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Tyre size setting in Lexia.

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smorphet   
Mon Aug 13 2012, 10:27PM
Joined: Apr 13 2010
Member No: #88
Location: Hertfordshire
Exploring in Lexia, I came across a configuration setting that I expect will be of interest to those of us who switch between 18" and 17" wheels as we change between summer and winter tyres. I know it's not quite time to be putting the winter tyres on yet, but it's August already, so it won't be long now.

The setting is in the configuration of the ABS/ESP system, and offers a choice of 225/55 R17, or 245/45 R18 tyres. I don't know what the effect of the setting is, but suppose that there must be some aspect of the ABS performance that can be optimized for the particular characteristics of different sized tyres.

The effect of tyre size on suspension performance has been discussed here before. Whether this setting has any effect beyond the ABS/ESP system, for example, on the suspension and damping, I don't know either.

The configuration screen can be found by selecting "Diagnosis" followed by "Global Test" in Lexia. When all the tests have run, select "Anti-lock Brake System", confirm the identity of the ESP ECU, and then choose "Configuration" followed by "Manual Configuration". You should see the screen in the first image below. Select the "Tyre Type" field, and the options are as shown in the second image.

Having selected the tyre type, and returned to the configuration screen, the job is not complete, as this configuration still needs to be programmed into the ECU. Notice, in the third image, that a "Configuration F3" button has appeared at the bottom of the screen. Press that, turn the ignition off and on as instructed, and the configuration will be stored.

Steve.



3 User said Thank You to smorphet for this Post :
 Dave-Retired (14 Aug : 07:53) , gmerry (14 Aug : 09:14) , cruiserphil (15 Aug : 22:01)
Dave-Retired   
Tue Aug 14 2012, 07:54AM

Joined: Oct 01 2009
Member No: #1
Location: Northumberland
Obviously the setting is there for a reason but what it does actually affect may be hard to ascertain as I have not seen any technical documentation for it.
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gmerry   
Tue Aug 14 2012, 09:22AM
Joined: Dec 11 2009
Member No: #21
Location: Scotland
Smorphet, nice find.

I wonder how many dealers know of this, would bother if one rolled up and said please fit winter tyres/wheels.

Regarding what the various ECUs do with the information, I speculate as follows:-

As the C6 is fitted with an electronic stability not just an ABS system, the ECU needs to measure the difference in wheel speeds for differing cornering situations via accelerometers and wheel speed sensors) and the stability program needs to have pre-programmd values of tyre lateral stiffness and slip angles and compare measured with predicted and then make the appropriate correction to the car's stability.

Obviously 245/45-18 tyres and 225/55-17 tyres have differing slip angles.

Good find

Regards
G
1 User said Thank You to gmerry for this Post :
 Henrik Almquist (20 Feb : 10:09)
furriegurrie   
Sat Feb 27 2016, 11:06AM
Joined: Dec 23 2015
Member No: #2448
Location: Roosendaal
Just tried to set this as I changed from summer to winter tires. Somehow it fails, gives me an error something like "update defect". Any ideas?
Switching off the tire pressure sensor sysyem seemed to work ok btw.
 

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