I'm considering 22" wheels and, as part of that process, was pondering how lower profile tires would impact ride quality and handling. It occurred to me that I was working from knowledge that I take for granted. There are a lot of people on TMC and thus a tremendous spread in experience and knowledge. I thought it might be useful to start a discussion on how wheel/tire combinations affect a car in hopes that I can learn from other's knowledge and that others might learn from the collective discussion.
I got into running cars on track to gain a level of competence as a driver (basically not be a threat to those around me) and to satisfy the nerd in me and educate myself on everything from engines/gearboxes/corners through suspension set up on high downforce cars. My comments below are based on those experiences extrapolated for street car use.
The ability of the tire contract patch to be pressed against the pavement is heavily dependent upon all the bits that exist between that patch and the main mass or bulk of the car. Following the patch to the car means you go through the tire sidewall, suspension bushings and spring/dampers. Two of the three of those elements are adjustable on track cars with the suspension bushings being that added element on street cars.
The general rule of thumb is that suspension compliance is directly related to mechanical grip. You will often hear something like "soften the rear to get more rear grip" with that softening taking the form of decreased tire pressure, softer spring rates or smaller roll bars. The same concept applies to street cars. With that in mind, let's look at the three elements that are "adjustable" on MS.
Tesla has recently started shipping the performance plus package where they have changed all three elements. They increased bushing stiffness, shock damping and changed the wheel/tire package. They also increased roll bar diameter but I'll pass on that for this post. Tesla has provided rough guidelines for the affect of the different elements in the P+ package and it goes something like-
40% - New control arms and links with revised bushings
40% - PS2's including wider rear tires and wheels
20% - Revised dampers and sway bars
The first item is new control arms with revised bushings. The bushings in question consist of a thin wall aluminum outer tube and a thick wall inner sleeve with rubber molded in between.
http://www.lolachampcar.com/images/Tesla/Rear%20Suspension/Bushing.JPG
The rubber annulus between the inner and outer aluminum bits serves to reduce vibration/noise from getting through to the chassis and acts as the bearing function allowing the link to rotate about the inner sleeve. You do not get a lot of rotation (before you tear up the rubber in the bushing) but then you do not need a lot for normal suspension travel. These bushings are incredibly cheap to produce and work very well in practice.
The bushings can be "tuned" by varying the amount and/or durometer (stiffness) of the rubber. Tesla has simply increased the durometer of the rubber to reduce the amount of travel between the suspension arm and what it is attached to (upright on the outside of the arm or car chassis on the inside for most links). The tradeoff being an improved control of the wheel/tire at the expense of noise and vibration getting through to the occupants. As a side note, changing the rubber used in the casting process has little affect on price apart from the fact that the P+ bushings are made in smaller numbers. Either standard or P+ bushings can be pressed into the suspension arms and, to my knowledge, the standard and P+ arms are identical (as in, only the bushings are different). Tesla appears to use a vendor to produce its arms as complete assemblies with bushings installed thus Tesla does not have the bushings themselves for purchase.
http://www.lolachampcar.com/images/Tesla/New%20Upper%20Arms/DSC03325.JPG
Going back to the compliance versus grip theory, these upgraded bushings will reduce compliance thus decreasing mechanical grip. They will also improve transient feel as changes at the contact patch will make it to the chassis quicker. Remember, Tesla says 40% of the difference you will feel with the P+ package comes from the bushings (as the arms are the exact same).
The next big item (40%) in moving to the P+ package are the wheels/tires. The P+ package has increased the width of the rear tires (larger rim and tire width) to increase rear grip. However, the Michelin Pilots have a stiffer tire side wall which tends to reduce overall compliance with an associated increase in "feel". I suspect the increase in width more than makes up for the increase in sidewall stiffness so the tires provide more overall grip while still improving driver feel. The increased grip of larger rear width is designed to offset the reduction of grip from stiffer rear bushings thus maintaining overall balance.
The last 20% is attributed to the dampers and sway bars. I've not driven the P+ package so I can not comment on the changes. It would be fun at some point to pull a standard and P+ damper and put them on a shock dyno. This would provide a graphical representation of damping as a function of shaft speed allowing the data from both dampers to be overlaid. Of course this would be purely a nerd activity as the dampers are crimp sealed and can not be rebuilt/revalved (per Bilstien USA). The increased dampening and increased roll bar diameter should decrease compliance and thus decrease overall mechanical grip. The very big caveat here is the assumption that all dampening was increased. I strongly suspect that Tesla increase low speed dampening (what you feel when you turn the steering wheel and side load the car) while leaving high speed dampening alone (what you feel when a wheel hits a bump). This would allow for better feel while still holding on to that lovely MS ride.
Most of the P+ changes result in reduced compliance. This should dramatically increase feedback from the road and driver feel. It should also reduce absolute mechanical grip. The increase rear tire size and improved rubber compound of the Pilots (and probably better overall construction) should more than make up for the loss of compliance related grip. It would be a fun exercise to get skid pad numbers from a P85 with 21" Contis, the same car with the P+ wheel/tire package and a full P+. I would not be surprised if the P85 with the P+ wheels/tires had the best skid pad numbers. It think it is important to note here that the P+ package is meant to provide a sportier car to drive and not necessarily provide better at the limit grip. It is very rare that drivers actually drive street cars at their limits (which is probably a very good thing) thus absolute grip is not really the main concern. For me, fun factor trumps grip. I would prefer a car that is less compliant providing very good feedback from a stiff chassis even if I give up grip at the limit. I simply do not spend any time at the limit and a lot of time just tossing a car around for fun.
Applying the above discussion to different wheel/tire combinations means we have to consider several things. First for me is that any change in sidewall stiffness must be accompanied by a matching change in contact patch. Shorter and stiffer sidewalls associated with 22" 30 or 35 series tires means the tire itself must be wider. Second, the stiffer sidewall means less compliance which means more road noise and bumps will get through to the occupants (downside) while the driver gets more feel/feedback (upside for me).
There are a lot of people on this forum with way more, or more varied, experience than me. Please feel free to chime in and add to or correct the above.
I got into running cars on track to gain a level of competence as a driver (basically not be a threat to those around me) and to satisfy the nerd in me and educate myself on everything from engines/gearboxes/corners through suspension set up on high downforce cars. My comments below are based on those experiences extrapolated for street car use.
The ability of the tire contract patch to be pressed against the pavement is heavily dependent upon all the bits that exist between that patch and the main mass or bulk of the car. Following the patch to the car means you go through the tire sidewall, suspension bushings and spring/dampers. Two of the three of those elements are adjustable on track cars with the suspension bushings being that added element on street cars.
The general rule of thumb is that suspension compliance is directly related to mechanical grip. You will often hear something like "soften the rear to get more rear grip" with that softening taking the form of decreased tire pressure, softer spring rates or smaller roll bars. The same concept applies to street cars. With that in mind, let's look at the three elements that are "adjustable" on MS.
Tesla has recently started shipping the performance plus package where they have changed all three elements. They increased bushing stiffness, shock damping and changed the wheel/tire package. They also increased roll bar diameter but I'll pass on that for this post. Tesla has provided rough guidelines for the affect of the different elements in the P+ package and it goes something like-
40% - New control arms and links with revised bushings
40% - PS2's including wider rear tires and wheels
20% - Revised dampers and sway bars
The first item is new control arms with revised bushings. The bushings in question consist of a thin wall aluminum outer tube and a thick wall inner sleeve with rubber molded in between.
http://www.lolachampcar.com/images/Tesla/Rear%20Suspension/Bushing.JPG
The rubber annulus between the inner and outer aluminum bits serves to reduce vibration/noise from getting through to the chassis and acts as the bearing function allowing the link to rotate about the inner sleeve. You do not get a lot of rotation (before you tear up the rubber in the bushing) but then you do not need a lot for normal suspension travel. These bushings are incredibly cheap to produce and work very well in practice.
The bushings can be "tuned" by varying the amount and/or durometer (stiffness) of the rubber. Tesla has simply increased the durometer of the rubber to reduce the amount of travel between the suspension arm and what it is attached to (upright on the outside of the arm or car chassis on the inside for most links). The tradeoff being an improved control of the wheel/tire at the expense of noise and vibration getting through to the occupants. As a side note, changing the rubber used in the casting process has little affect on price apart from the fact that the P+ bushings are made in smaller numbers. Either standard or P+ bushings can be pressed into the suspension arms and, to my knowledge, the standard and P+ arms are identical (as in, only the bushings are different). Tesla appears to use a vendor to produce its arms as complete assemblies with bushings installed thus Tesla does not have the bushings themselves for purchase.
http://www.lolachampcar.com/images/Tesla/New%20Upper%20Arms/DSC03325.JPG
Going back to the compliance versus grip theory, these upgraded bushings will reduce compliance thus decreasing mechanical grip. They will also improve transient feel as changes at the contact patch will make it to the chassis quicker. Remember, Tesla says 40% of the difference you will feel with the P+ package comes from the bushings (as the arms are the exact same).
The next big item (40%) in moving to the P+ package are the wheels/tires. The P+ package has increased the width of the rear tires (larger rim and tire width) to increase rear grip. However, the Michelin Pilots have a stiffer tire side wall which tends to reduce overall compliance with an associated increase in "feel". I suspect the increase in width more than makes up for the increase in sidewall stiffness so the tires provide more overall grip while still improving driver feel. The increased grip of larger rear width is designed to offset the reduction of grip from stiffer rear bushings thus maintaining overall balance.
The last 20% is attributed to the dampers and sway bars. I've not driven the P+ package so I can not comment on the changes. It would be fun at some point to pull a standard and P+ damper and put them on a shock dyno. This would provide a graphical representation of damping as a function of shaft speed allowing the data from both dampers to be overlaid. Of course this would be purely a nerd activity as the dampers are crimp sealed and can not be rebuilt/revalved (per Bilstien USA). The increased dampening and increased roll bar diameter should decrease compliance and thus decrease overall mechanical grip. The very big caveat here is the assumption that all dampening was increased. I strongly suspect that Tesla increase low speed dampening (what you feel when you turn the steering wheel and side load the car) while leaving high speed dampening alone (what you feel when a wheel hits a bump). This would allow for better feel while still holding on to that lovely MS ride.
Most of the P+ changes result in reduced compliance. This should dramatically increase feedback from the road and driver feel. It should also reduce absolute mechanical grip. The increase rear tire size and improved rubber compound of the Pilots (and probably better overall construction) should more than make up for the loss of compliance related grip. It would be a fun exercise to get skid pad numbers from a P85 with 21" Contis, the same car with the P+ wheel/tire package and a full P+. I would not be surprised if the P85 with the P+ wheels/tires had the best skid pad numbers. It think it is important to note here that the P+ package is meant to provide a sportier car to drive and not necessarily provide better at the limit grip. It is very rare that drivers actually drive street cars at their limits (which is probably a very good thing) thus absolute grip is not really the main concern. For me, fun factor trumps grip. I would prefer a car that is less compliant providing very good feedback from a stiff chassis even if I give up grip at the limit. I simply do not spend any time at the limit and a lot of time just tossing a car around for fun.
Applying the above discussion to different wheel/tire combinations means we have to consider several things. First for me is that any change in sidewall stiffness must be accompanied by a matching change in contact patch. Shorter and stiffer sidewalls associated with 22" 30 or 35 series tires means the tire itself must be wider. Second, the stiffer sidewall means less compliance which means more road noise and bumps will get through to the occupants (downside) while the driver gets more feel/feedback (upside for me).
There are a lot of people on this forum with way more, or more varied, experience than me. Please feel free to chime in and add to or correct the above.
