WCM Goldie gravel trap

[nc7s4]

I believe the left rear tire came off of the wheel, leading to this slow, unrecoverable spin.  Prior to this incident the right rear tire came off the wheel.

 

Next session I will be running a new set of wheels and tires.  Still not sure what happened with the existing setup but clearly it doesn't like being on the back of my car.

 

Existing setup: basset 15 x 10-in Wheels with Toyo R 245x15's.  18psi cold in rear, 20 psi cold front 

 

Still need to double check the alignment but I think it's ok.  I drove it home (on a different set of wheels and tires) and it was going straight.

 

Will get it cleaned up, checked and new footwear installed ASAP!  

 

Here is my full analysis:

 

What happened:

• I think the left rear tire came off that caused my spin into the gravel trap.  
• Similar to how the right rear tire came off in an earlier session.  
• This happened mid-right-turn, in the right hand exit of the S's.
• My opinion is based on how the car was behaving and the video.  The "slow, unrecoverable spin" was different feeling than normal.  I also recall a sensation of softness in the left side.  
• The left rear tire was dislodged from the wheel after the left side slid into gravel.
• There is a possibility that the tire did not come off the wheel on track and was dislodged at time of impact with gravel.
• Why do I think the tires are coming off?
  • Combination of undersized tires + low tire pressure
    • Low tire pressure  (23psi Hot)
      • Toyo minimum rating is 22PSI Hot
    • Oversized wheels/undersized tire.  
      • Toyo tire rated for 9.5" max wheels.
      • Wheels are 10".  0.5 inch out of spec
  • No safety bead in the wheels (surprising discovery)
  • Unknown method of tire mounting used - Ken used some kind of goop on the bead which may be necessary for track use.  I had mine mounted at a place that specializes in road cars
  • Unknown condition of wheels - they are old and came with the car
• Mitigation Plan
  • Run a staggered setup (245/275)
    • Weight balance is 43%F, 57%R
  • Run oversized tires (within spec)
    • Oversized tires will be safer for lower tire pressures (within spec)
    • 245 on 8 inch wheels (front)
    • 275 on 10 inch wheels (rear)
  • Use all new parts including wheels and tires.
    • Wheels: Basset with optional safety bead (dirt track spec)
    • Tires: Hoosier A7
  • Only use reputable, known technician to perform mount and balance
  • Run OTD with new setup at limit to test prior to race weekend

 

 

 

Edited April 28 by nc7s4
error corrected

[theDreamer]

“Ken used some kind of goop on the bead which may be necessary for track use.” Say whaa???

As you now know: Always use wheels and tires that YOU KNOW are suitable for purpose. Always use trusted technicians who KNOW THEIR JOB!

Back in my day, if I wasn’t doing the work myself I almost always was looking over the shoulder of the guy who was. He might not have liked me hovering but out on the track it was my investment and my well-being at stake not his.

[nc7s4]

3 minutes ago, theDreamer said:

“Ken used some kind of goop on the bead which may be necessary for track use.” Say whaa???

 

As you now know: Always use wheels and tires that YOU KNOW are suitable for purpose. Always use trusted technicians who KNOW THEIR JOB!

 

Back in my day, if I wasn’t doing the work myself I almost always was looking over the shoulder of the guy who was. He might not have liked me hovering but out on the track it was my investment and my well-being at stake not his.

 

Any recommendations on who to go to for this?

[theDreamer]

Sorry, I’ve been out of racing over 20 years now plus I’m in Ontario. I’d suggest you talk to people in the know like Time Attack clubs or other track day groups in your area. They will have the inside scoop on where to buy products and get services done specific to track use.

Edited April 28 by theDreamer

[nc7s4]

13 minutes ago, theDreamer said:

Sorry, I’ve been out of racing over 20 years now plus I’m in Ontario. I’d suggest you talk to people in the know like Time Attack clubs or other track day groups in your area. They will have the inside scoop on where to buy products and get services done specific to track use.

Oh ok it sounded like you had the inside scoop there for a minute!

[Dave W]

Whoa, one hell of a good motor! You get a really good drive off the last corner on to the front straight and drive all the way to right hander in same gear!!!

When getting new tires for the Seven, I get the old tires dis-mounted, take the wheels home and scotch brite the mounting surface, then return them to the tire shop.

[nc7s4]

9 minutes ago, Dave W said:

Whoa, one hell of a good motor! You get a really good drive off the last corner on to the front straight and drive all the way to right hander in same gear!!!

When getting new tires for the Seven, I get the old tires dis-mounted, take the wheels home and scotch brite the mounting surface, then return them to the tire shop.

Thanks Dave that's a good tip.  Honestly the more I process what happened and watch that video I'm leaning towards driver error.  The motor has a lot of juice and there is more in the car than I've been able to get out so far.  Gear changes on the front straight are still something I'm working on.  I can let it sit at 9000 for a while before turn 1 or upshift after the last turn onto the straight then downshift into turn 1.  That's a lot of footwork but may be faster when done right 

[Croc]

Interesting video.  Just watched your video quite a few times to get a sense of what went on.  My thinking (for all the f#$% its worth) is:

- you initiated the spin with throttle on a relatively slow switchback and held on to it a fraction too long.  There were other points earlier in video where I thought your throttle activation was aggressive and was moving the rear end around a little.   For the corner in question, you were swinging the wheel back to have the front go left while your rear end was still loaded up for going right - so car was unsettled and you had compromised the front end grip right when you needed it.

- that car did not want to come back from the spin even with you off throttle and I think hitting brakes about when you hit grass.  With such low pressures and no safety bead I think you released air via bead release but tire did not come off rim

- On grass you were on for the ride in a slippery surface.

- sideways into gravel took your tire off bead (finally). 

 

 

As for your remediation plan:

- ignore tire pressure - 1 psi does nothing unless you are in F1 and you are not Max Verstappen (probably more fun to have a beer with though!)

- undersized tire/oversized wheel - not really.  Half an inch is fine.  An inch or over gets tricky.  Reversed cantilever does stress the bead seal but you are not out of tolerance.  I have seen worse.

- Missing safety bead is not a surprise on a race wheel - its a Basset wheel.  Thats how they are - originally intended for oval tracks and banked circuits

- I like the idea of staggering set up and putting more rubber on rear.  You do have something like 260hp going through a 10 inch wheel on a light weight car.  Its easy to light up the rears with a heavy right foot.

- Hoosier A7s eh?  Hoosier has changed its recipe for its rubber in the last year.  It is now not as long lasting or as consistent in grip in a track session.  I used to get a year of consistent grip and the new formula would go off in the first day with them.   I have given up on Hoosiers and use Avon race rubber or Pirelli P-Zero F3 race tires.   Was the rubber giving up on the day?  What ambient temps?  What rubber temps?  

 

 

 

 

[nc7s4]

Hi @Crocthanks for taking the time to review this.  Tires seemed fine, tires were just warming up and the car felt basically great as I was passing the field.  I noted tire temps of ~160f rear and ~140f front in an earlier session.  Inside temp was 15 degrees warmer than outside on all corners with 1.5 deg camber.   Ambient 60f.  

 

I don't know much about the Hoosier a7, taking somewhat of a shot in the dark by selecting that product.  Also, reconsidered my wheel size choice and may end up with all 4 9.5 inchers.  Those will fit 245 and 275mm tires, and will give me the option to go back to a square setup (all 245)

 

End result of this day was good, despite the gravelly mess which I still have to clean out.  I received my novice racing license and a log book for the car.