HansB

Looking through my contributions of 2007 I saw I am overdue on a promise to publish some results on my wide body 951. I have done the widebody rebuild last winter, with the purpose to make the car faster with the same HP. Faster laptimes for less money, so to say.

To refresh your memory, we are talking about this car: 2.5l, 1050 kg, 340 crank HP at 1.0 bar (2350 lbs, 300BHP, 14,5 psi imperial)



The assumption was that lowering the Centre of Gravity, widening the Track Width and mounting wider tires could make the car substantially faster. Looking at what was realistically achievable we opted for 1.25 inch CoG lowering and 5 inches additional TW.

This was achieved by:

• F: custom A-arms, with raised subframe mounting points, and completely taking away the caster mount flanges
• R: Kokeln rear axle, Coil over suspension
• F&R: shortened shock absorbers and springs, custom 10-10.5 “ rims
• And topped of with widebody bodyparts

All very nice, but does it work?

First Question off course: Should it work in theory? There have been a number of PhD thesis written on the subject, but in essence it boils down to tire characteristics: The efficiency of a tire decreases as the vertical load per square inch increases:




In practice this graph means, that when you incline in a corner, the gain of traction on the outside tire is less than the loss of traction on the inside tire. By lowering and widening a car, you reduce the arm of the lateral force, with that the moment, and thus the lateral weight transfer.



Short: WB increases the G’s you can pull. Lateral acceleration is the formal word for G´s.

With some simple first order approximations it can be calculated, that the changes we have made, in the chassis, combined with the 10% wider tires (285 F, 305R) should result in an increase of total cornering force from 0.1 to 0.15 g, and an increase in cornering speed with 4-5%


This summer it was time to put the theory to the test. In August we spend two days on our favourite track, Spa/Francorchamps. As instrument I used a DL1 datalogger, which produces accurate results (speeds and g’s better than 0.5%) with dual accelerometers, combined with GPS. I could compare the results of the new setup to last year, where we spend some time under very similar circumstances.

To avoid random effects I took one turn, a double apex fast left sweeper called Pouhon



It is an interesting turn. The first time you drive it, it looks and feels like a highway exit, which you would normally drive at 80-90 km/h, say 50 mph. Over the years we came here, we have become more courageous, also helped by the removal of the intimidating gravel runoff area by a nice asphalt one, courtesy from F1. We now enter and exit with speeds over 185 km/h (115 mph). The trick is that if you turn in at the right point, you can keep your steering wheel in one position. So, once you have turned in you need very little steering action. Ideal for a lateral acceleration assessment.

In the next diagram I have plotted the lateral acceleration.



In the frequency diagram the vertical axis is the percentage of time that you spend at a certain level of lateral G. To get some statistical relevance, I used data from 6 passes through the turn. This frequency diagram shows the before and after results. You can clearly see the improvement from the new WB setup (orange) as compared to the old NB setup (black). You can observe that the peak in the acceleration distribution went from 1,12 – 1,15 to 1,24 – 1,27.
Maximum lateral acceleration value increased from 1.30 to 1,42 – 1,45.

So the practice concurs with the theory. The achieved improvements are in the expected range of 0.12 - 0.15


Let us now look at the resulting corner speeds.



In this diagram, that shows a number of passages through the Pouhon turn you can observe that this years curves (blue and green) are substantially faster than last years (orange and black).The minimum speed at the clipping point went up with a bit over 10 km/h (6 mph). The fastest exit speed observed overall showed a similar improvement from 192 km/h to 202 km/h.

I did obtain similar results in increase in the minimum speed in clipping point in a number of other high-speed turns. Eau Rouge: 155 to 167 km/h. Blanchimont 2: 178 to 192 km/h.

So, conclusion 1, does the lower – wider approach result in improvement in speeds and lateral acceleration: Yes, it certainly seems to do this, I got the expected results.

Does this lead to conclusion 2, to faster lap-times? Yes, if I compare these two track day sessions, driving with passenger, old tires, lots of fuel, lots of traffic. The best lap times went down 5-6 seconds. The real proof would be under racing conditions, lighter and with new tires. Last year my racing lap times were 5-6 seconds faster than my track day times. I did beat my fastest racing lap time in this track day session. (2:48.812 with over half a sec 2:48.25). The real proof is to get the same 5 sec improvement in racing conditions, so to go to 2:43 like times, and that proof has to wait.
For that I will also have to improve on my courage and driving skill. Using the 10 km/h = 3m/s advantage in the turns means braking 10 m later, which with speeds over 110 mph is scary…..


Conclusion 3: Is this a cost effective way of improving lap times? Well, by very unscientifically comparing lap times from different cars at Spa/Francorchamps it seems that an additional 10 HP will gain you a second at the current 3.5 – 4 kg/HP level. For a gain of 5 seconds I would need an additional 50 HP, so 390 crank HP, like the original 911 GT3 Cup. To build an engine that will last for over 150h track between rebuilds, this would probably mean mean going to a 2.8 /3.0 l drysump setup, with a cost of over $10,000.
The WB approach as presented here cost for me EUR : €7,000. But then I already had 3 pcs BBS’s and shortened coil over suspension. If you would do it in the US the cost would be probably over $10,000. So, not a lot cheaper than a plain engine upgrade. Biggest advantage is, that this is a one time cost, no regular rebuilds required. So in the end, for me this is a good step.

The engine I am going to do anyway sooner or later. Somebody has got those 911 GT3 RS guys some modesty, no?

Finally, I can recommend every track enthusiast a good data logger. Very instructional, you will quickly find out where the seconds are lost.

Lorax

Dont you know better than to post empirical test data on rennlist?

Good job!

HansB

Could not resist...

Rich Sandor

great post Hans! I just saw the pics of your custom suspension on your home pages. Do you have any more detailed pics of that?

HansB

Yes I have. I have been preparing a post on that.
Soon coming to this theather.....

pjburges

I wish more people would be more open about what they are acheiving in these cars and how theyve done it. I mean...hell - this is 30 year old technology, and the racing classes arent near what they were in the late 80's and early 90's for these cars IMO, even though they still are widely raced and lots of investment is at stake.

Perhaps this is the engineer coming out in me, not the businessman/race driver.

Great approach though, it obviously worked quite well!!! A 3-5 second increase from a 1" lower CG is pretty incredible!

Geneqco

Excellent post. Thanks for the info Hans. With your widebody setup, why did you limit rear tyre size to 305? Was it just because you already had the 10.5" rims or was there another reason you wanted to limit it to that? How do you think wider rear tyres would affect your results adn the balance of your car, especially once you do some engine upgrades for greater HP?

Thanks

2bridges

X2 - reading my mind.

Widebody setups I am familiar with run up to 3 inches wider in rear, 10.5 by design or cooincidence?


BTW: Nice post hans

evil 944t

Nice post Hans! Good job on the data.

Matthew DiRenzo

Well done, although the results are in no way surprising. The simple physics, as you hypothesized, dictate that those mechanical improvements will have such an effect. I do appreciate the satisfaction that comes with empirical verification, and applaud you for taking the time to quantify the improvement. Should you be motivated, the next step would be to quantify how much the increased frontal area cost you in terms of aerodynamic drag. Your data system should allow for that experiment given some simple before/after comparisons, and perhaps, a controlled coast down test. Probably a lot more than necessary, but if you're interested... Good luck!

thingo

Interesting reading, what was the tyre size before you went to 285f 305r?

krazE

good stuff here, almost like a cheat code in a game.


i think most people use the "drive it like you stole it" theory.

A.Wayne

Hans, fantastic post , with very good supported data. Could you confirm, where you have posted , that you believe 10 bhp to be good for a 1 sec a lap reduction at spa,is this correct ?

HansB

It is also the engineer in me that wants to share. Let us put our money where our numbers are. It is a pity that lots of people here waste there hard-earned cash on things that do not work.

It is not just the lower CoG, wider TW is an equal contributor.

HansB

The rear tire size is limited by design. I have measured my tire temps for quite some time now, and with my current HP, 285 R is probably the best.

In the previous setup 245F 285R the temperatures of the F tires were 20-25 C hotter as the R. In my current setup for the first time I get similar temps F &R. I find it strange that lots of people complain about understeer in a 951, but insist on driving narrow tires F. I can see no logic here. My setup is pretty neutral to over/under steer, though I could use some more stiffnes F.

The 10.5 R rims were intended to be 11", and in fact a combination of a measurement error and a mistake in the assumptions on how the BBS rims work. I could not find a BBS rim calculator, so I made one myself. Only after I took the rim apart I saw that I made an error here.

With 40 add HP I could perhaps use wider tires. Lots of space, just have to change the rims again. the temperatures will tell.