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Air or coils for the UK?

Discussion in 'The UK and Ireland' started by Afdyce, Jan 4, 2015.

  1. Afdyce

    Afdyce Member

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    My car went in to Gatwick for some minor fault correction before Christmas and I was given a loan car for two days. The perfect opportunity to compare the suspension options over a sensible distance on UK roads. These are my thoughts, I hope they might help any of you who are agonising over which set up to order.

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    Black P85 VIN 474XX Registration H15 AWF
    Coil suspension, 19” wheels, 245/45R19 Goodyear tyres. 42 psi cold

    Grey S85 loan car VIN 423XX Registration RK64 VAJ
    Air suspension, 21” wheels, 245/35R21 Continental Tyres. 45psi cold

    Date: 16[SUP]th[/SUP] December 2014

    Weather: 5 to 7 degrees centigrade, damp conditions on day one, light rain on day two


    Traction:
    Full power standing start with fast throttle roll on.

    The 19” Goodyears were fine, only activating the traction control occasionally.
    The 21” Continentals were very poor, only managing around 1 in 5 clean runs without traction control activating. From cold they were even worse.

    Applying full power at low speed, around 20 mph, in a straight line.

    The 19” Goodyears maintained traction every time. The Continentals broke traction around 50% of the time.

    The Continentals were surprisingly poor in both tests, especially as the S85 has 160 newton metres less torque than the P85.



    Road noise:
    The 21” Continentals are considerably noisier than the Goodyears everywhere with the noise varying markedly on different surfaces. It is the dominant noise in the car.


    Ride:
    The combination of coil springs and 19” wheels definitely rides better than 21” wheels with air. The air suspension does not have the ride subtlety of the coil set up. It has "sharp edged" responses over small ridges and feels just a bit too stiff everywhere. The end result is closer in feel to a mid sized car than a vehicle in this class.


    The only difference between the two suspension systems is the replacement of coil spring/damper units with air springs. As a result roll, dive and squat are identical and very good in both cars, largely due to the low centre of gravity in the basic design.


    The ride heights are different with the coil spring car set at a single compromise height, higher than the standard setting on the air suspended car. As the ride height setting reduces as the car lowers on the air car the front and rear tyres lean in at the tops as the car sinks, creating more negative camber. This transfers the weight more on to the inner portion of the rear tyres and is almost certainly the root cause of the poorer traction on 21” tyres. The full width of the rear tread is not evenly loaded across the tread, so effectively the load is concentrated on the inside edges of the tyre treads.

    This is born out by the very poor tyre life on air suspended cars with 21” wheels that has been reported on this site.

    Steering feel:
    A surprise here, because experience would say that the combination of increased front negative camber on the air suspended car along with the shorter stiffer side walls on the 21” tyres should give better steering feel. I had made sure both cars were on the “normal” steering setting for the test and re-checked I hadn’t made an error after I had gone 10 miles or so.


    Put simply, the air car’s steering feels more disconnected and artificial. It is precise enough, just devoid of feel about what’s happening between the tyres and the road surface.

    You do feel the benefit on initial turn in, which is more direct and sharper than the coil car with 19s, primarily as a result of the extra negative camber
    and stiffer side walls, but does this really matter? At track speeds I have no doubt that the air car on 21s will feel a bit more secure with less understeer when you are hauling towards an apex. The reality is that road driving rarely calls for such a fast rate of application of steering angle, so you don’t notice the benefit.

    The bit you always notice is the vastly superior steering feel on coils and 19s. Okay, it’s no 911 but you always have a sense of what’s going on and a feel for available traction.

    Grip
    At any sane road speed there is no discernible difference in outright grip because both tyres have 245mm tread width. The even front to rear weight distribution ensuring the tail slips out first under power. In fact, classic rear wheel drive handling.


    This is a big car on UK “B” roads and this is more than anything else the limiting factor to how fast you can go.

    My conclusions:
    This is a luxury car and it weighs 2.1 tonnes. The 21” wheels, 35 aspect tyres and low ride height of the air suspension give the appearance of a performance tuned model like an M5, but this isn’t the case. The suspension is set up for comfort and is actually compromised by the adjustable ride height. The increase in negative camber at the rear as the car lowers significantly reduces traction and increases tyre wear rates to near unacceptable levels. Of course the car looks great on big wheels with the low riding stance, most cars do. It’s only perceivable technical advantage over the coil spring set up is the ability to raise to clear steep driveways and self level with a large load in the rear.



    You have to decide if you want the best appearance and don't mind the increased road noise (plus a bill for £5,500) or the best ride, traction and driving dynamics.

    Whatever you choose, don’t mistake the Model S for an electric M5, it isn’t. It is great to drive in it’s own right and makes everything with an ICE feel clunky when you go back and drive them.


    These are my personal opinions, underwritten by 15 years of historic racing experience as a driver and in developing winning competition cars. I have written it to try to help future Tesla owners decide on the suspension specification they might want for UK (as opposed to US) driving conditions.


    I will be trying some different tyres over the next twelve months, starting with Michelin Pilot Alpin winter tyres, which are on order. Once the winter is gone I plan to go on to Michelin Pilot Sports and I’ll put my impressions of the differences up on the Forum in due course.







     
  2. mgboyes

    mgboyes Member

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    Interesting to hear your experience.

    However you're comparing a car on all season tyres to one on summer tyres, in weather conditions where summers are completely inappropriate. This is almost certainly the explanation for poor traction from the 21" wheels.
     
  3. smac

    smac Active Member

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    I've always felt the 19s were the way to go. Interesting to get some feedback on the air suspension too.

    Yes 0-60 it's faster than an M5, but show it some B roads, and my Elise with a fraction of the BHP will walk away. (And a 911 positively run)

    It is what it is, here in the UK. A BIG luxury car, one I'd never dream of taking to a track (other than for the giggle). It's just a shame they hadn't configured the goldilocks setting the coils are set at, to be a better compromise on looks. Effectively dropping ride height, without impacting caster

    There's definitely an "on stilts" look to a coil car on 19s :(
     
  4. Afdyce

    Afdyce Member

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    Hello mgboyes. The Contis are an all season tyre with an "A" rating for wet grip, like the Goodyears. The temperature wasn't really low enough to make a winter tyre necessary. I am convinced the poor traction was more a function of the tyre tread not sitting flatly enough on the road to grip across it's whole width than the tyre itself.

    I recently ran the same spec Contis on a 500bhp supercharged Mustang and found traction to be first class, so I was really surprised at how easily they slipped on the S85.

    I didn't touch on it in my original post, but a major difference is how the S85 launches in comparison to the P85. The P85 offers full linear power straight away. The S85 takes more time to ramp up to full power, almost as if it is artificially torque limited at first. I felt this should have helped the Contis if anything.

    To really answer the question I need to try an air car with 19s and a coil car with 21s!
     
  5. grahamsimmonds

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    @Afdyce a thought provoking post - I need to declare that I have a P85D with air and 21" grey turbines on order. The choice was a no brainer for me - I loved test driving the P85 with 21" wheels and air. I also drove an S85 with 19" wheels and air. Still a lovely car and very enjoyable to drive. However, I felt I did not want to own a car with 691 BHP 4WD with 19" wheels fitted but that's me. I am also aware that Tesla recommend that the P85D has the 21" wheels fitted.

    I am pretty certain that wheel and suspension choice is largely subjective - cost obviously also has a bearing here too. My own opinion is that the 21"s gave much more traction - not a squeak when launching and very reassuring handling. But then the 19"s were not bad either! But then the Model S is a unique car.

    Still I enjoyed reading your post even I am not sure I agree with your conclusions.
     
  6. Jamesteruk

    Jamesteruk Member

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    @Afdyce, thanks for the post - was very useful in trying to decide today when ordering my car. Thought the sensible decision was the 19s, and went for them on my P85D, without the air suspension as well. Order placed today!

    @graham - had no comment or pressure from the Tesla sales guy about debating 19 vs 21. Where have you heard that it is their recommendation?
     
  7. simonog

    simonog Member

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    On my P85+, I notice that the grip is phenomenal. That, I should add, is running on air not coils. I have only ever been able to get poor traction when there is loose material on the road surface.

    The upgraded suspension components may help here. My comments, by the way, apply both to my experience on the road and on track (yes, mad, but it wasn't my car :))

    I do also use winter tyres at this time of year and I agree with @mgboyes that the tyre/weather will have an impact.

    in the end, the coils/air decision seems to me to be a personal one based on driving style.
     
  8. J1mbo

    J1mbo Member

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    I have 19" on air and was loaned a 19" coil car. Both had standard tyres.

    After 8,000 miles on air, I immediately noticed the smoothness of the drive was gone with the coils. Yes, you get more driver involvement, but I found it distracting on the motorway where I didn't need to feel every crease and dimple on the road. Cornering wasn't any more involving with the coils (the P85+ wins here hands down), and London's road humps were just a bit more annoying. Not being able to raise the car in a spot where I usually do gave a moment of nervousness. Of course, the coils are slightly higher than air/standard so no drama in the end.

    Ultimately I think it comes down to what you are used to. After enjoying air, going back to coils made me appreciate the air more. However, it wasn't the sloppy, bouncy, sea-sick inducing ride that you might generally associate with big US sedans, and my overall impression was very positive.

    I'm glad I chose air, but I wouldn't say that it is an essential option like the Tech package.
     
  9. Afdyce

    Afdyce Member

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    Hello @simonog. Are you running Michelins on your + ? The rears are 265 wide on the + compared to 245s on the standard suspension, so there's a larger contact patch to help traction and my suspicion is that the Michelins have better grip in damp conditions than the Contis. I excluded the + from my comments for two reasons, the £11K price increase over a coil car on 19s and the fact that I have only driven one in London and never got over 40mph!

    All Model S variants that I have tried, regardless of tyre or suspension spec can make a full power start without activating the traction control in the dry. My surprise was how poor the Continentals were in the damp weather of my "test".

    - - - Updated - - -

    Hello @J1mbo. That is a really interesting comment as I haven't been able to assess air against coils with 19" tyres on both cars.
     
  10. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    This is not correct. Contact patch relies almost entirely on psi in the tire. The contact patch is determined by the load on the tire divided by the psi, so if you have 1500 lbs of load on a tire and inflate to 50 psi, the contact patch will be 30 in.sq. (numbers chosen for ease of calculation) Depending upon the tread width it may be short and wide or long and narrow, but the area will be the same. The traction differences between the two tires mainly depends on the tread compound and belt construction.
     
  11. Afdyce

    Afdyce Member

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    [email protected] Okay, I am officially envious. The P85D is top of my list to replace my P85 when the time comes. I am sure you will love it when it comes (and spend a lot of time on this forum while you are waiting). Traction is obviously not going to be a problem and as the suspension appears to be an updated version of the +, I would have gone with air and 21s myself. The P85D is really in a different class with a £25K premium over my P85 on coils.

    I hope I spot the D badge before I pull up along side you at the lights!

    - - - Updated - - -
     
  12. simonog

    simonog Member

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    @Jerry33 is right: the area of tyre contact is a function of tyre pressure and load on that corner of the car alone. As it happens, yes: I run Pilot Sports for the summer tyres and SottoZero for winter.

    PS As I understand it, the suspension for the P85D is identical to the P85+ design. That is optimised for handling with the 21" wheels. The optimum for range, however, is the 19" wheels, which get about 3% more than the 21" at the cost of cornering ability. It's about what matters to the prospective owner. I wanted optimised handling especially on such a high power car.
     
  13. Afdyce

    Afdyce Member

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    Yes, sorry, contact patch is the wrong phrase
     
  14. Alan

    Alan Member

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    #14 Alan, Jan 18, 2015
    Last edited: Jan 18, 2015
    Currently have a P85+ on 21 inch with air as a loan car for my Roadster. My wife has a P85 on 19 with coils so I have been swapping back & forth.

    Quick summary - P85+ is a slightly harsher ride but other than that no noticeable difference in grip, steering feel or handling. Would go for 19 inch and coils if ordering again. Only caveat is that the roads the last week were damp / wet all the time so not possible to push the handling to the limit (not that of course I would push a loan car to the limit :rolleyes:)
     
  15. Mark77a

    Mark77a Member

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    Slightly off topic - was it easy to get a Model S loaner while the roadster was in ? Any tips that might help get one when mine goes in for a service ?
    (oh and I hope the roadster is OK ? )
     
  16. Electric700

    Electric700 Member

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    I share your feelings on this, having also driven both types.
     
  17. Alan

    Alan Member

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    No tips really - I just asked nicely and they said yes. Suspect many Model S owners would be more than happy to try a Roadster out for a few days when their car is in for a service - no harm in asking.

    Roadster all OK - just a service and they have kept the car for a few days to try and find an intermittent issue with the tyre pressure monitoring system. Its great to see Tesla still look after / have not forgotten the Roadster owners!
     
  18. Electric700

    Electric700 Member

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    Yes, unlike other car companies where the dealerships try to get you to buy a new car when yours is only a few years old. This is from experience!
     

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