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I like to play with fire :-)
I like to play with fire :-)
Re: Chassis tuning with a pyrometer
Great information just in time for autocross next week. Thanks for an informative lesson.
Re: Chassis tuning with a pyrometer
Chassis tuning with a pyrometer
As I have stated before, the very best way to determine optimum tire pressures, camber settings, and sway bar settings is to use a tire pyrometer to measure the temperature of the tire treads. The following information is intended to help you get started with the process.
A tire pyrometer is a temperature measuring device which utilizes a thin needle probe to penetrate slightly into the tread of a tire. Measurements should be taken in a straight line across the tread at 3 different points: app. one inch in from the outer edge of the tread, in the center of the tread and app. one inch in from the inside edge of the tread. If the outer tread block is relatively small or narrow, use the next one in for your measurements. Insert the needle in the middle of a tread block so you get lots of rubber all around it. Insert the needle up to its "stop" and allow the temperature reading to more or less stabilize. Don't wait for the reading to stop changing entirely, just wait for it to slow down significantly, then move on to the next position on the tire tread. If your pyrometer has a memory function, let it keep track of the temperatures for you. If it doesn't have one, you must write them down as you go. The tires will be cooling down while you are taking measurements, so move as quickly as you can but don't rush to the point where you get innacurate readings. Always take the temperatures in the same order: start with the same tire, go around the car in the same direction, and always measure across the tread in the same order.
What you will end up with are four sets of temperature readings (one for each tire) with three temperatures for each set. It is imperative that you keep those temperatures in order since they will correspond to measurements taken on the inside, middle or outside of the tread. The last tire measured will probably have lower overall temperatures since it has had the most time to cool down. If you want to compensate for this, re-measure the first tire again and see how much temperature it has lost, then you will know how much that last tire has lost.
What we are looking for is the distribution pattern of temperature across the tire tread after a particular type of driving. If all temps are equal (not likely), the contact patch of the tire is being optimized for that type of driving since each portion of the tread is being worked equally hard. If the tire's pressure is too high, you will see the middle temp reading higher than the two side readings. This results because the middle of the tire is bulging out due to excessive pressure which results in more contact pressure at that point thus a higher temp reading. Try lowering 1 psi for each 4-5 degrees higher reading. If the tire's presssure is too low, the middle temp will be lower than the inner and outer readings. Try increasing pressure by 1 psi for each 4-5 degrees lower reading. If the tire's pressure is correct, the middle temp will be very close to the inner and outer readings.
In truth, three equal readings is good, but not perfect. The ideal readings would have the inner temp 5-10 degrees higher than the middle and outer readings, which should be very close to the same. This is because tires corner best when they have a small amount of dynamic negative camber, i.e., aren't rolling excessively onto their edges.
If your readings show the outer temps to be the highest, your tires are rolling excessively onto their edges so you need more negative camber, higher tire pressure or a stiffer anti-roll bar. Camber changes directly address this situation, anti-roll bar changes will also impact the car's tendency to oversteer or understeer, so be careful here.
If your readings show the inner temps to be much higher than the others, you are running too much negative camber or you aren't cornering hard enough to generate representative temps. If you drive in a straight line with negative camber settings, the inner temps will always be the highest since that is where the most tread pressure is being applied to the road. This is normal and explains why negative camber tends to wear out the inner portion of the tread first. What you want to measure is whether the amount of negative camber you have dialed in keeps the tire's tread relatively flat during cornering, thereby generating relatively equal tire temp readings across the tread.
If you cannot adjust negative camber settings, an adjustable anti-roll bar can be used to compensate for excessive tire roll. If the outside temps are too high, increase the stiffness of anti-roll bar at that end of the car. If the inside temps are excessively high, you can reduce the stiffness of the anti-roll bar at that end of the car, but remember we want the inner temps to be slightly higher than the outer ones. You can also try increasing tire pressure which will help resist the tendency of the tire to roll onto its outer edge but don't get carried away here or you'll sacrifice that optimum contact patch with excessive bulging in the middle.
Another technique requires averaging of the tire temps for each tire. Add up the three readings for one tire, divide by three, and compare that number with the average number for the other tires. This will give you an indication of how hard each corner of the car is working. If the front averages are much higher than the rears, your car may be understeering excessively or it may have too much weight on the front tires or you might need wider front tires or your front pressures are too low. If the rear averages are higher than the fronts, you may be spinning your rear tires under acceleration (get an LSD or wider tires) or there is too much weight in the rear or your car is oversteering excessively or your rear pressures are too low. If the averages are roughly the same, front to rear and side to side, your car is well balanced. Remember that the last tire you measured will have lost some of its heat and you must take this into consideration, especially when comparing averages.
Some caveats. Pyrometer readings are intended to help set up a car for racing where the ideal set up will be different for each track. On the street, we are looking for a good compromise set up, so don't go crazy trying to get everything perfect. I like to set up my car to work best during spirited driving. That means excessive negative camber and stiff anti-roll bars. My temp readings after normal street driving will not be well balanced. So I go out and drive very aggressively, then stop and take readings. If you want to optimize your car's set up for less agressive street driving, go for a nice drive in the country and then take your readings. Just remember that your car's suspension is active (camber, toe-in and weight distribution change during cornering) so don't expect perfection in your measurements. We are dealing with a series of compromises and just looking for the best balance for our intended use. One reason to lean towards the more aggressive optimization is then your car will provide the maximum traction possible during emergency situations. The trade-off will be slightly higher tire wear during normal driving.
A few things to remember when taking temps. Don't brake hard when stopping to measure temps. Come to a gradual stop. If the stretch of road you drive has lots of hard right hand turns vs few hard left hand turns, your readings will be skewed. If the last turn you made was a long, hard right hander, your readings will be skewed. I like to weave back and forth on a straight stretch to equalize the readings. This also allows you to determine just how aggressive you want to be and your readings will reflect that decision. I usually make 15-20 weaves at the limit of traction, then stop and take measurements. In any case, take your readings immediately after spirited driving, don't cruise around looking for a good place to park. Using tire temps well takes lots of practice, so don't rely on your first readings to completely tune your car. Take lots of readings under varying circumstances until you begin to get a feel for the process. Remember, too, that tire pressures will go up when you are driving and go back down when the tires cool off. If you are playing with tire pressures, add or subtract pressure until you achieve the temp spread you are looking for, then measure your tire pressure after the tires are completely cooled down (overnight) and use that setting as your starting point from then on. Ideally, you would have a source of pressurized air with you when testing so you can adjust tire pressures up and down. If this is not practical, I suggest starting with an elevated cold tire pressure, say 40 psi, and then gradually bleed off pressure until you get that middle temperature reading down where it belongs.
Also keep in mind that ambient temperature will affect your tire pressures, so if you optimize your settings on a cool day, they won't be ideal on a hot day. Just pick the most typical conditions for your testing and live with the compromise on atypical days, unless you are racing, in which case, you will make adjustments to compensate for the differing conditions. If you aren't pushing the car really hard before taking your readings, you might want to move your measuring points in towards the center of the tread slightly so you get a more representative spectrum for a milder driving style (eliminate measurements at the extremes).
My favorite pyrometer is the Longacre Digital Recording Tire Pyrometer available from Pegasus Auto Racing Supplies (www.PegasusAutoRacing.com) at 800-688-6946 part#3174 for $299. This is a very high quality unit which achieves its readings very quickly (very important) and has a memory feature built in. Intercomp (www.Intercomp-racing.com) also makes a great pyrometer #360012. I do not like the infrared pyrometers because they tend to give innacurate readings since they measure surface temperature which changes quickly and you must take your readings from exactly the same distance away each time (hard to do) to get consistancy. If you already have one, you can give it a try to see what the process is like.
This is a very complicated subject and I have purposely left out some of the more esoteric considerations. If you follow these guidelines, you will get off to a good start. Then just take lots of readings and pretty soon things will start to come into focus.
If all of this is making your head hurt, take heart! If you have the original tires or new ones that are the same size on your Z8 and haven't altered the suspension set up, just run 35 psi front and 35 psi rear and sleep well at night. The engineers at BMW chose those settings because they represented the best all-round compromise for street driving. With those settings, you can still push your car to its limits on the twisties with no worries so don't feel like you're missing out on something by running the factory recommended pressures. In my experience, they're right on. Also, while using a pyrometer is the best way to dial in your tire pressures and chassis settings, a simpler method is to check for tire wear towards the outer edge of the tread after spirited driving. There should be a band running around the edge of the tire just beyond the tread area. If you see scuffing within 1/4 to 1/2 inch of this band, you are definitely in the ball park with your tire pressures. If you don't see any scuffing beyond the tread area, you are probably running too much pressure. If you have scuffing beyond the band, you are definitely running too little tire pressure.
If you have specific questions, I will try to help. Have fun!
Chassis tuning with a pyrometer