Oversized Front Tires on Magnum Chassis

[steve zannella]

Has anyone installed 315r80 r22.5 tires on the front of a early 2003/4 marquis on a magnum chassis.

Stock size is 295r80 r22.5.

A truck tire shop told me they run about 3/4 inch wider and 1 inch taller?

[Tim Bentley Co-Admin]

If you do a search for (315) using the exact phrase you will find several posts discussing tire seizes.

[Bill Sprague]

Why do you want to do that?

[steve zannella]

because

higher gross weight rating on the front axle and lower tire pressure for the same weight rating with softer ride quality!

[Steve Jewell]

Steve
       How much does your front axle weigh?

Steve

[steve zannella]

My front axle weighs 14,950 and my tire pressure is 115 psi. I would like to use 315/80r22.5 tires so
I can lower tire pressure to 105 psi and maybe improve ride quality with lower pressure.

[Gerald Farris]

Steve,
I do not think that you will find that the 10 PSI drop in pressure will be that noticeable, but if you have the clearance for 315s, there are no down sides to using them other than the expense. Be sure to measure your coach for adequate clearance before installing the 315s, because the rear air bag for the front suspension is located directly behind the wheel housing, and excessive tire contact on the wheel housing can damage the air bag.    

Gerald

[Steve Jewell]

Steve,
    I agree with Gerald. The 295 is a load range H and the 315 is a load range L. The 315 tire casing is much heavier. It will not ride better even if you could run 30 psi less in it.

Steve

[Gary Winzenburger]

Steve,
I run 105 psi on the front and 100 psi on the rear and it's still rough riding on highways with a lot of joints, dips and pot holes. The previous owner installed a size larger tire all the way round also.
Good luck,
Gary

[Edward Buker]

Steve,

The specs/dimensions on the tires you have now should be online as well as the 315s that you are considering. Knowing the offsets you should be able to go lock to lock with the current tires and verify if the adder for the width and the diameter change can be accommodated given the clearances that you have now. Measurements would need to be done fully aired and with the suspension fully bled of air with the wheels set straight.

I have had several brands and tire models over the years and the Michelin XZA2 Energy tires seem to be more compliant with a softer ride then others I have had while still having good handling. I do not know if there is a safe more compliant sidewall manufacturer or tire model out there. I think that Gerald is right regarding the 10lbs of air reduction. I believe that the air pressure within the 8% range change will be less of a player then the design of the tire and sidewall construction.

This may be a good question to take to Michelin and ask them which suitable model of tire would provide the softest ride and then if you could accommodate 315s at 105psi or 295s at 115lbs would you feel any ride difference between the two.

As an aside I have a Mini Cooper with Goodyear run flat tires on it and they ride like they have no give at all, a very rough ride On the other hand they handle very well, like a go kart...this is all a tradeoff. If the sidewall is too compliant you can have a sense of vagueness and unresponsiveness in handling.  In the shock department I have kept the Bilstein shocks for what I expect is a  softer ride at the expense of handling that the Konis might provide. Another piece of the puzzle.... Not sure that you will find a magic bullet here.

Later Ed

[Karl Welhart]

Ed brings up some interesting points about tires/shocks.  I will share my experiences.  Switching from Toyo to Hankook, I notice a much better ride with the Hankook.  Sidewall construction is the main reason for the difference.  Also, recently tried different tire pressure settings and to my surprise, I found running higher pressures in the front (105 to 115) made the ride better, not worse...  On the shocks, I switched from Bilstein to Koni FSDs and they made a huge improvement in ride and handling.  

BTW, you can get much better handling and ride on the Mini Cooper by replacing the "run flats" with a good high performance summer tire.  Be sure you carry a can of tire puncher sealant and air pump for good measure.

[Jeremy Parrett]

Hi Ed,
 my 2011 Cooper S has DRAG D-31 rims and 205 55 EZ16 DWS Continentals .  The ride is hugely improved over the Bridgestone Turanza runflats.  I carry a spare and a pump !!

[Edward Buker]

Jeremy and Karl,

My butt would love to dump the Goodyear run flats but they were put on by the prior owner in order to sell the car for $1000 plus. They are loaded with tread and I will run them until either the tires or my butt gives out. Every once in awhile I get frisky and put this car through its paces and I have to say they are the best handling tire I have ever driven on. Hard to even get a squeal out of them. Thanks for the Mini help...

Later Ed

[steve zannella]

 Karl

What part no did you use for Koni FSDs' ? I called Koni for FSDs' part # on my magnum chassis and they only made adjustables .

[Karl Welhart]

Steve,

The part numbers are as follows:

8805-1004-SP1 for the front
8805-1005-SP1 for the rear

These are only for the 8 air bag magnum chassis.  Not sure about your application.