gravity height

The center of gravity height, relative to the track, determines load transfer, (related to, but not exactly weight transfer), from side to side and causes body lean. When tires of a vehicle provide a centripetal force to pull it around a turn, the momentum of the vehicle actuates load transfer in a direction going from the vehicle's current position to a point on a path tangent to the vehicle's path. This load transfer presents itself in the form of body lean.

Height of the center of gravity relative to the wheelbase determines load transfer between front and rear. The car's momentum acts at its center of gravity to tilt the car forward or backward, respectively during braking and acceleration. Since it is only the downward force that changes and not the location of the center of gravity, the effect on over/under steer is opposite to that of an actual change in the center of gravity. When a car is braking, the downward load on the front tires increases and that on the rear decreases, with corresponding change in their ability to take sideways load, causing oversteer.

Lower center of gravity is the principal performance advantage of sports cars, compared to sedans and (especially) SUVs. Some cars have light materials in their roofs, partly for this reason. It is also part of the reason that traditional sports cars are open or convertible.

Body lean can also be controlled by the springs, anti-roll bars or the roll center heights.
[edit] Center of gravity forward or back

In steady-state cornering, because of the center of gravity, front-heavy cars tend to understeer and rear-heavy cars to oversteer, all other things being equal. The mid-engine design offers the ideal center of gravity.[citation needed]

When all four wheels and tires are of equal size, as is most often the case with passenger cars, a weight distribution close to "50/50" (i.e. the center of mass is mid-way between the front and rear axles) produces the preferred handling compromise.

The rearward weight bias preferred by sports and racing cars results from handling effects during the transition from straight-ahead to cornering. During corner entry the front tires, in addition to generating part of the lateral force required to accelerate the car's center of mass into the turn, also generate a torque about the car's vertical axis that starts the car rotating into the turn. However, the lateral force being generated by the rear tires is acting in the opposite torsional sense, trying to rotate the car out of the turn. For this reason, a car with "50/50" weight distribution will understeer on initial corner entry. To avoid this problem, sports and racing cars often have a more rearward weight distribution. In the case of pure racing cars, this is typically between "40/60" and "35/65".[citation needed] This gives the front tires an advantage in overcoming the car's moment of inertia (yaw angular inertia), thus reducing corner-entry understeer.

Using wheels and tires of different sizes (proportional to the weight carried by each end) is a lever automakers can use to fine tune the resulting over/understeer characteristics.

 

After searching countless magazines, I didn't find exactly what I was after, I did find that I made a mistake in the bank angle, it was 106 degrees not 111 degrees although Renault officially wont say exactly what angle it was. Bob Bell Renault's technical director said that the COG was less than 10 mm difference between the 72 degree engine and the difference was measurable but sacrificed due to reliability issues with the 106 degree motor and the need to make the engines last longer under the rules.

Now the COG change was when measured in terms of the car not just the engine. He said that it was the slow speed handling or mechanical grip that suffered. That is probably quite relevant to us as we don't have the 3000 pounds of downforce at 150 mph that the Renault has.

So the change to a different bank angle was a sacrifice worth making according to Renault. I will keep looking for the article but it is not a priority as there is plenty of other things to concentrate on.

By TV
Yes it will be done by then, first is the body, i.e new paint work vents, and CF turret. The full underbody will be last as I will just use the 928 INTL one first up.

Then the interior, bit of altcantara, changing the colour from blue to black and dark grey I think, should work well with the outside colour scheme, which is shieffer blau. I hope this will be done in the new year.

Then the engine, exhaust and the major job there is the intake system and oiling system, I did figure out that CF oil tank. In relation to the intake I have the design but it is a big fabrication job. I am a bit scared of that cost. I have spoken to AT Power, their throttle bodies are 1600 pounds, I would be happy to pay 2000 pounds or so to have them adapt them but the carbon work which will look similar to the OEM design, i.e spider and the current GT3 engine which again looks like a spider but with six legs

Then another very big job in the engine management as it is tied into other systems like the data logging ABS PSD and the video system.

The plan was always to take the car to the UK when I am fifty, visit Nurburgring etc. Only a few years to go