Saturday, January 6, 2018
Camber/Caster and Toe for the iRacing FR2.0
Camber/Caster and Toe for the iRacing FR2.0
In the FR2.0, Caster is fixed and not adjustable. It is most probably affected by other settings. Based on experiments, there is a small increase in positive Caster when increasing the negative value of front Camber. An increase in positive Caster pushes the inside front tire down which causes weight to be transferred to the outside rear tire and in most cases this makes the car oversteer more, or understeer less.
The Camber settings on the iRacing FR2.0 are generally lower or less negative than the car in real life, and much lower than you typically see on F1 cars.
Camber adjustments and their effects has a great deal to do with tire characteristics, as the “tire patch” is deformed and moved by cornering forces.
Camber and Toe-In or Toe-Out are essentially settings that affect cornering force, overall stability, traction for braking and power application, and mechanical drag.
Our goal is to achieve the highest cornering force and good stability while minimizing loss to traction for braking and power, and having the acceptable mechanical drag.
Negative Camber is simply the angle that the tire leans IN at the top.
The photo above is a real test car with an extreme amount of negative camber. Interestingly, the test results confirmed that with “rounded” motorcycle type tires, the more negative camber the better. On “rectangular” race car tires, the optimum amount is much less because as the tire leans more, the contact patch becomes smaller.
Most race cars use a form of double wishbone suspension. Note that the upper rods are shorter and tilting at a greater angle than the lower ones.
This difference in length and initial angle cause the camber angle (relative to the chassis) to change as the tire moves up and down, whether the movement is caused by aero downforce, bumps, braking, power acceleration, or chassis roll. The particulars of the design, determines how much the camber angle changes relative to the ground.
In the picture below, the outside tire has moved up relative to the rolling chassis. The chassis roll of 2 degrees does not cause the outside tire’s camber to change 2 degrees (less negative) relative to the ground—instead the shorter upper rod causes the camber to only change by one half of a degree. If the outside tire (left in the picture) had started with negative 1.5 degrees camber, than during cornering it would have negative 1.0 degrees. This is good.
The opposite happens on the inside tire. The chassis roll of 2 degrees does not cause the outside tire’s camber to change 2 degrees (more negative) relative to the ground—instead the shorter upper rod causes the camber to only change by 1.5 degrees. If the inside tire (right in the picture) had started with negative 1.5 degrees camber, than during cornering it would have negative 3.0 degrees relative to the ground. One will immediately realize that in a turn, the inside edge of the inside tire will be doing more “work” than the outside edge. This is not good and is really not good for the rear where power is applied to both tires. (That is one reason why camber needs to be limited and rear camber is usually less than front.)
The primary benefit from Camber is from Camber Thrust. Read the article at this link for more explanation: https://en.m.wikipedia.org/wiki/Camber_thrust
The primary benefit from Toe Out/Toe In is tire wear and stability. Read the article at this link for more explanation: https://en.m.wikipedia.org/wiki/Toe_(automotive)
We took the FR2.0 to the Centripetal Track and tested five different combinations of Camber:
Low Medium High
-0.5F/-0.5R -1.5F/-1.0R -2.5/-1.8R
The “Low” settings did not produce the optimum cornering force. The “Medium” settings provided sufficient cornering force to allow a 1.5-2.0% faster cornering speed.
The -1.5F/-1.0R combination had the best tire wear—more uniform across the face of the tire tread. The -2.0/-1.4R had a marginally higher cornering force.
The “High” setting produced unacceptably uneven tire wear, although in “high grip” track conditions with higher chassis roll, might be acceptable.
We next checked to see the relationship, if any of camber to top speed—a way of measuring mechanical drag. On to the Talladega Speedway.
With the -2.5F/-1.8R “High” setting, we reached a maximum speed of almost 158 mph. With the -1.5F/-.1.0R “Medium” setting, we were almost 1 mph faster with a 0.5% faster lap time.
Actually, this confirms the “theoretical” mechanical drag produced by the “Camber Thrust” that occurs from tire patch deformation. This Camber Thrust adds to cornering force, but also produces mechanical drag.
Next we checked the relationship, if any of camber to braking power and stopping distance. It is assumed a similar relationship for traction under power would exist.
Running at 155 mph, with threshold braking, the -1.5F/-1.0R “Medium” setting allowed the car to stop 6.5 feet or 1.6% sooner than the -2.5F/-1.8R “High” setting. Running at 100 mph, a similar result with the “Medium” setting stopping in 7 feet or 4% less distance.
My theory on why the larger difference for lower speeds, is that at the lower speed, there was less downforce and impending wheel lockup occurred over a larger relative part of the braking zone—where traction would be more critical. (i.e. there was “surplus” traction with the high downforce at higher speed.)
Finally, while at Talladega, we tested various Front and Rear Toe combinations.
0/32F/0/32R produced 158+ mph with a lap time of 1:00.749
-1/32F/0/32R produced fastest speed of the day with lap time of 1:00.678 (only 0.12% better)
-2/32F/0/32R saw little or no change from the 0/0 settings.
-2/32F/+1/32R saw little or no change from the 0.0 settings.
-2/32F/+2/32R still achieved 158+ mph and lap time of 1:00.749
-2/32F/+3/32R saw a 0.5% reduction of speed
Essentially the slight toe out of -1/32F offset or cancelled some of the drag from the negative camber.
-2/32F toe out in the front did not produce any significant drag and would be suitable for improving initial turn in according to driver preference.
-1/32 or -2/32 toe in for the rear (each tire) did not produce any significant drag and would be suitable for increasing stability and decreasing oversteer under power during cornering according to driver preference.
The chart below illustrates the relative grip with various camber settings.
The ideal front setting is in the 1.4 to 2.0 range. With lower settings, you will not be optimizing cornering force that is gained by camber thrust. With higher settings, you will begin to risk overheating the inside edge of the tire, but more importantly will be adding to mechanical drag and reducing traction for braking application. (There are exceptions depending on all the other chassis setup settings, depending on the importance of braking power, and for qualifying vs racing trim.)
The ideal range for the rear is in the 0.8 to 1.6 range. With lower settings you will not be optimizing cornering force that is gained by camber thrust. With higher settings, you will begin to risk overheating the inside edge of the tire, but more importantly will be adding to mechanical drag and reducing traction for power application. (There are exceptions depending on all the other chassis setup settings, depending on the importance of traction for power, and for qualifying vs racing trim.)
Note that the rear range is lower, and the “average” difference between front and rear is generally 0.4-0.8 degrees. This is partly due to the individual car’s particular suspension design, but generally, rear wheel drive cars are using the rear tires for braking, cornering and power application, and therefore having both tires with maximum rubber on the ground is the goal. Too much rear camber (which rises on the inside tire during cornering) can result in lost traction on the inside tire during corner exit.
Toe Out on Front should be -1/32 to -2/32 each tire to suit driver preference on initial turn in.
Toe In on Rear should be 0/32-+2/32 each tire to suit driver preference for reducing “power on” oversteer at mid corner and on corner exit. (Throttle application can be theoretically more abrupt.)
Higher Camber will result in a larger difference in tire temperature and wear from inside to outside. More Toe Out will increase this tendency. Toe In will decrease the tendency. Look for an ideal where the Inside and Center Tire Temps and Wear are close to the same with lower Temps and Wear on the outside edge. And, of course, as mentioned in earlier articles, a balanced race car exhibits similar tire temps and wear on the front and rear tires on the same side of the car. (You will often see a difference between the right and left sides of the car—that is dependent on the track design.)
Please remember that these settings all are interdependent on other chassis settings and driver preference. Test, make minor changes, and retest. Each driver/car/track combination will be different.
Posted by Donald "Wayne" Strout at 4:24 PM