What tyre pressures for none original tyre sizes on R129 SL 500 ?

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macabethiele

New Member
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May 16, 2020
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4
Location
Derby,UK
Car
R129 SL 500
I have recently changed the wheels on our R129 to a set of Monobloc 8J ET 31 x 18" rims the tyre size is 235/40 R 18 but not sure what tyre pressures I should have, the previous wheels/tyres were 245/45 R17" they ran at 32 psi F & 35 psi R.
Should I be running the lower profile tyres at higher pressures ?
 
Tyre pressures are determined by axle load. Start with the original pressures and monitor wear, adjust accordingly.
 
I have recently changed the wheels on our R129 to a set of Monobloc 8J ET 31 x 18" rims the tyre size is 235/40 R 18 but not sure what tyre pressures I should have, the previous wheels/tyres were 245/45 R17" they ran at 32 psi F & 35 psi R.
Should I be running the lower profile tyres at higher pressures ?

As mentioned, use the standard pressures in the filler flap.

As an aside ... on the R129 8J x 18 rims would normally have 245/40 tyres, and would be fitted with 9 Jx 18 ET 35 at the back (with 275/35 tyres):

 
As BTB500 says this was around the start of MB introducing staggered rim widths on their performance models. Before this wheel rim widths were often the same all round and conservative rim widths meant even wheel offsets were often the same front and back. Mercedes did however have a propensity back then to run higher pressures at the rear than the front -- counter to what you might expect from a RWD front engined car-- this was to do with tyre slip angles -giving the car natural understeering characteristics- deemed safer/ more predictable for most drivers
 
.................... Mercedes did however have a propensity back then to run higher pressures at the rear than the front -- counter to what you might expect from a RWD front engined car-- this was to do with tyre slip angles -giving the car natural understeering characteristics- deemed safer/ more predictable for most drivers

About a week ago I was sorting my tyres and it registered with me that the rears have higher pressure than the front and it did seem counter intuitive to me. I put it down to the fact that the rears are bigger than the fronts.

I'm not contradicting your explanation as I really don't know but my car also has torque vectoring to help counteract understeer 😳 .
 
The raised rear pressure slip angle argument only applies to older models with same size rims all round and no electronic driver aids which indeed may put an entirely different complexion on staggered tyre pressures on newer models which as you say may be for other reasons
 
FWIW our S203 has higher book pressures at the back whereas the 'light load' settings on the Vito are the same all round. Both RWD with ESP etc. and same build year - the S203 does have staggered rims though..
 
Tyre pressures are determined by axle load. Start with the original pressures and monitor wear, adjust accordingly.
At present I am running on 32 psi at the front with 36 psi at the rear as the stated tyre pressures in the MB handbook make the ride quite firm at the rear. I was told the higher rear pressure was to prevent the tyre casings from overheating at very high speeds 140 mph etc. My concern is that the lower aspect ratio has less volume of air in the cushion area so might need more like 35 psi all round. Methinks MB made some special edition models during the run out period that had 18" wheels as standard - yes they were wider at the rear than the front. I have gone for my tyre width to keep the rolling radius as close as possible to the original wheel size. rolling radius Thought someone here might have a R129 Silver Arrow edition with the appropriate handbook. I'll take a look at some US websites as they seemed to have beefier wheels fitted.
 
At present I am running on 32 psi at the front with 36 psi at the rear as the stated tyre pressures in the MB handbook make the ride quite firm at the rear. I was told the higher rear pressure was to prevent the tyre casings from overheating at very high speeds 140 mph etc.

You should have a separate set of pressures for 135+ mph:

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Methinks MB made some special edition models during the run out period that had 18" wheels as standard - yes they were wider at the rear than the front. I have gone for my tyre width to keep the rolling radius as close as possible to the original wheel size. rolling radius Thought someone here might have a R129 Silver Arrow edition with the appropriate handbook. I'll take a look at some US websites as they seemed to have beefier wheels fitted.

The Silver Arrow had 8.5Jx18 ET25 (245/40) at the front and 9.5Jx18 ET23 (275/35) at the back.
 
Do you have the tyre pressures ? At the oment I have put 38 psi in the frone & rear.
It’s on the sticker, with a passenger it would be 36 Front and 39 Rear. Are you really planning on doing more than 135 mph?
 
I run mine at the lowest pressures shown above (30 front, 35 rear). It never carries 4 people with luggage, or gets driven at over 135 mph :)
 
I thought higher tyre pressure = better fuel efficiency. And if you do not go faster than max speed does it really matter that much ? Am interested to know.
 
I thought higher tyre pressure = better fuel efficiency. And if you do not go faster than max speed does it really matter that much ? Am interested to know.
There is indeed less resistance at high speeds but the ride will be incessantly firm, traction may be reduced and the tyre will wear prematurely in the centre.

The tyre is designed to operate most effectively at the standard pressure (or close to) and similarly the car is designed to do the same.

Hence inflating to higher pressures for high speed high load and returning to standard pressures for the 99.99%+ of the time it’s not actually required!
 

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