November 15, 2014

[Loren]

Not great but it had better feedback at the low PSI. Not sure how that works.

Here's how it works:

The street tires that most autocrossers tend to think about these days are the high-end "summer" "ultra high performance" variety. They all have really stiff sidewalls and tend to respond to tire pressure adjustments in the same ways.

The difference you feel in all-season tires has as much to do with the tire's construction as it does with the rubber compound. Your tires have softer sidewalls. This is readily felt, and we usually want to add air pressure to prop the tire up and improve the steering response. We also do that to keep the tread from rolling over onto the sidewall in hard cornering. (don't forget to CHECK that closely if you've lowered pressure) What we don't often think about is that not only is the sidewall less stiff, but the whole "carcass" of the tire is less stiff, including the part supporting the tread.

Raising the tire pressure beyond a certain point causes the tire tread area to balloon out. Lowering it beyond a certain point can cause it to go concave in the middle. EVERY tire does this. All-season tires do it a lot more.

Every tire/vehicle combination has a tire pressure "sweet spot" that you have to find. Ideally, it's that place where the contact patch stays as flat as possible. The best way to find that is with a tire pyrometer and try to get the tire TEMPERATURE as even as possible by adjusting the tire pressure.

So, what we can conclude is that whatever tire pressure you were running before was above the "sweet spot" of your particular tire on your particular car with your particular setup. The tread was bowing out, effectively giving you less contact patch and less grip. Lowering the pressure got you into the sweet spot. More contact patch, more grip. And because it's an all-season tire, you get the side-effect of less steering response.

Now, this is important: The relationship of "less pressure = more grip" is NOT always true. It depends on where you are relative to the "sweet spot". If you're already at or below that magic pressure point, going LOWER will decrease grip. That's why we always recommend that people start with pressures that are artificially high. Start BEYOND the sweet spot (also saves your sidewalls vs. erring on the low side), and bring pressures down slowly to find the grip.

[lddavis91]

Loren, you linked Sean run twice though you labeled the second one as my run. Here is my real run

https://www.youtube.com/watch?v=dpNghzzVxuw

Fun course and a great day. That little Vee on fresh "purple crack" is a whole new level of fun. I was gradually scrubbing the molding compound off the tires as the day went on and actually did manage to find and exceed the limits on my fifth run as the course workers can attest (sorry course workers). A more perfect run from me in that car I could have been a bit more aggressive in the first segment and was on my fifth run, that car slaloms like you wouldn't believe. I also probably should have downshifted to 2nd in the turn around for a bit more acceleration coming out, but shifting isn't really the "forte" of that car so I'm a bit skittish to throw too much more shifting in if I can help it. Generally, I'm a believer in the "tight is right" philosophy, but in this case I slightly "looser" line through the turn around might have paid off since that would have allowed slightly more exit speed into the fairly long straight. Tough to say sometimes, but in this case, probably so. All in all I think with a couple more cracks at it I could have gotten deep into the 43.XXXs which still wouldn't have been fast enough to "beat" Greg according to Greg. ;-) According to him I needed to beat him by 7 seconds and would have required a 41.8, probably not going to happen even with Darren Seltzer behind the wheel.

Anyway, I'm off to the FL state championships next Sunday with Sean, Jesse and a few others. There is actually another Solo Vee signed up so it will be interesting to see how I stack up against another car that is actually similar.

[Loren]

Sorry, Les. I'm usually better at proofing than that!

[twistedwankel]

Miatas after 1994 have a "fake" oil pressure gauge. It uses the same sensor as an oil light. Trips at 7 psi. You either have oil pressure and the gauge reads in the middle, or you have less than 7 psi and the gauge drops to zero.

But, a well-maintained Miata engine is good for 300,000 miles or more before it needs a little head work. The bottom end is stout!

Yeah. My RX8 uses that same gage and sender. I note that Jamie's car never shows any blue smoke under stress. I also have a lame temp gage with irratic higher meaning. Doubt it matters unless one road races or never checks their oil/water? Just goes to prove Mazda learned not to upset the normal mechanically brain dead drivers unnecessarily:) I see their point. They are tops in customer satisfaction and reliabily again now that Ford doesn't call the shots. All my first gen RX7's were bullet proof just like the Miata.

[gammaradiation]

Id like to thank everyone involved in finding me a battery tie down. Even though I failed miserably on the course due to inexperience, lack of sleep, and lack of breakfast I still had a good time. I will be back with an OEM tiedown, more camber, and breakfast in my belly.

[Carracer]

Not great but it had better feedback at the low PSI. Not sure how that works.

Here's how it works:

The street tires that most autocrossers tend to think about these days are the high-end "summer" "ultra high performance" variety. They all have really stiff sidewalls and tend to respond to tire pressure adjustments in the same ways.

The difference you feel in all-season tires has as much to do with the tire's construction as it does with the rubber compound. Your tires have softer sidewalls. This is readily felt, and we usually want to add air pressure to prop the tire up and improve the steering response. We also do that to keep the tread from rolling over onto the sidewall in hard cornering. (don't forget to CHECK that closely if you've lowered pressure) What we don't often think about is that not only is the sidewall less stiff, but the whole "carcass" of the tire is less stiff, including the part supporting the tread.

Raising the tire pressure beyond a certain point causes the tire tread area to balloon out. Lowering it beyond a certain point can cause it to go concave in the middle. EVERY tire does this. All-season tires do it a lot more.

Every tire/vehicle combination has a tire pressure "sweet spot" that you have to find. Ideally, it's that place where the contact patch stays as flat as possible. The best way to find that is with a tire pyrometer and try to get the tire TEMPERATURE as even as possible by adjusting the tire pressure.

So, what we can conclude is that whatever tire pressure you were running before was above the "sweet spot" of your particular tire on your particular car with your particular setup. The tread was bowing out, effectively giving you less contact patch and less grip. Lowering the pressure got you into the sweet spot. More contact patch, more grip. And because it's an all-season tire, you get the side-effect of less steering response.

Now, this is important: The relationship of "less pressure = more grip" is NOT always true. It depends on where you are relative to the "sweet spot". If you're already at or below that magic pressure point, going LOWER will decrease grip. That's why we always recommend that people start with pressures that are artificially high. Start BEYOND the sweet spot (also saves your sidewalls vs. erring on the low side), and bring pressures down slowly to find the grip.

Thanks for explaining this. I just figured ~31 would be way to low for the tires to autocross but they seem to like it. I was at 33~34 before and I noticed that for street driving the car felt best at 30~31 so I figured I would try it.

The grip was better but what surprised me was the feel was better as well ( compared to 34 PSI starting pressure ) after the cars 2rd run. On my last run the car felt really good.

[Loren]

What will really blow your mind with some of the high performance tires is that they will often not feel perceptibly different throughout a WIDE range of pressures. I first learned this with the old Falken Azenis. You could run them at 26 psi or 36 psi and they really wouldn't feel that much different, and the sidewalls were so stiff that they wouldn't roll over. But, there was still a sweet spot somewhere in there that provided the best grip. (and even that was a pretty wide range... at least 4-6 psi) For THAT tire, the happy grip range was closer to 40 psi, though.

A lot of people get too wrapped up in tire pressure. SOME tires are sensitive to minute tire pressure changes (generally not street tires), and SOME drivers are good enough to detect and take advantage of that difference. (and these should be the same drivers that are good enough to drive fast IN SPITE OF the tire pressure being slightly off) But, generally speaking, if you're "in the ballpark", plus or minus 2 psi isn't going to make or break you... and that .5 psi adjustment that people make between runs is all psychological.

Example: I was running consistent times low 50's at this event. On my last run, I was trying very hard to get it all right and beat the 49.8 that I knew Rob had ( didn't even know about his 49.6). And I did. I put it all together and nailed a 49.2, a full 8/10ths faster than my previous best.

Now, if I was a "tire pressure guy", keeping a log of my tire pressures and checking and changing them between runs and all that... if I'd made a 1 psi change before that last run, I'd have said "oh, that change in tire pressure made a difference"! But, I didn't make that change to tire pressure or anything else. It was all me... and that is almost always the case!

My tire pressures aren't even optimal. We tried to get them to optimal by checking tire temps, but if we go low enough for the front tires to have even temps, we get too much shoulder wear (because my wheels are slightly too narrow for the tires). So, we're running 5psi more pressure in the front. I'm at 35 front and 30 rear... and those are cold pressures. What are they hot? I don't know and I don't care. I don't clutter my mind with such things. Knowing the "correct" cold pressures is enough info to allow me to be relatively consistent with the setup, that's all that matters. I've found something that works well and I stick with it. After 4 months of fiddling with alignment, shock adjustments and tire pressure, I'm at the point where I "just drive the car".

[Tim_M]

Fun event even at Brooksville!

I hope the newly minted Autocross School folks all felt better after getting an actual event under their belt...I rode with a couple and the speed of a true autocross was likely pretty exhilarating for them (and I!).

#2 Joshua W. was getting it...cut ~5 seconds off his time in 5 runs...best run was by himself which is a great indicator.

Glad my Subaru was back together with a couple of clunks, but the clutch is likely fully bedded in now!

My best run...just call me hamfisted!

https://www.youtube.com/watch?v=3eVP_0d597k

Can't believe Drew put me on the trailer in the afternoon...great driving.

First real event with a Go Pro...I can now obviously see how far I am from the majority of the cones...some work to be done. Pretty handy little deal (expensive though!).

Jamie: I believe you actually drove both laps...one with a wig and one without... Also, 200,000 is a milestone...I turned 167,000 (didn't take a picture) on the way home with the Subaru...I'm proud of that, of course.

[Jamie]

Wow. 8 times around the world and fantastic oil pressure. Love the "in the pink" gages.

What Loren said about the oil gauge...tempted to replace it with a very large idiot light that I'd actually see if something blew up. And the gauges light up in "yes, I can pretend I'm a PT boat on night patrol" red/orange.

I believe you actually drove both laps...one with a wig and one without....

Could be that Sophia drove both...who'd know under the helmet?

[ExecutiveHydra]

AutoX School Graduate! Joe rode with me on my first run at the school, and then throughout my runs at this event. Scrubbed 6 seconds off my time. I need to stop being afraid to step on the gas pedal and trust the car.

[twistedwankel]

AutoX School Graduate! Joe rode with me on my first run at the school, and then throughout my runs at this event. Scrubbed 6 seconds off my time. I need to stop being afraid to step on the gas pedal and trust the car.

Squeeze on the gas pedal and "trust your tires". You have a lot of torque there young man. Respect it or at least say you do and drive the bitch home. My favorite band on Put-In-Bay when I worked there was "Ben Dover and the Screamers".

[jesup16]

I forgot to post the link to pictures. Here they are: http://www.apexpix.net/p266413820#h4f556d1

[Loren]

Thanks, Jesse!

I hate to gripe about stuff, but in the interest of better photos: I keep seing Sarah's logo on top of things that I WANT TO SEE in the photos. Namely, tire contact patches and sometimes cones. Call me weird, but I like to see what the tires are doing. And I'll be looking at a nice clear photo where the lighting is right and I could see the contact patch... and there's the logo obscuring it.

Just a little something to pass along.

Example:
http://www.apexpix.net/p266413820/e1d5e3b45

But, I still love Sarah and her photos!

[jesup16]

Thanks for the feedback Loren. I think she uses a program for the watermark/logo. I could just have her shift it up one of the corners...there should never be a tire in that portion of the pic.

[Loren]

Yeah, she just needs to think about where the watermark is going to be planted when she crops.

[jesup16]

OR...you could purchase the pic and there would be no watermark. Just sayin...

[Loren]

True.