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How different wheels/tires affect MS using the P+ package as a guideline

Discussion in 'Model S: Driving Dynamics' started by lolachampcar, Jun 15, 2013.

  1. lolachampcar

    lolachampcar Active Member

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    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.
     
  2. Realist

    Realist Member

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    #2 Realist, Jun 17, 2013
    Last edited: Jun 17, 2013
    Hello :)

    The Michelin Pilot Sport 2 is a soft compound tire with excellent grip and feel on a dry circuit. The Contis are better suited for wet conditions.

    A 22 Wheel will not improve grip or lateral g until you are not changing the size of the tires. Steering feel can improve but the higher unsprung mass will offset that since ride will suffer.

    For maximum grip set toe and camber at it's most negative settings. Take a look at the Tesla Standard range and go a bit further on the negative side.

    Than look at the tire pressure under cold and hot conditions and adjust if necessary. These are the very basic instructions before doing anything regarding tires/wheels/suspension and damping settings.
     
  3. Zextraterrestrial

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    I had a chance to try out my new wheels ant tires autocrossing this weekend (Potenza RE-11). A world of difference!
    the 275/30R19 rears make the rear end hold quite a bit better I really want to try a P+ package to compare the stiffness

    just bushings will really add 40%, or the same improvement as wider rears?
     
  4. lolachampcar

    lolachampcar Active Member

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    Zex,
    I was regurgitating what I have been told by Tesla when I stated the bushings were worth 40%. I do not know that for a fact myself.

    I will say that there was a significant change from the P85 Conti test drive I did and the P85 Pilot that Tesla delivered to me. That being said, I still found the rear of the car to be vague to the point that I sold it and ordered the P85+ when it became available. The rear of the car needs to be less compliant and I suspect the bushing change is the answer.
     
  5. derekt75

    derekt75 Member

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    In this thread: Would 19s on a P85?
    some argue that 19" wheels will generally perform better than the 21"s.
    It seems reasonable to conclude that 22s would do worse.
     
  6. lolachampcar

    lolachampcar Active Member

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    22" tires/rims have less side wall = less compliance = better feel = less absolute grip. The tire sizes proposed have a larger contact patch to account for the reduced compliance. This is why Tesla went to slightly larger rear wheels when they increased bushing stiffness = less compliance.

    Putting really sticky tires on 19" rims should give better overall mechanical grip. The car would launch harder and may even have a greater skid pad number. However, I think you would find that the slalom number would fall as the transient response would suffer with the increased tire sidewall flex.

    Again, I do not use the ultimate grip of my MS much apart from hard acceleration from a stop. Cornering at the limit on the street is no longer in my bag of tricks. However, I am very interested in feel and feedback as this I experience every time I am in the car thus my interest in different wheel/tire options. Of course, the first stop for me is the P85+ as delivered. Perhaps this will do the trick for me.
     
  7. Doug_G

    Doug_G Lead Moderator

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    The PS2's are pretty good in wet conditions, in my experience (previous car). Some racers use them as their rain tires.
     
  8. Zextraterrestrial

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    Thanks LolaCC
    I have 30 series 19" though for the rear 275 mm so the sidewalls are even thinner than the P+ setup
    and the car is lower. But it still is a bit loose in the rear on bad pavement. I really need to learn to drive it with TC off at the next autoX. It is tricky!
    The RE-11s on the front makes a big difference in getting into corners over the Conti 21's

    I didn't want suspension changes until I started autocrossing. It is pretty good but could be stiffer. Cant you add more pressure air suspension or change something a little?
     
  9. Realist

    Realist Member

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    A race driver will use a wet compound race tire which is in a completely different league to any standard road tire.

    The PS2 is not bad in the wet,but the Conti is among the best under such conditions. The new Michelin SuperSport is eben better than the PS2 with more consistent and predictable grip levels on the limit.

    This tire might improve the Model S' handling even further.
     
  10. SouthJerseyJon

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    I have a non-perf and my thought was to put staggered 20's on my car to prevent slip during the launch phase from a red light. I live in NJ and I was concerned about going to large on the rims to avoid the pothole damage but large enough to get an aggressive wheel/tire combo. They are being forged and I should have them in a couple of weeks. Once they are installed, I will not the performance and then see about having the bushings upgraded as well.
     
  11. lolachampcar

    lolachampcar Active Member

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    Zex,
    I've not looked into the exact details but I think you will find air pressure changes in the suspension air springs simply change ride height. Spring rate itself is a matter of bag geometry and thus is set at bag design. Anyone here have direct experience with air spring design or design in?
     

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