mx5cartalk.com

Firstly, I have researched with the search facility, but have ended up more confused, so please forgive my lack of knowledge.

BARMY has 205/40/17 Yokohamas now. I did a test and tune day at Winton last week. Pressures were set at 32 at the servo nearest the track. After the first session, when I was learning the track for the first time, they went up to 38 all round all on their own. For the second session, the back end was getting loose half way through and the pressures became 40 at the end. I don't know whether this was the right thing, but I guessed that the tyres were creating their own internal heat by squirming around. So I added another 4 psi and was way quicker in the next (dry) sessions. The tyres were 46 at the end of the last of those.

Then it rained. Hard. And I decided I could have more fun at lower speeds in the rain so was the only car to go out. The extra fun bit was true until I reached the edge of my talent when the back end gave way flat in 3rd and I changed up to counter the sideways-ness only to find it did the same thing in 4th except in the opposite direction. Good fun, but not quick! Plus eventually there was an excursion onto the grass. Wiser heads than mine saw what happened and wondered what I'd done to the tyre pressures before going out in the wet. Yup, you guessed, not dropped from the previous 46 end-value and still with the shockers fully hard. Doh!

So please, as well as getting a pyrometer, give me opinions on what I need to do to get tyre pressures right for various different situations.

What type of Yokohama tyre do you have? They make street tyres, S-specs and R-specs, and the preferred pressures vary for each. Assuming you've got regular street tyres, it's best to aim for a hot pressure of 40psi. This pressure is enough to keep the tread flat on the track under various loads. This means starting at around 35psi cold on a dry track, or 38-39psi on a wet track. The tyres will increase in pressure as the air inside heats up due to lateral stress from cornering, longitudinal stress from braking, and brake heat too. You don't need to weave around to heat your street tyres either - they will have good grip from cold. And once you've got 40psi hot, just leave them alone! Lots of "newbies" endlessly fiddle with tyre pressures during the day, and chase their tails in the process. It's better to focus on developing your skills with a stable set up, rather than trying to fix a "set up problem" by tweaking the car. Yes the car will squirm around on track - its meant to do that because you're going beyond the grip available. Revel in it and have some fun with it. Improve your driving skills while you're at it.

Oh and Winton in the rain is enormous fun, especially the sweeper. You'll learn MUCH more about car control at a wet track than a dry one. The car will be constantly moving around under you, and you've got to be quick to react to it.

Guran wrote:What type of Yokohama tyre do you have? They make street tyres, S-specs and R-specs,

A-drive R1s. Thanks for the advice.

davekmoore

I run an NB8A with 15x 7.5 6UL's fitted with Kumho KU36 - 205-50-15

Last Lakeside I ran 34psi cold that built to 38psi hot with almost 7 laps - normally I start cold at 35psi but never had temp reads at that pressure. My PB came from 35psi previously and even though only 1 psi difference I prefer the 35 cold start

As a matter of interest the original set of Kumho's at 35psi gave 325 track laps and 16,000 road km's and those on the front would still be 65-70% and are now on the rear of my NA. As for the rears I run quite a bit of rear 'toe-in' so those were right on the limit and I gave them to a friend who might get one meet out of them! The NB now has newbies all round that have done only one meet

I do know others that run from 26psi up, but looking at their tyres they aren't getting much life out of them

Watch 'Guran' and his 'careful' driving style, if he gets life out of them go for HIS pressures!

Guran wrote:What type of Yokohama tyre do you have? They make street tyres, S-specs and R-specs, and the preferred pressures vary for each. Assuming you've got regular street tyres, it's best to aim for a hot pressure of 40psi. This pressure is enough to keep the tread flat on the track under various loads. This means starting at around 35psi cold on a dry track, or 38-39psi on a wet track. The tyres will increase in pressure as the air inside heats up due to lateral stress from cornering, longitudinal stress from braking, and brake heat too. You don't need to weave around to heat your street tyres either - they will have good grip from cold. And once you've got 40psi hot, just leave them alone! Lots of "newbies" endlessly fiddle with tyre pressures during the day, and chase their tails in the process. It's better to focus on developing your skills with a stable set up, rather than trying to fix a "set up problem" by tweaking the car. Yes the car will squirm around on track - its meant to do that because you're going beyond the grip available. Revel in it and have some fun with it. Improve your driving skills while you're at it.

Thanks Guran. These specs worked perfectly. Plenty of squirming as you say, but mostly controllable even by a learner. The most interesting bit is seeing the car go in four different directions under heavy braking for a couple of the corners. Some of the directions even go towards the corner, which is good.

With some weight out of the car and better tyre pressures I was a few seconds less slow this week. Better drivers with less powerful cars and semi-slicks are quicker than me and I may want to upgrade the tires eventually especially as the car doesn't do the very long trips I used to do. But, apart from more track time for me, the next important improvement is to replace the flat slippy-slippy leather seat which throws me all over the place in the twisty bits. So a grippy bucket seat goes in this weekend.

Meanwhile, since you've proved so accurate with info on tyres, please consider how I deal with this:

The car had a roadworthy a few months ago, for which the ride height had to be raised (it's got Tein Flex coil-overs). After the roadworthy the nearest spannerman to my work lowered it again, after which it was almost undriveable on the road - so stiff that over bumps it was hard to steer it or to avoid jerking the throttle. As it happens this was good at Winton last week (except in the rain as mentioned), but it couldn't stay like that as a daily driver. So although I don't normally do spannering, I had a look at what had been done .....

Yes, he'd partly lowered it by screwing the shocker bodies down into the lower brackets. But then he seems to have run out of available adjustment and decided to wind up the spring seat locks, perhaps thinking this would create more lowering, whereas all it did was take away the lovely compliant and controlled ride the Teins normally give and create instead a car with locked suspension. No wonder I found it "interesting" in the wet with 44psi and nil skill to contend with as well!

Anyway, I've now reset the Tiens to a good lower setting and with springs that can actually move. Because I'm very time-poor this was done between sessions and one shocker at a time at Winton today (yes, I know a corner at a time is very wrong, but not as wrong as how I loosened the seized bits!).
I did the fronts last and, as you'd expect, putting some travel in the front shockers where there was none before has created understeer in the slower corners with the dampers on all four corners set at 8 clicks out of 16. I suppose this understeer might be safer but it does seem to involve a lot of steering input and it doesn't feel quick.

So what do I do to get neutral or tail-happy again (if there needs to be a compromise, as a learner and a wimp, I'd love tail-happy in the slow corners and neutral in the quick ones as opposed to the other way round)?

Good to hear Dave!

davekmoore wrote:a few seconds less slow this week.

I love this phrase!

Better drivers with less powerful cars and semi-slicks are quicker than me

It's a humbling experience isn't it? I was in that position several years ago when I started tracking my MINI Cooper S. It had heaps of power, but I was much slower than guys in more modest rides. With the benefit of more track experience, I can now lap Wakefield Park and Eastern Creek about a second faster in my standard 85kW NA6 than I ever managed in my modified 140kW MINI on R-specs. The benefits of seat time!

Meanwhile, since you've proved so accurate with info on tyres, please consider how I deal with this:

I run on stock NA suspension and don't have any direct experience with coilovers. However, my understanding is that when you adjust the ride height via the threaded body and/or spring seats on coilovers, it will change your wheel alignment. It's probably not a great idea to be playing with ride height on the go like that, unless you also measure the camber, caster and toe, and adjust it too. Adjusting the damper setting is a different matter, because that doesn't change the alignment. That dartiness that you felt under brakes is possibly due to some front toe out. I highly recommend you get yourself a proper wheel alignment ASAP, so that your car is baselined again and returned to more predictable and balanced neutral handling. You can then find some tail happiness with trail braking or a sharp throttle lift (doesn't get much happier than this!). Guys with experience running coilovers could suggest a good alignment setup that would suit your purposes.

Darting under brakes is often a pitch error in the setup where the rear end is sitting up too high relative to the front. For track work the rear should be around 1/4" higher, once you start getting beyond that then it gets darty. You can tame it a bit at the track by stiffening the rear shocks.

manga_blue wrote:Darting under brakes is often a pitch error in the setup where the rear end is sitting up too high relative to the front. For track work the rear should be around 1/4" higher, once you start getting beyond that then it gets darty. You can tame it a bit at the track by stiffening the rear shocks.

I like the darting though.

You obviously haven't taken it to Sandown like that yet, have you?

manga_blue wrote:You obviously haven't taken it to Sandown like that yet, have you?

Sandown sprints today. The darting made aiming it quite interesting over the bump at turn one at 180kmph. Especially when combined with front pad wear that was so bad I had to give up two-thirds of the way through and drive home before I had no brakes left at all. Bit of a shame really because I'd just reached the point where I not only knew that I was really wimpy and slow, but also had half an idea where on the track I could improve (I cheated of course by following better drivers who went round curvy bits way better than me but then 174rwkw lets me catch them up in time to watch them at next corner).

So brakes next then finalise the ride heights and another alignment - unless semi-slicks arrive earlier.

At the end of a track session all four tyres are at around 40 degrees (and psi) with the front rotors at 200 and the rears at only 100. So I'm also wondering whether the darting might be caused by the front brakes doing too much of the work.

I drive an AE82 and went to a track day at Winton last year. I used Kumho KU36 tyres and found them to be a bit loose with higher pressures. I ran the car with 30psi all round, i found the grip wasnt progressive at all and loved understeering!

I lowered the front down to 24 and the rear to 26 and it was much better, the tyres leveled the pressures out themselves due to the extra stress on the front and felt grippier and more progressive. I ended up shaving 0.6 off of my time, some of that time was due to improving lines though.


Tyre pressures are trial and error though, as with any part of the setup it depends on the track, the car setup and the driver.

4ABHGE wrote:loved understeering!

Taken out of context but still.. You said it hahaha. FWD guy

Dann

4ABHGE wrote:I drive an AE82

That would be a Toyota Corolla then. With front wheel drive. Not an MX5.

BARMY has 205/40/17 Yokohamas now. I did a test and tune day at Winton last week. Pressures were set at 32 at the servo nearest the track. After the first session, when I was learning the track for the first time, they went up to 38 all round all on their own. For the second session, the back end was getting loose half way through and the pressures became 40 at the end. I don't know whether this was the right thing, but I guessed that the tyres were creating their own internal heat by squirming around. So I added another 4 psi and was way quicker in the next (dry) sessions. The tyres were 46 at the end of the last of those.

First tyres increase approximately 1psi for every 10 degree celcius change. So you went from about 20 degrees air temp in the tyre to about 80C after the first session, then 100C. Less tyre pressure means more tyre deflection which makes the tyre heat up faster. The tyres will only produce their highest grip in a certain temperature band. This is where the black arty/tunability comes in with tyres. Low pressure generates heat faster but also overheats easier and gives you a smaller window of optimum grip. High pressures take longer to heat up, may stay in their heat range longer but now the tyre shape is wrong. This is why it's so complicated as you have to find the magic holy grail window, recognise and use it effectively. So it actually takes 3 big factors to get the most of your tyres. Your driving, your alignment and your tyre pressure. For now I'd say your priorities should be fun and tyre durability so to that end try below first, your driving will come with time/experience.

Using a pyromometer the temperatures across the face of the tyre will show you where the work is being done by being hotter. There is no set pressure for a profile/width tyre, the optimal pressure is the one that gives you the most evenly distributed contact patch based on the weight on your car (also most even wear and longest tyre life to boot) and tyre design. Personally I've found different tyres requires different pressures with the same weight car and same width/profile (e.g D03's/RE55's/V70A's all had different pressures to make the same surface temperatures). Plus to make it more complicated I usually find an alignment adjustment is required too due to different sidewalls/tyre shape (kumho's have a very square shoulder, most others are very rounded etc).

The temperature differential between the outside and inside edge will teach you alignment. Starting off unless you have a very aggressive alignment you probably won't have enough camber and this will cause the inside of your most loaded tyres (what's most loaded will depend on circuit design/direction) to heat up most (or opposite if you have too much camber for those street tyres). My rule is basically add castor until you can add no more (I can explain this further if you want but basically it gives you camber only when under load at the expensive of steering weight which won't matter because you have power steering) . Then add static camber to the car until you either max it out or the inside edge of the tyre is only marginally hotter than the outside edge (this is trial and error unless someone else already has a very similar car in weight/tyre/suspension and I'd want it within %10 ). Reason for this is the inside should always be a little hotter as when the car is not cornering the wheel angle will distribute more load on the inside edge/sidewall while you drive into the pits. Static camber has the negative effect of reducing your straight line braking/acceleration grip which castor doesn't. Keeping the whole tyre face the same temperature after a session means you won't go through a tyre edge and lose a tyre prematurely. If your going to the same circuit and it happens to be biased (e.g Wakefield Park up here is all clockwise so the passenger side gets more hammered) you can just rotate the tyres as they get lower so the highest tread is on the loaded side. This is slower in absolutely lap times but you can get many more trackdays out of tyres (lower tread depth = lower tread block movement so the temperature increases slower which gives more time in the sweet spot if you reach it with increased contact surface area to boot . Not something to worry about until you get really fast ).

The temperature differential between the centre and edges of the tyres will teach you correct pressure, I'd be keeping my centre temperature as close to the outside edge temperature as possible. This means that the pressure in the tyre isn't ballooning the middle which means your not taking away grip from the inside of the tyre . As you've probably seen overpressuring a tyre wears out the centre on the road and on the race track your making a pivot point between the outside/inside of the tyre.

That all works for the dry but it also gives you a figure for the wet. I run my ideal hot temperature in the wet cold and just wind the shocks to full soft with the EDFC.

As for dartiness it could be HEAPS of things....I'm gonna leave it here as it's getting far too late...

Blimey! But thanks.