ANALYSIS : Benefits of Shaving Road/Treaded Tires.
AUTHOR : Malcolm Edeson (www.driver-coaching.com)
Shaved vs Unshaved
Over my 20+ years as a Race Driver, Test Driver and Driver Coach a key aspect of what I do is advise on tyre strategy. I’ve known from my earliest involvement with working with tyres that a shaved tyre will ‘outperform’ an unshaved tyre in the dry.
What I mean by outperform is that it will grip better and generally last longer (as a usable tyre) than a brand new tyre. Tread is only required to disperse water, it is not needed in the slightest for grip.
Back in 1990 when I started racing the vast majority of racing was carried out on standard road tyres, even many formula cars where still running treaded tyres, it was only the high profile Formula and Saloon series (i.e. BTCC) that were running on slicks. So there was a lot of ‘knowledge’ on how to get the best from a standard road tyre.
I also worked for the Jim Russell Racing School from 1991 to 1995 at Donington Park who ran a fleet of Vauxhall Astra Saloons for their 7 x 7 days courses, additional training days and end of course race weekends. All the cars were looked after by the school and intended to be equal. They ‘could’ have just used standard tyres straight off the shelf but they PAID to have them shaved/scrubbed, the reason? they lasted longer as a usable tyre.
Back to grip levels. A tyre will generally give more grip if it has more rubber contact with the track surface (hence slicks grip better than treaded tyres). You actually have a very small contact patch with the track on each tyre, not much wider than the width of your hand (front to back) and as wide as your tyre width. It is this that is keeping you on track, is used for braking, accelerating and cornering.
A new treaded tyre will have ‘slightly’ less rubber in contact with the road as the tread blocks are slightly narrower at the top than near the bottom but this isn’t the main reason a shaved scores over a tread tyre. The maximum ‘lateral’ grip from a treaded tyre of Radial construction comes when the tyre is in a slight slide, from memory a road tyre needs about 8 degs of slip to be at it’s optimum. Above or below that it will gradually tail off. This figure is called the ‘slip angle’.
It doesn’t matter what the exact figure is but generally this means, if the tyre is sliding it is also squealing. What causes the squealing is the tread blocks moving laterally across the tarmac. The ‘taller’ these blocks the more tyre squeal you will hear as the blocks are moving around more. It’s possible the tyre will be at the same slip angle but the treaded tyre will be much harder to keep at this angle consistently and will feel vague in comparison to the shaved tyre. As the tread moves more turn-in will feel a little less sharp.
Because there is more movement, more heat is generated, what’s more this heat is higher on the very outside of the tyre (the ends of the tread blocks) which means it is much easier to overheat the tyres surface, even though the tyre itself is not overly hot. This can lead to it being much harder to set pressures correctly.
It’s also this heat that wears the tyre MUCH more than a shaved tyre, this is why a shaved tyre can ‘last just as long as a full treaded tyre BUT worse than this the full treaded tyre often wears unevenly and suffers from ‘tread curl’ which further reduces the contact patch.
So, I know this, have seen it ‘in action’ over many, many years but it would be nice to be able to provide more concrete evidence of this. We decided to try and do a quick tyre test on a recent training day I did.
I used data & video logging extensively in my coaching work and we were planning to run both tyres on the same car, same day, collect data & video and use this to demonstrate the above.
I have been involved in back to back tyre testing before and it’s a laborious job and requires a lot of setting up and preparation. I was involved in testing the new ‘at that time’ Toyo tyres when they first came to the UK (T1Rs). Our main aim this day was training 2 race drivers for a coming race and we didn’t have long to do the tests. As it was the ‘actual’ back to back test had to be rushed and wasn’t ideal but we did collect enough data & video to back up what I already knew and can be clearly seen in the data.
The data was captured on a Race-Technology DL1 and Video was also captured via an integrate VIDEO4 system. http://www.race-technology.com
The track was Donington Park GP Circuit, the day was dry and cool. The car was an MX5 racecar approx 970kgs with driver and running on Hankook Ventus V12 Evo road tyres. Drivers were both very experienced and the times set good enough for pole position by almost 2 secs over the last time the MX5s raced on that circuit.
Tyres : Hankook Ventus V12 Evo (Before and After shot below). The tyres are the exact same size but you can see in this photo that the shaved tyre has a much greater contact patch (actual rubber in contact with the track surface)
Notes on the above tables
NOTE 1 : Max Speed slightly higher due to less rolling resistance.
NOTE 2 : Shaved can maintain slightly higher minimum speed (more grip)
NOTE 3 : Shaved can maintain higher lateral g (l/h is most prevalent at Donington)
NOTE 4 : Shaved can maintain higher lateral g
NOTE 5 : Unshaved provides slightly better braking g (more compliant)
NOTE 6 : Shaved has the higher average Total G figure, this is THE figure for performance comparison
NOTE 7 : Again we see unshaved can give slightly higher max speeds
NOTE 8 : Shaved maintained on average a higher min speed but unshaved sweet spot gave higher min and the best lateral g performance from Unshaved which actually led to the fasted lap. After that the lateral g performance dropped off considerably.
NOTE 9 : Shaved can maintain higher lateral g
NOTE 10 : Shaved can maintain higher lateral g
NOTE 11 : Shaved now giving slightly better braking as they get more bedded in.
NOTE 12 : Shaved giving higher average of total g (accel/brake & corner)
The Solo Unshaved run was the very first time the tyres were run that day and it is interesting that they behave similar to slicks (of which I now have much more hard knowledge) in that they have a sweetspot around lap 2 or 3. At the time I was perplexed that the unshaved tyre had set the fastest lap, I had not expected that but you can see it was strictly a one lap deal. The shaved tyre had already done some laps so had already been through a heat cycle and short cool down.
Readers may be interested to see the higher top speeds for the unshaved tyres, fact is the more lateral grip a tyre has the more it sticks to the track surface which goes for when you try and push/pull them forwards, so the ‘rolling resistance’ is higher.
That’s why running low powered cars on slicks often leads to slower or at least none improved lap times (dependent on circuit).
So what does this all mean in terms of time? I suspect the best thing to look at would be the fastest 2-up times and using that reference we could estimate that the shaved tyre is around 1.5 secs a lap quicker. If I was every asked before I would have said ‘a second a lap’ (based on experience) but that would have been for most circuits which is around a 1:30 lap, with the GP Circuit lap being 2:00 then this would tend to bear that out.
What I think is more important is that they are MUCH more consistent as can be seen from the table above.
Videos with data overlay (showing speed and g-forces) will be available shortly.
About the author:
Malcolm Edeson is a professional race & test driver, driver coach and ARDS instructor.