Super Jamie Posted July 23, 2004 Report Posted July 23, 2004 i used to have to run 42psi because i had 185 tyres on 4.5" rims. silly me. now i have some nice wide 6s, i have done a little experimenting with 42psi in the back, the car just bounces around alot. with 38psi it will spin the wheels at first but seems to get more grip and arc up once traction is gained. today i lowered them to 36psi, not a huge difference but i didn't get up it much i raised the fronts from 28psi to 36psi, steering response was noticably improved, the car noses down less, i guess from less sidewall flex? i think less bump steer as i drive around on our shitty nsw roads too so what are some general rules and theories for tyre pressures? what set of conditions are you trying to attain and what are you trying to avoid? Quote
irokin Posted July 23, 2004 Report Posted July 23, 2004 42psi!!!@$## WHAT THE FARKBOT!!! Id be running 35psi tops...shit thats what we run in the commodore when we're towing Recommended for 155R12s on the KE20 from the factory is something like 25psi!! ok its not much good for performance. I run 32psi all round (higher in the rear if i want some sliding action) Be VERY careful running more than 35psi...you'll have a contact area around the size of a bike tyre (I have seen an example of this!!!) very dangerous. Same with running lower pressures (say lower than 25psi) the tyre will only wear oun the outsides and will have a tendancy to try and roll itself off the rim... meh...I'll post more later Quote
demuire Posted July 23, 2004 Report Posted July 23, 2004 Erm... Short and lazy and not-so-useful answer: Read How To Make Your Car Handle. Even less useful to you because I think I have your book :) With my road tyres I generally run something along the lines of 36psi on the front and 38-40psi on the back. With my motorkhana tyres I sometimes even bump the backs to like 44psi. With track rubber (stiffer sidewalls etc) I tend to run slightly lower, usually like 34/36psi, although tonight I'm going to give 32/34psi a go. I guess the main things about tires and tire pressures is: - Underinflated or overinflated you lose grip - The grip levels drop off a fair bit slower on the overinflated end of things - With more air, sidewall flex is less giving more direct feel and less chance of rolling the tire off the rim - With more air less heat is generated due to less sidewall flex - this can be a good thing or a bad thing, depending on application. - With more air the "threads" open up, giving bigger channels to drain water So generally, it's better to err on the high side of things. Ideally you'd want the "perfect" pressure, but what on earth is that? Quote
demuire Posted July 23, 2004 Report Posted July 23, 2004 Be VERY careful running more than 35psi...you'll have a contact area around the size of a bike tyre (I have seen an example of this!!!) very dangerous. Same with running lower pressures (say lower than 25psi) the tyre will only wear oun the outsides and will have a tendancy to try and roll itself off the rim... This isn't really the case for most newer radial tyres these days, the contact area actually doesn't change too much with tire pressures anymore. That's nto to say it doesn't change - you'll still wear the center / edges more with too high / too low pressures, but it isn't all that obvious. I tend to run reasonably high pressures, and I've got negative camber on the front of the car, yet I tend to wear the outside edges out before anything else. Could be toe settings too I suppose, but Pedders recon that's fine... Quote
irokin Posted July 23, 2004 Report Posted July 23, 2004 The guys I met who proved this ran a yokohama tyre on 45psi round a track for a while the wear pattern was astounding :) Quote
Teddy Posted July 23, 2004 Report Posted July 23, 2004 ..What do you guys recommend i have my tire pressure's on? Remember - Stock Ke70 which isn't a race car :) Quote
Redwarf Posted July 23, 2004 Report Posted July 23, 2004 Teddy, 32 psi all day everyday, you can't go wrong. Will be running the Falkens at 38 psi cold tomorrow. My rally tyres grow around 6-8 psi when hot. Quote
demuire Posted July 23, 2004 Report Posted July 23, 2004 Took the car out for a spin at 32/34psi cold today. Apart from feeling a little less direct, *maybe* they stuck a bit better. But they were still a bit sketchy and Hal drove sideways a lot. Interesting... Don't really know what to do now... Might pump them up to 34/36 again tomorrow. Maybe. I can always drop the pressure again, but can't always pump them up. Quote
irokin Posted July 23, 2004 Report Posted July 23, 2004 Took the car out for a spin at 32/34psi cold today. Apart from feeling a little less direct, *maybe* they stuck a bit better. But they were still a bit sketchy and Hal drove sideways a lot. Interesting... Don't really know what to do now... Might pump them up to 34/36 again tomorrow. Maybe. I can always drop the pressure again, but can't always pump them up. ive got some 165 retreads that might stick better lmao Quote
demuire Posted July 23, 2004 Report Posted July 23, 2004 Haha, perhaps. They do seem to be getting rather sticky, and sand and little stones etc are getting stuck to them, yet they seem to still slip and slide a fair bit. Maybe I've just got too much power :) Yeah right :D Quote
Super Jamie Posted July 23, 2004 Author Report Posted July 23, 2004 hmm, very educational, thanks fookie :) and yes you do have my book. ironically, since you got it back for me i've seen ken more than i've seen you! i really really like the steering with the fronts at 36psi, it is a shitheap more stable at 150km/h, and it just turns in SO well it's beautiful, it also seems to jolt less in the back too. i might try pumping the fronts up a bit more. i had an idea that you'd want to run "perfect" pressure, which would be a sweet spot where you get what you want best out of the tyres depending on what i find, i'll probably run slightly higher rears, ~40psi, on the hillclimb track just to be safe (though i don't think i'm going to roll a 185/70/13 off a 6" rim) but i find it amazing the difference a few pounds in the treads has made to the road feeling of my car. free comfort and handling, this rocks! Quote
demuire Posted July 24, 2004 Report Posted July 24, 2004 Well, ended up running 32/34 today, car went awesome. Rears have stickied up a fair bit. Quote
Super Jamie Posted July 24, 2004 Author Report Posted July 24, 2004 36psi all round rocks! maybe it's my rear tyres balding down, maybe it's me learning greater car control, maybe it's the tyre pressure cos that's the only thing i've changed, but if i try to slip the back out around a corner, it will more often kick two wheels and drift around, gettting traction with a good driving line depending on where i'm pointing the steering wheel. i like this :) Quote
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