Advanced Driving: 

Learning About Tires, Brakes and Uncontrolled Friction Interfaces

By Nigel Etherington, P.Eng.

The steep learning curve continued last summer at several Advanced Driving Schools. The incline was more jagged than smooth as I pursued more track time. One instructor had intimated that intermediate students were an instructional challenge and I proved the rule, not the exception as I’d hoped. In year 2 I’d almost equalled my inaugural of 10 track days with two schools at Mosport, L’Est Fest at Calabogie and some Dunnville lapping days. However near the end of the summer, rising confidence closer to the edge of the friction circle did exceed car control at one point. So this fall provided time to ponder what was needed to reach the next level, and answer confounding equipment questions about:

• R – Compound Tires
• Intermediate Brakes
• Uncontrolled Friction Interfaces         
  

R- Compound Tires

The move to the intermediate level at ADS was accompanied by an upgrade in both brake pads and tires.  The tech inspection for the 2007 season opener surprised me. I had to install a set of more aggressive ‘intermediate’ brake pads (Ferrodo DS2000). I’d only had 4-5 track days plus some six months of daily driving on the last set but they were below the 50% remaining threshold. I was beginning to understand why they were called consumables.  I’d not factored in the refresh cost of $400 plus in my ADS budget. Hmm!

I came to Mosport for the April 21-22 weekend with the emerging confidence and excitement my first season had instilled. I recognized the familiar faces a few of my novice classmates, Jesper Throft (540ia) and John Hansen (race-prepped E36 328i) to name two who had graduated with me to the intermediate class.  After a round of golf at near the Le Circuit at Mt. Tremblant last summer, our class instructor, Peter Clifford, was a familiar face too.  It was fun to be back learning and swapping stories with my track buddies.

On the Day 1 track sessions I was content to run mid-pack with the intermediate group. With street tires on I focused on remembering the racing lines.  I was privileged to be instructed by Alan Lewis, regularly at the top of the BMW club racing circuit, and, after the previous winter layoff, surprised to find I had to relearn the track. When I hitched a ‘student ride’ in Alan’s E46 M3, he set a gold standard in smoothness and consistency. I knew then what to aspire to at ADS.

The biggest excitement of the weekend was the introduction of R-compound tires onto my E36 M3.  In my first year I’d been advised by John Dimoff to stay on high performance street rubber until I’d gained more experience. However, I was itching to strap on a set of K1 Racing RIMS (17x8.5 in) I’d procured from Covell Brown’s old E36 racecar. On Day 2, Year 2, I decided I was ready.

So on went the heat-cycled 245x40 mm Hoosiers. The impact was immediate and dramatic. The momentum carried through the corners was significant, and the satisfaction of drifting through Corner 5B on occasion was thrilling. Now I really could catch people under braking. The previous benchmark in our class was a trio of Porsche’s (of the Cayman S variety) who on Saturday had consistently dispatched me on the Andretti Straight. Sunday, I was able to return the favour on more than one occasion in the afternoon session.

However all was not smooth sailing. A strong and persistent vibration occurring at speeds over 100 km/hr was unsettling. As was the smell of burning rubber percolating into the cockpit.  Back in the pits, an inspection revealed that a thin but deep (1 mm) groove had circumscribed the inside tire wall.  Being cautious, I mounted my street tires back on and then struggled to keep up with the Porches again in the last session of the day.

Alan’s summation in my student evaluation was he’d seen progress but he felt I was ‘pushing’. With R-compounds I’d shown him ‘fast’ but ‘smooth’ – like that Santana song – I wasn’t so. But approaching Moss Corner with Hoosiers on had taught me the meaning of “brake hard”! And the lesson this ADS weekend: good tires make a difference, but not all E36 coil-over suspension setups are the same. However, the consequences of my fun experiment with Hoosiers were to be delayed until Round 2 at Dunnville Autodrome on May 20.

Intermediate Brakes

I presumed a spacer kit would solve my problem of running wider tires/rims in the front. A quick call to the Raven Peformance secured some 5 mm H&Rs and I headed off to Dunnville to test the solution. But the new longer wheel bolts proved excessive for the 5 mm spacers and clanked against the rotor backing plate. First track session lost.  Equipment issues were becoming a recurring theme.

With no other option, I put the standard wheel bolts back on (with an extra 10 lbs of torque) and made it out on track with the Hoosiers. The short Dunnville track had been vastly improved with new ‘turtles’ and paving. Soon I was circulating respectably fast with the other lappers. Hard braking was a constant necessity and the brakes were ‘pinging’, even after a cool down lap, dissipating the heat they stored from the tremendous amount of braking each session. 

Nearly the last to pack up, I heard a horrible grinding noise from  the rear wheels as I pulled out to drive home. I waved down John Dimoff for assistance just as he was departing.  Perhaps a stone caught in the brake shield ?  He recommended we pull off the rear rims to be safe.

“No brake pads”, he stated matter of factly. “Your kidding, right? “ I responded. “No, there is abolutely no pad material left”,John continued,”…see the shiny backing plates?”. It was then I remembered I’d not put in a brake wear sensor with the last pad change. Fortunately Covell Brown had stayed behind to donate some used HT-10 race pads from his former E36 race car. It was a very good example of BMW Club comeraderie at work. Both John and Covell were very deserving of the fine bottle single malt it cost me to get out of the jam.

I returned home questioning how I could possibly have gone through a new set of ‘intermediate’ rear combo street/track brake pads in 3 track days – plus 4 weeks of daily driving.   Something had to be wrong with the car. The front pads were still half worn so maybe it was a brake bias problem? Or perhaps the rear Brembo cross-drilled rotors were acting as ‘cheese graters’. Then again, maybe it was my technique - excessive hard braking ? (And I still had reduced but not eliminated the tire rubbing problem at the front.) This ‘upgrade’ in equipment was seeming like a downgrade.

So back to Bavarian Motors / Raven Performance for some car therapy. John Dimoff suggested graduating to even more aggressive street/track pads – Performance Friction Gold’s  – and wider 12 mm spacers plus some fine tuning of the coil-over suspension. By the time the June 6-8 ADS at Mosport rolled around I was hoping I’d sorted my brake & tire problems. With Instructor Bruno LaFosse, I focused solely on smooth driving. However there was one drama that could have become a crisis. Vibrations appeared (again) during a track session and got progressively worse. Perhaps my tire rubbing problem had returned? I felt discretion was the better part of valour and pulled into the pits. Inspection of the driver’s side front wheel found all five of the (longer) wheel bolts loose!  Somehow I”d forgotten to torque one wheel prior to the session.  An alarming rookie error.  After that fright my adherence to routine pre-track checks became religious.

I neglected to mention this incident to my wife as we headed to Calabogie for our first driving/holiday weekend.  Liz ‘camped’ out at the Calabogie resort, making guest appearances for the inaugural Club Race, the ADS dinner and a few of my track sessions.   This was part of my indoctrination plan to convince her that of course performance driving was safe, fun and social.  But ‘Houston’, we were to have another problem or two before the weekend was out. 

            Late Saturday, my instructor and namesake, Nigel (Mortimer) was very cool after I put two wheels on the dirt at the ‘Quarry Hairpin’and induced a graceful 180º spin. Coming to a screeching halt facing the on-coming cars, Nigel offered a perfunctory ‘well you didn’t catch it, did you!” . 

No. I didn’t. The first time in nearly 17 track days that I’d lost control of the car. I was embarrassed and unnerved as we went into explain ourselves to the Chief Instructor, Bennett Leckie. And what was the ADS lesson here ? Well there is a friction circle and if you unsettle the car’s balance mid-turn you too can exceed it. I just wasn’t expecting it then and there!  Fortunately, Liz didn’t see that bit of advanced driving but our dinner table guests were kind enough to bring it to her attention in the day’s review.

The next day, after replacing a slightly bent rim with four rebalanced K1 Racing wheels & tires, I had two wonderfully smooth - meaning vibration-free - sessions.  Checked out and signed off by two instructors, Ralph Hansen and I agreed that we would have some fun with the 25 turns, hills and dales of Calabogie next session.   After a couple of laps leading his silver E36 328i racecar, I pulled over and let him through.  A lap or two later we got separated by a slower car, and as I tried to reel him in I found a trouble-some vibration appearing again. Having learned my lesson the last time, I immediately ended our game of cat-and-mouse and pulled into the pits.  An inspection of car after cool-down revealed I’d worn my rear-brake pads down to the backing  plates. Again ?! ….‘What the #%@&’ !! 

            With no time to swap pads I casually informed my wife that we wouldn’t return to  Toronto as quickly as we came as I wanted to practice a new-technique I’d just learned: engine braking. I also  vowed to solve the riddle of two complete sets of (rear) brake pads in 8 track days. That equation didn’t compute on several levels.

Uncontrolled Friction Interfaces

Late that Sunday night I went to all the on-line forums looking for clues. StopTech’s website, in particular, has some very good whitepapers on brake system design[1] and so I fired off a letter to them in the hopes they might be able to illuminate the situation. I wasn’t disappointed as an answer back the next day from Dan Barnes, the OE Manager of StopTech.  It turns out the answer to the riddle is very logical for those of us who claim to have studied engineering. 

According to Dan: “[My] situation is almost to be expected. The thing that changed between having brakes that performed to your expectations and brakes that are disappointing was tires. When brake pads are operated above their maximum operating temperature [MOT], a variety of problems can arise, which all fall under the umbrella of "uncontrolled friction interface." The one you seem to be seeing is accelerated wear.”

So how do better tires make your brakes stop working? StopTech’s explanation was basic high-school physics. By increasing your cornering speeds, you increase the speeds at the end of the straights heading into the braking zone. As kinetic energy is proportional to the square of your speed, braking from 80-40 involves three times as much energy as braking from 40-0,   even if a vehicle's power and weight are unchanged. Increasing average speed around a track increases the energy that brakes have to deal with.

With the Hoosiers, the brakes had to deal with a lot more energy, and their increased grip meant I was able to get my braking done in less time. The front rotors [13.5 inches] are larger than the rears [11 inches] and though they do more work, they seemed to be hanging in there okay with more air getting to them. It's sometimes said that the front brakes do 80 percent of the work, but according to Dan, that's not true. With my E36 M3, the rears might have been doing as much as 35 percent of the work.  The result was much higher peak in-stop brake temperatures.

Dan provided a compelling example to prove his point:

 “I have seen another situation when a very fast ‘time-attack’ car with our brakes on the front received a poor recommendation for rear pads, and burned a set to the backing plates in about 12 minutes on track. With the right pads on the rear, that car has carried on to set lap records at various tracks!”

StopTech’s recommendation was to get brake pads with a higher rated MOT.  They typically try combinations such as Hawk DTC-70 front and HT-10 rear, or PFC 01 front and 97 rear when a car is MOT challenged.   However, they are not inexpensive pads. StopTech’s engineers also suggested trying to get more cool air to the rear rotors, for example by trimming the dust shield/backing plate or creating aerodynamic devices to move air toward the brakes[2].  

To StopTech’s credit they didn’t say anything about their competitor Brembo’s product on my E36 in their response: “We really don't think [cross-drilled rotors] are your problem”.

Regrettably, owing to a family illness, the Calabogie L’est Fest event was to be my last as an ADS participant.  I did attend the PBOC (Porsche-BMW Owners Club) event in September as a spectator but my bad luck with equipment continued. I borrowed a Michelin jack from Ralph Hansen to return my front coil-over suspension to a street setting. Unbelievably, the jack failed as I was sliding the jack stand into position. I recoiled as hydraulic fluid seeped silently out onto the paddock and then watched helplessly as my BMW M3 sank down onto the misplaced stand. The fender sill crunched and slowly contorted until the front tire touched the tarmac.   Damn lucky I thought afterward that I didn’t try the ten second suspension adjustment without a ‘safety net’. It was a $1000 life insurance lesson.

*           *           *           *

Looking back on my 8 track days in the summer of 2007 it seems that I’d pushed the envelope a bit, suffered a few ‘teething’ problems but learned some valuable lessons.  It's been said that a race/track car is basically a machine to convert tires into speed. If so, lesson one might be don’t mismatch intermediate (street/track) brakes with advanced R-compound tires. Second, don’t short-change car-prep safety routines or you could risk learning things the hard, dangerous way. Third, expect to find equipment limits as you progress. Uncontrolled friction interfaces, whether those between the road and your tires or between your pads and rotors, will not be conducive to trouble-free advanced driving.  And remember as you continue down this road, it’s probably not a question of if, but when.