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'14 CLS63 AMG S, '10 GLK350, '03 SL5000
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I got some interesting and surprising results from pyrometer tire temperature measurements while running on the racetrack with my SLK55 (see an earlier post, “On Track with Porsches and BMWs, 3/8/05) I’ll be back on the track in early May and will file a follow-up report because I was not able to establish what I consider “final� tire pressures.

CONCLUSONS: (Preliminary); SLK55, standard-size Pirelli P-Zero Rosso:

- For spirited driving at track events a good starting point for cold tire pressures is: 36 psi in the front and 34 psi in the rear (EDC Off.) More testing is needed.

- Mercedes has done a fantastic job on the SLK in designing the suspension geometry to minimize tread wear on the outer edge of tires under hard cornering.

BACKGROUND:

Most of you already know all this, but just in case, here goes: Tire temperatures across the tire tread are useful in determining tire pressures and the amount of camber required to maximize cornering force and minimize tire wear. Typically the car is driven hard for 10 laps or so and then immediately brought into the pits to measure the temperature on the outside edge, center and inside edge of each tire. Adjustments are made to equalize these temps. On a race car, camber can be adjusted -- for example, add more camber to reduce the temperature on the outside edge. Although camber is not adjustable on our stock SLK, we can adjust tire pressures -- ideally to yield even tire temperature across the tire tread.

Factory recommended pressures for the SLK55 are 36 psi in front, 39 psi in the rear. These typical soft front and stiff rear tire pressures likely reflect input from MB’s legal staff rather than it’s test drivers. This setup promotes understeer which, generally is one way to keep inexperienced drivers out of trouble and MB out of court. I’d expect that 39 psi is really too high and will lead to rapid tread wear in the center of the tire. I’m running 34 psi on all corners for everyday street driving.

INITIAL TRACK SET-UP:

From my previous experience (primarily with stock and race-prepared BMWs), tire wear on the outside edge of the front tires is the major concern. This means adding significant camber and/or bumping up the front tire pressures to 40 psi or higher to prevent the front tire from rolling under during hard cornering. So I started my 1st �Getting To Know You� track event with the SLK55 with 42 psi, front and 38 psi, rear.

The track is run in a clockwise direction. Normally, this would suggest using higher pressures on the left side, but in the interests of keeping things simple, I started with equal pressures in the front and in the rear tires. And actually, I wanted to see the difference from side to side. So I expected (and observed) higher temperatures on the left side of the car. No surprises there.

TEST RESULTS: (Temps in degrees Fahrenheit):

Test #1: EDC On, 42 psi Front, 38 psi Rear

____________________Front of CAR_________________

_____LF (42 psi)________________________RF(42 psi)

Out___ Middle___In_______________ In___Middle___Out

131____138____148______________148___148_____132


_____LR (38 psi)_______________________RR (38 psi)

Out____Middle___In_______________In___Middle___Out

126_____128____131_____________125___126_____128

=================================

Reduce front tire pressure 4 psi and turn off EDC via dash button:

Test #2 EDC Button OFF, 38 psi Front, 38 psi Rear

_______________________Front of CAR

_____LF (38 psi)________________________RF(38 psi)

Out___Middle___In________________In___Middle___Out

129____131____140______________134___128____119

_____LR (38 psi)_______________________RR (38 psi)

Out___Middle___In_______________In___Middle____Out

119____127____150______________126___112____106

==================================

ANALYSIS & COMMENTS:

It is very unusual that the inner edge of the front tires is the edge which shows the higher temps with these pressures. Also, there was little evidence of tire wear on the outside edges. This is wonderful news and shows that Mercedes has done a fantastic job in suspension geometry design on the SLK. It looks like that tire pressures can be reduced another 2-4 psi without inducing substantial tire wear on the outer edge.

Note the differences in rear tire temperatures for the two tests. With the EDC on, the tire temps are very uniform across the tread. However, when the EDC is switched off the inside temps rose significantly, likely due to increased wheel spin coming off the turns. If I were going to run with the EDC on, I’d stick with 38 psi in the rear. But with the EDC switched off via the dash button (as recommended in my post about the on-track experience), rear tire pressures need to be reduced.

Next time I’ll start with 36 psi in the front and 34 psi in the rear with the EDC switched off and make adjustments in tire pressure from there. I’ll make a follow-up report from Pikes Peak International Raceway (road course, not the oval) in early may with the BMW Club.

Hope you found this interesting and helpful.

Earlier related post, “On Track with Porsches and BMWs, 3/8/05
http://www.benzworld.org/forums/forums/thread-view.asp?tid=1202849&posts=10&fid=27
 

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SLK 350
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2,327 Posts
Gordon,

Thanks again for your informative threads. Though I don't think I'll ever drive my 350 on a race track, I find all technical information about the SLK to be very interesting.

Mel
 

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'05 SLK350
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73 Posts
I have really enjoyed reading these threads. I take every new car to the track for a day just to better learn its dynamics and my SLK350 will be no exception this spring some time. Tire wear has always been an issue for me at the track. I learned a lot from this message. Now I think I'll have to buy a pyrometer![:)]
 

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2005 SLK350 2011 GLK3504X 2006 ML350(selling when GLK arrives)
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1,940 Posts
Gordon...what can I say....great posts...thanks....[:D][:D][:D]
 
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