Help me 'test and tune'

[Jack Olsen]

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).

[Paulporsche]

Can't really offer much advice--just some questions:

Did whoever advised you about 200 degrees refer to both fronts and rears? Can Nitto offer any help? Have you played w/ tire pressures? Have you tried chalking the sidewalls? How about increasing front downforce via additional splitter, valance or trimtabs, while maintaining the rear angle? Revisions to front camber?

Short of an output mapping your acceleration, throttle position and slip angles and track position, maybe your trial and error, "butt dyno" and lap time approach may be your best test.

Either way, I just wanted to say that I never get tired of seeing and reading about your car and garage!

[Dave Colangelo]

I am no expert in this but ill throw some ideas out there.

I know you have a bigger engine in your car and as you know the 911's are rear heavy, my instinct would tell me that the rear tiers would get much hotter than the fronts simply because that is where the power is getting put down and that is where there is most likely more forces being applied from a variety of sources. I would be interested to see air flow over the tires as well, Im sure that the different air flow patterns account for some temperature differences.

I also know that different suspension set ups will improve different things. For example cars that are fast on the straights may not be as fast in the turns so some of your adjustments may be making you faster in the turns but slower on the straights. Some one with more suspension knowledge should chime in here.

Also how are you measuring tire temp? One interesting way I can think is those new fancy laser temp sensors. They care cheap and easy to get and dont actually have to come in contact with the surface to measure temp. I would set up 4 arrays over your tires, connect them to a micro controller (arduino mega should suffice) and log some real time temp data to see what the tire temps are in real time. I would be curious to see if they are getting up to temp in the corners and then cooling off quickly.

Just some ideas.

Regards
Dave