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Originally Posted by batterylow 1. OK that should have been bottoming out under full load, not scraping.
2. I'm sure the "arbitrary" value quoted by the tyre shop is backed by trial and error, after all its not rocket science. |
1. A car with a stock tyre cannot bottom out and rub the fender liner under full load while going over a big pothole/bump even at higher than usual speed. A suspension system is designed and only certified to be roadworthy in most countries only when it passes such criteria as having a safety margin of an absolute minimum 10mm or 12.7mm in some cases, normaly the margins are even greater especially with tyre sizes approved for winter use (some countries need additional snow chain clearance too).
If your car does have this problem, the suspension buffer should be checked for damage and replaced if required. Furthermore, in the front suspension of most modern cars, the lower retention plate of the spring is normally easily visible. Any tyre that does not touch the underside of the plate itself cannot possibly touch the fender without literally breaking the suspension and its mount! The rear is a different story though and must be inspected for maximum travel before the clearance is correctly established. That said, in normal circumstances, the front and rear typically have similar clearances in the vertical plane.
In reality, an increase in width beyond the OEM spec is likely to cause more problems than an increase in diameter (within the said mechanical limits).
Here is a schematic of the front suspension setup and the retention bracket/plate I was referring to:
2. Well, it's just a matter of perspective. The 4 tyres are the final point on which every control aspect of the car are dependant. If hearsay is good enough for you without even an attempt at understanding the logic of what should be the correct pressure, then the whole discussion is a moot point in the first place. Still, you know you're on this forum simply because you want better than hear-say. You want the answer to the question 'why'!
If my first post did not explain the concept of pro-rated calculation, here is an example:
Say you replaced the stock 205/55R16 91V tyres of a Skoda Superb with 215/60R16 99V tyres, here is what the basic pro-rated calculation would look like:
Recommended Pressure by OEM: 2.4 bar or 34.8 PSI. FRONT; 2.5 bar or 36.25 PSI REAR. Note: 1 bar = 14.50377377 PSI.
Load rating in Kg of Original Tyre @ Pressure: 615Kg @ 51 PSI
Load Rating in Kg of New Tyre @ Pressure: 775Kg @ 50 PSI
Here the load in Kg is taken from the Load Index Table from the ETRTO Standards Manual, which can be reffered here:
Wikipedia - Tire Code - Load Index
The Max Load Pressure is taken from the tyre spec sheet and can also be seen on the tyre sidewall.
New Front Pressure = 2.4 x 615/775 x 50/51 = ~1.87 bar or ~ 27.1 PSI.
New Rear Pressure = 2.5 x 615/775 x 50/51 = ~1.95 bar or ~ 28.2 PSI.
That said, there are a lot of other factors at play in tyre pressure computation, the main aspects of which are:
a) If sidewall increases, tyre pressure typically increases.
b) If sidewall height decreases, tyre pressure typically reduces till upto a sidewall height of about 100mm / 4". Ultra low profile tyres with sidewalls lower than 100mm typically need increased pressure.
c) No tyre should be inflated less than 22 PSI or over 36 PSI for Standard Load Tyres and over 42 PSI for Extra Load Tyres to carry the vehicle in partial load operation, unless approved by the vehicle OEM.
d) No tyre should ever be inflated beyond the recommended Max Pressure published on the sidewall (Typically this pressure is 44, 50 or 51 PSI).
e) Increased rim width for the same tyre normally needs a slight increase in pressure.
All in all, from what I have gathered from a lot of reading, in normal circumstances, an increase in the volume of air in the tyre normally needs an INCREASE in tyre pressure and vice versa (I know this may seem counter-intuitive).
Also, it is better to keep the load rating changes in a close range as far as possible and a reduction in load rating should be entirely avoided if possible.
In the example, since the sidewall is a fair bit taller, I would keep the pressure at 27.5 PSI & 28.5 PSI (front & rear) and observe the tyre contact to see if any tweaking is required.