Here at UQ we're running 10" wheels for the first time and we're curious to hear about others experiences with the 18x7.5-10 we're using on the rear of our car. A brief outline of our car is as follows:
- 190kg wet (no aero yet)
- Yamaha R6
- 60% rearward weight bias
- 60-70% Rear LLTD
- Spool rear driveline
- Rear beam axle
- Front Tyres: 18x6-10 (R25B/R35A)
- Rear Tyres: 18x7.5-10 (R25B/R35A)
A quick intro to our car - we choose to run a 4 cylinder for reliabilities sake. We've seen other teams have horrible experiences dealing with reliability of singles and their shorter rebuild intervals, we're just not interested in having those troubles with the resources we have. As such we've chosen to try and build a car that takes advantage of the power a 4 cylinder provides. We built a car with significant rearward weight bias with the intention of giving the rear wheels enough normal force to be able to make use of all the power we have. The end result is a car that puts power down incredibly well but also drives with significant oversteer and rear wheel slip on corner exit.
The oversteer itself doesn't seem to be slowing us for the first few laps, infact the corner exit is far faster than our previous cars. But if we try to tune the balance of the car back toward neutral we end up with mid corner understeer and can't unload the spool and make the car work. It doesn't appear to me as a loss of grip so much as the conditions the tyre and car tend to want to operate at. The oversteer is a steady controlled power slide. When we look at scatter plots of longitudinal force against wheel slip for different lateral acceleration cases we get plots like the one below. The following plot has isolated all the data points between 0.875 and 0.925 lateral G's using a colour channel; corner exit data.
My reading of this plot is that up to 10km/h of rear wheel slip is providing significant gains in longitudinal force and even up to 20 is still providing stable grip. When we sync our gopro footage to this data we can also see that this wheel slip is happening at significant body slip angles - I'd estimate 20+ degrees. The issue that we're encountering is that our rear tyres don't seem at all happy with this style of abuse.
The first issue is the amount of rear tyre deflection we see during corner exit is ridiculous. The following picture is midcorner through a hairpin and is a tame example of what we've been seeing.
Initially this wasn't an issue and just looked funny, but as we've started tuning our car and making it faster the deflection has gotten worse to the point it appears to be causing loss of tyre pressure. There are times where from the side of the track the deflection is so much you can see the bead of the rim, but as the car straightens up it reseats. Normally we run our tyre pressures at about 10 psi, but even up at 14 psi we have this exact same problem. The bead of our rims isn't crap either. It takes a 5t arbor press and a strong arm to debead the tyres when we change them. We're at the point now where we're going to run ATV tubes in the rear tyres to try and keep them pressurised.
We're also struggling quite a bit with tyre temperature. Our front tyre of choice are R25B and the 13" R25Bs we've run on previous cars normally come back at about 50-60 celcius carcass temp. By the time they're at 65+ on hot days with high track temp you can see them shedding rubber off the inside of the tread section on gopro videos.
Last track day we were running R25B compound rear tyres and stopped the car on track to measure the carcass temperature. We measure temps by digging a thermocouple into the bottom edge of the treadwear marker and measured 107 celcius for all except the outter most marker that read 108 celcius, top layer of rubber had a consistency somewhat like bubblegum. There were marbles all over the track and the tyre itself looked absolutely destroyed, especially the inside 30mm of the tread. The track temp was only about 30 celcius. In hindsight I wish I'd taken a picture of it.
With the way our car is setup I would expect to see a peak normal load of between 120kg and 130kg on the outside rear tyre. I've seen teams such as Monash run these on a 200kg car making 80-100kg of downforce through some sections of the track. Before weight transfer the normal load for a 50/50 car of that weight and downforce is 95kg. I can't imagine that the amount of normal load on the tyre is the issue, I think the issue is high slip angles and 10-15km/h wheel slip on corner exit. We're finding R35A is just as fast as R25B, assumedly because of the way we're using the tyre the advantage of the soft compound is offset by the fact we overheat the tyre and leave it all over the track.
The wear pattern is somewhat confusing to me as well. My understanding is that when crossplys are loaded laterally they tend to peel up the inside edge and heavily load the outside edge. We're seeing the outside shoulder become very rounded as we'd expect, but we're finding the inside edge is rapidly destroyed despite running 0 camber at the rear. The tyres were fresh at the start of the day with 4mm of tread depth on each wear marker. 30km of driving and 100+ celcius tyre temps later we measured the following tread depths:
- Outside 3.5mm
- Outside Centre 2.65mm
- Centre 2.65mm
- Inside Centre 2.65mm
- Inside Marker is gone
Initially I wondered if the temperatures caused the overheated rubber on the tread to migrate towards the inside of the tyre and fill the marker up, but one of the rows of markers still has the inner-most marker just visible and clearly not filled up.
So getting to the point of my rambling, we're about to add aero to a car that already destroys soft compound tyres on a cold day. We'd like to keep running them because in skidpan, AutoX and especially acceleration R25B was faster than R35A. We're concerned that if we have a hot comp with 50 celcius track temperatures the R25B's will be destroyed in endurance and end up slowing us down significantly. Has anyone else encountered debeading levels of tyre deflection, crazy tyre temperatures and heavy inner shoulder wear on these tyres? Has anyone else had any other weird experiences with these or other tyres? Any theories as to why we're having such issue with them?