JackOlsen

At the outset, let me make it clear that I don't have much of a clue. But I got a day off on Friday and went down to my local track to do some testing and tuning on my car. I've got a big wing and a front splitter that I bolt on for track days, and I wanted to take some tire temperatures and look at some ride height data to see if I could coax a little better handling out of my car on the track.

Let's start with tire temps. I don't know if it's even a smart goal to want front and rear tire temps to be the same, but I am having trouble getting heat into my fronts. They were about 50° colder than my rears, initially. This was with about a 3° differential in pressure from front to rear. I ended the day getting the fronts within about 25° of the rears, inching up to a 6-lb differential (fronts 6-lbs less pressure than rears).

With the wing, I've been getting an interesting (and slightly frustrating) effect from changing my angle of attack. I have four incremental settings, with a 3-4° difference between each. I have ride height sensors on my car that produce consistent data within a single run, but may be unreliable in comparing one run to the next, since their absolute position is probably not reliable (zip ties hold them in place, so there's probably some shifting around happening when I jack up the car and let it back down). That said, I get very consistent readings that the steeper my rear wing, the more my front end comes up. It's an understandable effect, like a teeter-totter with the rear wheel as the pivot point. The front is still much lower than it would be without the front splitter, but it's 'less low' as the rear wing does more work.

My options to get the front lower (and presumably doing more work) are to back off the angle of attack or (at least, this is what occurred to me) to increase the rake of the car by raising the rear end. I have right about 1.5° of rake when the car is at rest, which is normal for a track car. But I'm guessing that is changing at high speeds when the rear wing is pushing down more effectively than the front. So I tried two cautious changes with the rear ride height, raising it about 1/8" for one run, then up a little bit more, to about 3/16" total increase, for another.

My lap times will generally fall off as the day progresses (more heat in the tires and engine, the track getting hotter, whatever), but in the course of Friday's testing, my average lap time came down very slightly. But we're talking less than half a second average change.

A part of me thinks that getting the most downforce at the rear is going to be more important than trading it off for improved performance up front, because the weight is back there and the rear end of a 911 is really the thing you're 'driving' around the track. My non-expert impression was that by the end of the day I was a little loose in the rear on the lower speed sections of the track (60-70 mph corners), and experiencing some very minor push on the faster corners (95-130 mph). I'm okay with this, and I ran some respectable lap times (relative to what I've done in the past), but I can't help wonder if I could do better if I knew more about suspension set up and tuning.

That's where some of you come in. Am I right to be trying to get similar temperatures front and rear in a 911? I'm told my tires (Nitto NT01) do best close to 200°, and that's where my rears were (although the inside shoulders would typically get about 30-40° above that). Will increased rake provide better grip in the front? From a cursory look at my ride height data, the additional rake was improving downforce in the back, but not having as clear an impact on the front (at lease partly because I need to improve the way my ride height sensors are mounted).

GuyIncognito

by no means am I an expert on....well, anything but I would think that you'd want similar temps across the tread, on all 4 tires. maybe slightly higher on the rear since that's doing most of the work on a 911. but even then I'd think they'd be within 5-10 degrees (please for the love of racing and all that's holy, please experts correct me if I'm wrong)

I heard an interview by the guy who did the ground effects development on the 956 say that you knew your aero package was working when a change on the rear of the car manifested itself on the front, and vice versa; that's why aero balance is so important.

Matt Romanowski

Tires work best at a specific temp (usually around 180-200), so you want them to work hard enough to be at those temps. You can do that through mechanical grip and/or aero grip. For most of us, we're really only working with mechanical grip for two reasons - 1. most corners are slow enough that it's the mechanical grip that provides traction and 2. We don't have real aero packages.

Now that you are playing with both, you'll have to find the sweet spot in both. It's much easier said than done. You can cover mechanical grip issues with downforce very easily. The best way to work through this is either work with an engineer who's "been there, done that" or to do lots of testing. You want to have a good mechanical balance and then add aero to compliment the car.

My suggestion: If you're going to use any sensors, they need to be accurate and repeatable (more than accurate). It's a classic case of GIGO.

J richard

Jack,

IM(VERY)HO, you will never see temps consistent F/R in a 911. If you consider the static balance of the car at 45/55 you would at best be within 10% and the corners would be working as well as they could. Added the fact that a 911 is driven at the rear of the car it is doing more of the work and will have higher temps, it's the nature of the beast. Just look at pics of your car in action and consider the amount of time three wheeling. That tire is cooling off in the breeze...

Aero is a tricky thing, I'm not sure tire temp is the right metric, I think it would be more a balance between downforce increasing mid corner speed and aero drag at the top end on the straights. Handling balance and speed trump tire temp.

winders

Jack,

I would be looking at car balance and the stopwatch. What good are perfect tire temps if the car handles terrible and is slow?

Scott

analogmike

Are you running the same pressures front and rear? Lower front pressure could help add heat in the front tires. I would not be raising the rear end on a 911 unless it was pushing badly. I take it the front is as low as it can go?

333pg333

How big is the splitter and do you have Canards? Can you go larger? Do you have underbody aero? Skirts?

bobt993

Jack, I have been working on some aero over the later part of last year and a couple of tuning parameters I am using. Most DAS systems allow pressure/ride height sensors that will help quantify downforce. Without sensors you can look at your G plots throughout a corner and recognize the increase in sustained G's along with the typical oversteer / understeer traces. I also then compare the friction circles of before and after changes to see how the outer edges are populating. I see aero effects on corners exceeding 75mph being pretty obvious. You also want to quantify your drag effect. You can do this by rolling off the throttle on a front straight. The technique is define pretty by Buddy Fey in "Data Power". Keep a diary to for confidence effect felt as you get aero results. You want a stable car the actually is faster and predictable.

winders

This is pretty simple really. The chassis needs to have decent balance at low and high speeds. You have more aero downforce at the back than the front. Adjust the mechanical grip so the low speed balance is right. Your front aero is fixed so adjust the rear wing until your get the high speed balance you want.

Adjust tire pressure (within reason) to get the temps as close as you can.

If you move the wing closer to the rear axle, you will reduce the cantilever effect....you get more rear downforce with the same lever effect.

Scott

JackOlsen

Thanks for the replies so far. I'm going to type up some follow-up later today, but in this minute I've got right now, here's a video of the last session of the day. It's 10 minutes, and a computer glitch on my part means the frame rate is too slow. But it will give you all another data point to (maybe) help assess the car's handling.


Thanks! I've got more questions I want to ask, but I'm right between child care tasks.

IcemanG17

great thread

I also agree that the goal of even tire temps probably is unattainable in a 911....with much smaller fronts and such a rear weight bias you probabluy would end up with a near street alignment to get even temps in the front tires, which of course would handle horrible

I'm with scott...I would worry more about peak grip and balance-feel than actual temps.... If your tire temps suck front to rear but your lap times improve....honestly who cares.....the reverse is true as well....you get nice even temps, but lap times are slower....thats no good either....

granted my racer is NOT a 911....but about 52.5% front.....running square 275 tires all around at about 2850lbs off track... Even at my relative low HP (243whp) I find I get consistently more heat in the rears vs the front...with a delta of around 20F at every position..... I haven't tried aero yet.....that is NEXT

One last thing....have you read "chassis engineering" by Herb Adams....great book I learned a lot from it

tlarocque

Sick car, have long admired it. Get a HANS though.

J richard

+1000

winders

Have you tried not shuffle steering?

Scott

JackOlsen

Thanks for the input. This is very helpful.

The rule of thumb (as I've seen it play out in my testing, at least), is that whatever you do to the front generally improves the numbers at both the front and rear axles. A wing in the back only gives you downforce at the rear (although I think the testing on the early ducktail also showed some improvement at the front axle -- but it's a spoiler, not a wing). Rake is one of those things that -- I think -- can help the aero at both the front and the rear, although at a certain point you've gone too far (a kind of stall, maybe?) and it has some weird aero effects that make the rear end get light at high speeds.

I already had 1.5x a stock model's rake -- which I'm told is typical for a track set up. So I added a little more, cautiously, and it did improve the tire temps up front.

Of course, it's possible that I was also seeing the heat sink effect, where the tires don't cool down as much between runs and it appears as though they're getting hotter, when in fact they just started out hotter.

They're surprisingly consistent, considering they're $5 parts pulled off of an 18-year-old Lincoln Continental. But when the car is jacked up, I think it sometimes shifts their position slightly, and it makes it misleading to overlap runs made before the jacking with ones made after.

I've got an idea to improve my readings, putting two more sensors in place (so they're on both sides of the car), and then averaging the readings from each axle.

I suspect you're right. The car is lapping the track as fast as it ever has. I'd hate to make it even more tail happy (mechanically) only because it evens out those numbers. The problem I have is not knowing what the delta would normally be, front-to-rear, for 911 tire temp readings.

I agree. I didn't see a drop-off in lap times yet, but that might happen if I keep working to equalize temps. I've thought about loosening up the front swaybar, but that might fix the temp differential and also make those slower corners really crazy.

I started out with the fronts 3-lbs lower than the rears. By the end of the day, I'd bumped that to a 6-lb gap. (I made a mistake in my initial post where I used the degree sign when I should have written pounds -- I fixed it.)

Yes, the front is pretty low. But as I've been told, rake really only changes a car's aero, with the other effects pretty minor.

The splitter comes out 5" from a vertical curtain that goes up to the forward-most portion of the bumper. And yes, I do have canards to generate a vortex along each sidewall. I've also got a flat sheet of aluminum running from the nose to the back axle. I've thought about lining the front wheel wells to improve the aero there, although I'm not willing to cut venting holes in the top of my front fenders, at this point. (Is there any other way to relieve the high pressure around the wheels?) I've also thought about lowering aero dams down in front of each tire, although the splitter is already pretty low and it might have an adverse effect on the air scoops I use to cool muy front brakes.

I've got a stable car that's predictable. I've been working on aero stuff for it for a decade, now. But if there's anything I'm leaving on the table, I'd like to learn about it. I'll look for that book.

As I understand it, putting the wing farther back gives the thing mechanical advantage, and puts it in a more effective zone for air. But I could be wrong about that. Why does everyone run their wings so far back if there's no benefit to it? (I don't ask that to sound like a smart-*** -- I'm genuinely curious.)

I don't want to start any arguments, but I've got an Isaac, which I believe offers more protection than a HANS.

Will it make me faster? In other cars, my hands stay planted. This is an early car with no power anything, and the front track has been pushed out by 70mm. It's actually pretty nimble around town, but it's work in the sweepers.