Please Help: Slick Tyre Issue

[ja9]

220s are too wide for a 7 inch rim. You need at least 8s to run those.

You could try the Kumho 205s which are also a bit cheaper, or buy some 8's (which might actually be cheaper than new slicks!).

I'm with zossy1 on this one. I think they might be too wide also for a 7inch rim. The guy at Dunlop did say it is within spec but that may be more a sales pitch than anything. He did tell me that it would wear the outside part of the tyre which it is. Surprising considering I'm running
-4 degrees camber. Anyone know why this is or is it because of what zossy1 said in his other post.

220's will be fine on a 7" rim if that's what the Dunlop sold you knowing the rim width.

I respectfully disagree... A tyre that wide, especially at a 580 profile (which is what I'm sure these must be), will just work the sidewalls to buggery. The car will wallow all over the place as the wheels work back and forth inside the wide tyre carcass. IMHO, even 8's are only barely wide enough to secure the sidewalls of the 220 properly.

Remember that a 7 inch rim is only 175mm wide at the bead.

I'll grant you, that may have something to do with my driving style though, coming from karts.

I always ran the 220 slicks on 7 inch rims with no problems.
28 hot sounds a bit high, I used to start at 19 or 20 cold.
This is the reason I don't buy slicks any more, the first lap you do on them is the fastest and it's downhill from there, usually about 10-15 heat cycles loses about 2 seconds, I didn't ever wear out a set, they always just ended up being really slow.
We had a set of Kumho slicks that were good for one day, the next day the car was undriveable.
If you get a set of new slicks that are a bit old they will be crap.
Just get yourself some Yoko 050 semi slicks.

Lou my car is so difficult to hang on to without p/s when I run 19psi. I've even been told to start with 18psi but how do I reach my goal of 28 degrees hot which was the figure given to me by Dunlop. Having shelled out $1500 for a new set of tyres a month ago will make it difficult to cure this problem with a new set of Yoko 050. This is why I'm persisting with all the questions.

To work the tyre harder and get it to bead, do I need more or less tyre pressure?

[zossy1]

I have heard of at least one very quick driver who prefers lower pressures with the Dunlop... I won't reveal too much but suffice to say some amount less than 28psi hot.

So, you may try shooting for a lower hot pressure...

Think about how a tyre with a 220 max width will look on a 175 rim. Think then how it will distort when the wheel exerts outward force on that tyre. The inner bead will drag the inner wall outwards. This will drag the inner edge downwards and away from the road. Meanwhile, the outer bead will straighten and push the outer tread corner up and towards the road surface, creating the outside wear you are seeing.

Now that isn't always a bad thing, as long as the slip angle of the tread is still optimal (which itself is a function of alignment and driving style). But it's the responsiveness and float of the car while this is happening... Personally, I hate it.

We experimented a lot with narrower and wider wheels in karting and we noticed all of this. We did find that on wet tracks, running narrower rims was faster because it worked the sidewalls and, presumably, this allowed the tyre to retain more heat. Still, while it was faster, it felt like crap. The kart felt like a hovercraft comparatively speaking. You could feel the rear flopping when you turned in and it inspired little confidence. We had to consciously overdrive the kart to yield the times. We also had to run fairly extreme alignment settings. EDIT: This was only with one specific type of wet tyre too...

In the dry however, it was always slower to run anything other than the widest rims available. An inch of wheel width cost us up to 0.5 sec on some tracks, and also a lot of lap time consistency.

[ja9]

Zossy1,
There are two main reasons I've avoided the extra low tyre pressures.
1. As mention above the steering is too heavy and it wears me out
2. Wouldn't lower tyre pressures exacibate the issue of wearing the outside part of the trye.

[Boyracer]

If you get worn out running the car you should put the power steering back on, if the Tyres need the lower pressure to give you the grip then you need to run lower pressures... I know all about getting worn out in a race car, I did about 60 laps at Wakefield in my white car which has a NB rack with no p/s.

Sometimes these things get way over analized, it's just a MX5, not a formula 1.

chances are you just bought a sh*t set of slicks... It does happen but the tyre mob won't tell you that...

Check the age and compound of the Tyres you bought.

Forget about wearing the outside edge, like I said you will replace the Tyres a long time before they wear out.

[toppertee]

Sometimes these things get way over analized, it's just a MX5, not a formula 1.

Post of the year!

I'm running 295s on 9' rim. LOL

[jeffreyg]

I'm running 205 50 15's on 8 inch rims (A050r's) and was previously running the same tyre(Dunlop and Yokohama) on a 7 inch rim-the improvement in handling and feel with the wider rim is amazing.
Anything wider on an 8 inch rim is too big in my view as the tyre wall needs to be as close to vertical as possible to work at its best. I usually start at 24psi cold to cycle up to 28 psi hot-19 is too low.
I do run lower pressures on lower speed stuff(hillclimbs) though
Cheers

[MattR]

I'm running 205 50 15's on 8 inch rims (A050r's) and was previously running the same tyre(Dunlop and Yokohama) on a 7 inch rim-the improvement in handling and feel with the wider rim is amazing.
Anything wider on an 8 inch rim is too big in my view as the tyre wall needs to be as close to vertical as possible to work at its best. I usually start at 24psi cold to cycle up to 28 psi hot-19 is too low.
I do run lower pressures on lower speed stuff(hillclimbs) though
Cheers

Slicks and semi slicks are very different animals with the starting pressures and hot pressures. When I used super tourer slicks starting pressures were 21psi, when I changed to porsche cup fronts the pressures went up to 23psi for sprint races and I started at about 18psi for hill climbs to try to get as much heat as possible in the slick. Usually I ran Toyo RA1 semis for hillclimbs.

[CT]

Hi John, Nice to meet you down at Wakefield. Just to re-iterate a couple of things we chatted about. Ambient temp was 34deg C (from my data logger) that day so expecting really fast times is perhaps not realistic. Wakefield also gets quite dusty in the summer with the lack of rain so that affects overall grip and times too. 28 hot is a good spot for those tyres, I must have used at least 20 sets of those in the old days and 28 is really their sweet spot but higher track surface temps will slow any tyre down. There is also an art to driving a slick tyre. You cannot drive them like a semi slick with a lot of slip angle, slicks do not like to slide too much. They have maximum grip just prior to generating slip angle so it's a delicate balance. I think I mentioned needing to feel the slick load up, which is where they are generating maximum grip prior to slipping. They need a very clean driving style. Alignment wise, your numbers are not crazy. I never ran any from toe - always zero and like the castor around +5 - but that's possibly personal taste too as more castor makes for a more stable car at speed. The other area where slicks are unforgiving is shock and spring settings. To load the slick, the cars needs to shift it's weight just enough to the appropriate tyre during a transition. If the car is too stiff, it may not transfer enough weight onto the tyre. Springs that are too stiff have that affect as does very stiff bump and rebound damping. Again, it's a balance. Also, I will refute the internet folk lore that a slick tyre will automatically find 2 seconds over a semi. Never has in my experience. Perhaps at a longer track with higher lateral loadings like Phillip Island, but not a Wakefield that's for sure. Maybe pick a cooler day for your next run and take a tyre pyro and get some measurements across your tyres to see what temps they are - that should help provide some setup guidance.

[PeterB]

I agree on time improvements at Wakefield.

On Fanta (NA6 racecar with 1600 normally aspirated) I ran 225x45x15 Kumho V70A mediums or Dunlop medium slick on 8" rims at supersprints/trackdays - no racing. It was running on Konis with King Springs, so was a softish setup with stiff swaybars.

My best on slicks was a 1.09.25, compared to a best on semis of 1.10.0. However, the semis were much more consistent in their times, whereas the slicks needed a good day to perform well, on hot or dusty days they dropped lots of time.

At hillclimbs I ran semis on the front and full slicks on the rear, using a burnout to get adequate rear tyre temperature.

The radial slicks needed an aggresive -ve camber, whereas the crossply slicks only needed a couple of degrees -ve.

Racing or the faster tracks would be a different story.

[ja9]

Hi John, Nice to meet you down at Wakefield. Just to re-iterate a couple of things we chatted about. Ambient temp was 34deg C (from my data logger) that day so expecting really fast times is perhaps not realistic. Wakefield also gets quite dusty in the summer with the lack of rain so that affects overall grip and times too. 28 hot is a good spot for those tyres, I must have used at least 20 sets of those in the old days and 28 is really their sweet spot but higher track surface temps will slow any tyre down. There is also an art to driving a slick tyre. You cannot drive them like a semi slick with a lot of slip angle, slicks do not like to slide too much. They have maximum grip just prior to generating slip angle so it's a delicate balance. I think I mentioned needing to feel the slick load up, which is where they are generating maximum grip prior to slipping. They need a very clean driving style. Alignment wise, your numbers are not crazy. I never ran any from toe - always zero and like the castor around +5 - but that's possibly personal taste too as more castor makes for a more stable car at speed. The other area where slicks are unforgiving is shock and spring settings. To load the slick, the cars needs to shift it's weight just enough to the appropriate tyre during a transition. If the car is too stiff, it may not transfer enough weight onto the tyre. Springs that are too stiff have that affect as does very stiff bump and rebound damping. Again, it's a balance. Also, I will refute the internet folk lore that a slick tyre will automatically find 2 seconds over a semi. Never has in my experience. Perhaps at a longer track with higher lateral loadings like Phillip Island, but not a Wakefield that's for sure. Maybe pick a cooler day for your next run and take a tyre pyro and get some measurements across your tyres to see what temps they are - that should help provide some setup guidance.

Thanks for taking the time to write such a lenghthy reply Chris. Btw, you should check out Chris's new car, what a beast of thing and looks awesome.

I agree on time improvements at Wakefield.

On Fanta (NA6 racecar with 1600 normally aspirated) I ran 225x45x15 Kumho V70A mediums or Dunlop medium slick on 8" rims at supersprints/trackdays - no racing. It was running on Konis with King Springs, so was a softish setup with stiff swaybars.

My best on slicks was a 1.09.25, compared to a best on semis of 1.10.0. However, the semis were much more consistent in their times, whereas the slicks needed a good day to perform well, on hot or dusty days they dropped lots of time.

At hillclimbs I ran semis on the front and full slicks on the rear, using a burnout to get adequate rear tyre temperature.

The radial slicks needed an aggresive -ve camber, whereas the crossply slicks only needed a couple of degrees -ve.

Racing or the faster tracks would be a different story.

I think you proved my point here Peter that you can do a time of 1.09 in an normally aspirated NA6 when all I could manage was a 1.11's with 124rwkw Now you can understand my disappointment. Anyone who knows the car has told me it's good for 1.07-1.08's.

[Deckspeed Racing]

124 rwkw?? from what i remember about this car when Col owned it , I'd be prepared to put a small wager that 90 to 95 rwkw would be much more realistic. unless of coarse you have changed the engine . if this is corect your not that far off the cars potential , put 8 inch rims on it with those tyres and you'll do a high 9 at best . I've dynoed a lot of these cars with engines from all the top builders the best I've seen with std intake 118 rwkw and close to 130 with itbs and these where no expense spared engines. I would be happy to dyno the car for you John and overlay power and torque graph from some of these other cars .

Regards

Daniel

[ja9]

124 rwkw?? from what i remember about this car when Col owned it , I'd be prepared to put a small wager that 90 to 95 rwkw would be much more realistic. unless of coarse you have changed the engine . if this is corect your not that far off the cars potential , put 8 inch rims on it with those tyres and you'll do a high 9 at best . I've dynoed a lot of these cars with engines from all the top builders the best I've seen with std intake 118 rwkw and close to 130 with itbs and these where no expense spared engines. I would be happy to dyno the car for you John and overlay power and torque graph from some of these other cars .

Regards

Daniel

Hi Daniel,
Thanks for your offer to dyno the car. I think the best thing I can do is to let you have a drive of the car when we're next at WP. Your feedback would much appreciated.
In regards to the kw claim I'm happy to say the engine has had a lot of money spent on it. Both Col and myself have spent our fair share. The engine is built with forged rods and pistons, big valves, large cams etc. Obviously you know what it takes to build these motors to that sort of horsepower.
Don't you hate people who exagerate about their figures. Looks like I did a little, sorry.

[Deckspeed Racing]

Hi John
by the sounds of it the engine is not your issue . my 2 cents on your setup , I've found 4 degrees camber a bit excessive I would knock it back to 3 , 0 toe all round caster irrelevant to lap time, ride hieght on the car can have much more affect, it may be droping onto the bump stops, this will dramatically accellerate the spring rate and make the car feel very nervous . 8 inch rims would be a help aswell , i can lend you a set to try before you spend money on any, I've raced a lot on the 15 inch dunlops , they are a very good tyre , I personally have not found performance drop that others have mentioned until the've worn out after multiple race meetings , for me they have always been very consistent, I would be happy to do a couple of laps to help you dial it in. Not having powersteer should be an advantage you shouln't tire out if the car behaves well, my guess it does'nt turn in well, try stiffening the rear bar then try it disconected, the difference should give good feed back on which way to go.

Regards

Daniel

[ja9]

Once again thank you. I'll come and visit you to borrow those wheels you mentioned and have a chat about how we are going to sort this car out once and for all.
Cheers - John

[Magpie]

Have you looked at your sprung/unsprung weights, required/optimal frequency, spring rates and damper setting/valving etc?

Tyres should not be done in isolation to other suspension components. I'm also interested as I may make the move to slicks sometime this year and hence are interested in reading other forum members experiences.