Tire tutorial

[Qgrappler]

In a long ago post replying to me, Q45tech wrote:

We warned members about the Symetrical D5000. Did you notice it was a 51 psi [weak side wall tire]......hopefully you ran them greater than 40 psi cold [1521/51=30 pounds per psi and the front static weight is 1200 so 40 psi minimum COLD!With 51 psi tires you better check the inflation once a week.

Check you FRONT camber and make sure it doesn't exceed negative 0.7 degrees with you sitting in drivers seat!***************I need some tutoring here on the tires.

Looking at the Michelins the group seems to favor, they have a sidewall rating of 44 psi. I dont understand how having a higher rating of 51 psi makes the Dunlop a weak sidewall tire. [I'm not arguing, i just dont understand]

Also, if it is a weak sidewall tire, why am I supposed to run them at such a high pressure.

I am redoing the shocks and tension rods and upper links, and will have an alignment done, but I am inclined to simply replace the two from Dunlop SP 5000's that i wore the corners off, since the ones that were on the rear look good. (still have the snow tires on as of now)

IS the basic message underlying this that the camber in the front of the Q eats your tires alive unless they are overinflated to keep them up on the tread and off of the corners of the tire? Comments welcome

[Q451990]

The idea of higher PSI is that you use air pressure rather than sidewall stiffness to hold the tire in shape. Think of a baloon or air mattress - the more air you put in, the stiffer it is. With lower PSI rated tires, the sidewall has to be thicker or stiffer to hold the car up so you don't have to use as much air pressure.

I think Q45Tech's point is that you're buying less rubber (and protection from failure) with a higher PSI rated tire.

You're right, underinflation can cause inside tire wear to accelrate because of the negative camber the Q uses. This is an overexageration, but wheels are essentially tipped like this / \ in the front to improve handling.

What size wheels are you running?

Heath

[Qgrappler]

Heath

I'm running the stock wheels, which I think are 15"--its a Q45T, 1994.

On the sidewall strength, max pressure question, I see what you are saying. However, when I go back and read the explanations linked to the headings on the tire specs section at the tire rack site, it says that the load rating of the tire is done at a standard pressure for all tires. It also says the mac pressure is unrelated to the load pressure. The way I read their explanation, I would say that the Michelins and the Dunlops load handing ratings were equal, but that the Dunlop can be inflated to about 7 psi higher (15% above the Michelin 44 psi max).

Having said that, I also see a table where they show the relation between the maximum load and tire pressure, and the load rating decreases materially as you lower the pressure from the measurement standard of 35 psi. Now,... I dont know if the decreased load rating means that you'll hit the rim in certain potholes, or that the tire will wear out quickly from overheating and excessive flexing. I suppose both. ....In any case, it shows the importance of keeping the inflation pressure high. I wonder what pressures group members try to maintain in their cars.

[Q45tech]

To make it theoretically possible for all tires in the same size to be substitutable a standard load size was created. Tire and Wheel Handbook used 35 psi [for 44 psi tires] 80% of maximum cold inflation as a benchmark for testing the load index.

Load index is a wishy term in that a weaker tire can support some over load [safety margin] at under pressure for some time depending on temperature and oxygen degradation [storage time since manufactered].

The Q front tire normally sits [4300 * 0.54/2=1161-1200] pounds with a 1521 max load tire.

The tires are tested with ZERO camber so that the weight is equally distributed arcoss the tread structure of the tire.With oem camber [negative 0.7 degrees] the tread patch [contact patch area] becomes trapezoidal in shape and the inside of the tire is subjected to a greater percentage of the force [weight].

For the oem Michelin which wore out the tread in 17,000 miles at best this was ok because enthusiast knew to be safe in water [rain] they should change tires early........all this occurred before internal damage [from heat] caused the tire to be un safe at its real world load USED 1 year.The Q tire placard had 2 pressure readings 29 psi [200Kpa]Cold for town comfort and 35 psi [240 Kpa] COLD for high speed driving.

Assuming that all tires in the same size and speeding rating are equal is dangerous in that the tires only have to meet tests which are not severe enough to reflect the loads of a Q with all its camber.

Worse yet is very few Q meet the ideal camber and suspension wear creates even more camber so the stresses are even more.........Some tires can tolerate more camber than other.

The heat builds up to critical levels in summer where it accelerates the external rubber tread wear but worse inside the heat destroys the protective rubber coatings around the steel wires exposing them to oxygen [rust]......ultra premium and Michelin H rated use a very impermeable interior liner to keep the inflation air from passing into the tire core and attacking the steel wires.

51 psi tires are the result of government pressure to reduce highway rolling resistance [and traction as a by product] to improve gas mileage.

Unfortunately states [highway departments] have created rougher highways [larger harder stone specs and reuse of tire byproducts and open asphalt structures [better drainage] to increase road life [time between resurfacing]........these things fight against each other with newer tires.

Anyway 35 psi cold on a 44 psi tire.

Michelin caused a heart attack in the tire industry when it suggested that all tires be tested at 110% of their rated load at camber................everyone else knew their tires would fail such a test...........when you are oem supplier for BMW and MB you are use to such demands and loads.

[natsoundup]

i inflate tires to infiniti's specs.... go to Wal-Mart every 30k miles and get a set of thier home brand....

...then get the suckers rotated and balanced religiously (for free) when I get my Wal-Mart oil change every 3k miles.

I am not a very aggressive driver however...and if I was would probably invest in a higher quality tire. I just haven't found the need... Initially I invested in higher qulaity tires... they lasted the same 28-30k miles.... tires seem to wear just like the brakes...never get more than 30k miles...

I would say there's 15 percent rubber and break lining on them at that mileage...

[Q45tech]

An enthusiast would never allow the tires to wear more than 40% of new depth [down to 4/32"] and the same with brake pads..............as the fade resistence is a function of lining thickness.

Being an enthusiast requires money and time.

Getting the full brand new capacity of a Q [or any car] requires lots of both, why most people settle for 70-90% in their used cars.

For sure you cannot count on stopping from 60 mph in 129 feet as the car did new but 150 feet is ok for most people.Same with an emergency lane change at 55 mph what's 5 mph among friends.

After seeing so many Q in the shop and seeing what standards most owners keep them to, when I see an older Q on the highway I floor it and get away as fast as I can, never follow or let one get behind you...........1,000 foot safety margin please.

[natsoundup]

Q45 tech....

I just like to put the non-enthusiasts point of view on from time to time...

My enthusiasm goes to my sons education which cost many times more each year than the Q enthusiast puts into their car