Tyre pressures for W124 on 17s

BlueQuinn

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Hi all,
I've searched but cannot find a definitive answer. I'm not really sure how this works.

I've put W210 Mekhab 7.5x17 inch alloys on my W124, with 225/45x17tyres.
And they look sensational. :)

Should I inflate these to the same pressures as indicated on the petrol flap (34 front / 39 rear) as with the 15s, or isn't that appropriate for 17s?
 

trapperjohn

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I have 16 inch 8 hole alloys on my 124 estate and keep them at "petrol flap" pressures and have no wear or grip issues.

Others will be along with their ideas soon.
 

grober

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In theory the load bearing capacity of any tyre will depend on its inflation pressure. This will vary according to its wall thickness and load rating. The higher pressure at the rear in cars of that vintage to a certain extent is to vary front to back slip angles making the car primarily understeering [ back end holds on longer] That said the rear of an estate will be heavier than a saloon so some of that pressure may be in anticipation of load carrying?? However I imagine a w124 is a slightly lighter car than than a W210? You need to experiment a bit I think but the fuel flap pressures would be a good place to start.
 

hotrodder

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Tyre size is pretty much irrelevant unless making massive changes* like say back halving the car and going drag racing. Pressures are mostly about stuff like the weight the tyre needs to support hence unladen and laden figures

Very crudely (ignoring what happens dynamically when the car is moving and pretending stuff like load rating and side wall stiffness are identical) lets say the corner weight is 800lbs all round... with 10psi in the tyre it looks flat because each square inch can only support 10lbs making for a contact patch of 80 square inches. At 32psi the contact patch will be 800/32 = 25 square inches. Width of the contact patch is obviously a function of the tyres width so when you fit wider tyres and keep the same pressure the static contact patch gets shorter and wider as opposed to the myth that wider tyres = more rubber on the road

The narrower tyre with it's longer & narrower contact patch has to flex more than a wider tyre. More flexing = more heat generated and since more of the tyres circumference is in contact with the road cooling is hindered. Beyond this/when the thing is moving things start to get complicated due to stuff like slip_angles

I run the same pressures with my 225/45r17 that i did with 205/60r15

* Mountain bike versus 'racing' bike is a good example, the later needs higher pressures because the tyres are extremely low profile and typically around 1/6 of the width at most. They aren't physically big enough to run them with the same size contact patch. Drag racers do things the other way around... massive tyres with very flexible sidewalls run at extremely low pressure so they actually do put more rubber on the road track. Especially when the tyre 'winds' up during launches
 

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