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Thread: Hoosier 18x7.5-10. Tyre deflection, pressure loss, overheating and strange wear.

  1. #1
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    Hoosier 18x7.5-10. Tyre deflection, pressure loss, overheating and strange wear.

    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?
    Last edited by Menisk; 10-12-2015 at 05:07 AM.

  2. #2
    Definitely overheating. In my experience, Hoosier tires run best at 60-70 degree Celsius. They can withstand temperatures to about 90-100C but after that they will disintegrate. What kind of temperatures are the front tires? Do you have an idea what kind of slip angles the rear tires reach? (Except large)
    Last edited by DMuusers; 10-10-2015 at 11:38 AM.
    Daniel Muusers
    Formula Student Team Delft
    2010-2015

  3. #3
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    If you are seeing around 20deg of lateral slip and what seems to be around 40-50% of longitudinal slip I'm going to suggest that the cause of your rear tyre durability problem is probably somewhere between the seat and the steering wheel.

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    Just My Inclination

    Add a few degrees of static camber to the rear. Make one driveshaft hollow to reduce stiffness or put a tunable isolator in one.

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    Next Question

    What does GoPro say about your rear dynamic toe change ?

  6. #6
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    Quote Originally Posted by BillCobb View Post
    Add a few degrees of static camber to the rear. Make one driveshaft hollow to reduce stiffness or put a tunable isolator in one.
    Is the idea behind the static camber to reduce the deflection and try to keep the tyre on the bead? Any idea why we might be destroying the inside of our tyres and how the camber will go with this? Also I don't follow you with the driveshaft isolator. Is the idea to allow a little flex between the two rear wheels to reduce how much they fight each other during corner exit as the inside wheel finds imperfections in the road surface?

    Quote Originally Posted by BillCobb View Post
    What does GoPro say about your rear dynamic toe change ?
    Toe relative to the beam is rock solid. We noticed at the end of the day that the m6 rod ends in our trailing links were shagged and the beam had some noticeable lateral compliance relative to the chassis, it's entirely possible this added to the oversteer. We intend to replace them with M8s. From your experience do large slip angles tend to overheat a tyre or is it more likely to do with the amount of rear wheel slip?

  7. #7
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    Tire wear

    Yes the shaft stiffness difference is for a bit of relaxation in turns that exceed the tire grip. This should be considered a tunable item. It will also help with launch ("power hop").

    Consider this to be an engineering issue. Separate the toe and camber degrees of freedom in your tests. There is no such thing as rock solid, that's how Mt Everest got where it is. A weak spindle or wheel bearing will give you unanticipated camber and toe-out angulation (is that realy a word ??). its Compliance.

    Bolt a flat plate to a wheel and apply 100 Nm to the wheel in camber and then toe axes. Measure the deflection(s) with a dial indicator and convert to degrees (That's why the use of a large plate or beam. Expect a degree or so of deflection, any more is unfortunate (yet typical) and needs to be fixed.

    Do cornerng only tests at first in just one direction. Then check for tread migration or wear considerations.

    The goal is to reduce the overturning moment (My) and make the tire "happy". A split mirror that can give you views of toe and camber change with your camera in your cornering vs launch tests will be very helpful. ("Say, that's not supposed to do that.." is sometimes heard in the background).

    Another fun factor would be to swap the tires front to rear, just to make sure you have not gone too far out on the rim width. That can happen. Think like an optometrist: Is a change Better or Worse ? If better, keep going, (ask someone who wears glasses, they'll know how it goes.)

  8. #8
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    Menisk,

    The oversteer is a steady controlled power slide.
    The first and most important thing is ... BE HAPPY! The fact that you can wear out your tyres in "a steady controlled power slide", at this stage of your testing, is a good thing!

    Some more suggestions, in no particular order...
    ~o0o~

    ... we're about to add aero to a car that already destroys soft compound tyres on a cold day.
    ...
    We're concerned that if we have a hot comp with 50 celcius track temperatures the R25B's will be destroyed in endurance...
    ...
    Any theories ...?
    As is almost always the case, aero will fix your problems. Or, at least, it will push them up to a much higher speed.

    Do this simple test. Go to the big grinding-wheel or belt-sander in your workshop, and make sure it is switched OFF. Now push the palm of your hand firmly against the rough surface (ie. for a large Fz), and drag your hand SLOWLY across the surface. Result is high Fx,y, and maybe a very small amount of wear. Next switch the grinder/sander to ON, preferably at its highest speed setting. Now very gently place your hand against the surface, for a very low Fz. Result is low Fx,y, but very rapid wear!

    As you add more aero Fz downforce to the rear tyres, their Fx grip increases to a level that the engine can no longer break them free, at least not on the higher speed corners. So you get even MORE EXIT SPEED out of the corners, but LESS WEAR. On the lower speed corners (= less aero-df, more wheel-torque from higher gear-ratio) you have to ask the driver to modulate the throttle based on experience gained from previous full-length Enduro tests.

    Also just leaving the gear-box in a higher gear in Enduro can help (= driver can floor it, and minimal wheel-spin because lower wheel-torque).
    ~o00~

    My understanding is that when crossplys are loaded laterally they tend to peel up the inside edge and heavily load the outside edge.
    This behaviour is heavily influenced by the relative widths of tyre and wheel rim. The behaviour in above quote is typical of a relatively narrow rim. However, your tyre wear pattern seems to indicate a relatively too wide rim. So how wide are your rear rims? Maybe a half-inch, or inch, narrower might be better? But also many other things to play with.

    EASIEST TUNING ADJUSTMENT OF ALL is tyre pressures. I would be trying some much higher tyre pressures, perhaps up around 2 bar, 30 psi? (Hey, it only takes a few seconds, and costs nothing! ) This might be even more necessary as you add more aero. Of course, do it in steps of say 5 psi at a time, just to get a feel for what happens. Roughly speaking, a very stiffly sprung car works better with low tyre pressures, while a softly sprung car can work better with much higher tyre pressures. Your car, with rear-beam, should be able to run fairly soft springs all around, so...

    The proper solution to tyres debeading is "bead-locks" (ie. internal ring + many bolts, as seen on most off-roaders), but they are a lot of work. I recall Carroll Smith mentioning in one of his books the once common practice of drilling multiple holes in the periphery of the wheel rim and then screwing self-tapping screws directly into the tyre-bead! If you must run low tyre pressures, then my preferred "fix" is glue. Put a bead of RTV-Silicone-sealer (acetic cure, smells like vinegar) between outer-rim and tyre-bead when mounting tyre. Use a press + razor-blade + patience to remove tyre.
    ~o0o~

    ... the m6 rod ends in our trailing links were shagged ... We intend to replace them with M8s
    I would go directly to M10 rod-ends, and maybe heavier-duty tubing for the four links. And maybe put doubler-plates on the various clevises (ie. to make them thicker). And study the beam very closely for any yielding or tiny cracks, and add more gussets.

    But BE HAPPY! All these are good signs telling you that your car is indeed going very well!

    Z
    Last edited by Z; 10-11-2015 at 08:31 PM. Reason: More EMPHASIS!

  9. #9
    Here is a video of the car in action:
    https://www.facebook.com/UQRacing/vi...53668851001197

    The hard launches show how stable the toe base is (and how stable the muffler mounting isn't). This is the second year we have done a rear beam and this years design is significantly stiffer than last years due improved beam geometry and triangulated 4 link (last year was peg & slot, 4 trailing link). Will do physical testing on the camber and toe compliance as it's data we need for our own information and design event. I am certain that this car has less toe/camber compliance than any double wishbone car we have ever built - the load paths are far superior.

    Last year we did destructive testing of ~5 beams to collect camber compliance data and compare with FEA. I was surprised by the accuracy of the FEA as I generally doubt the results. The camber compliance was under 1deg/G, closer to 0.8deg. This was not including the wheel.

    Bill, how do you recommend attaching a plate to the wheel? I can easily collect the data bolting to the spindle but the wheel makes it a bit more complicated.
    UQ Racing

  10. #10
    A suggestion regarding the tyre wear pattern, I think the spool might be the culprit.

    Try running a clockwise skidpad continuously, till the driver gets dizzy. Keep the radius tighter than the competition standard to magnify the required differential action.
    Then measure both the tyres' temperatures, across the tread width. My guess is that the inner side of the inner tyre might be warmer.
    ( following Z's idea that a lightly loaded, higher slipping tyre will wear faster )

    Definitely interested in the solution for this problem !

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