A few days ago a friend who is a Ford mechanic and I were chatting about my 04 XJ8. It has the aftermarket wheels on it that were there when I bought it. They are okay, I suppose, but I am not thrilled with them.

I told him that I had read a few times that Jag owners suggest not buying original Jaguar wheels for replacements because they seem to be soft and more easily damaged than a lot of other wheels. As a possible example, I showed him the spare tire in the trunk/boot which still has the original Jag wheel and a lot of damage marks around the edge of the wheel.

He suggested something about those original wheels that I had never considered or read. He qualified his statement by saying that he does not know if this is true of Jaguar or not. He was just thinking out loud. He said that he wonders if Jaguar intentionally designed the wheels with a softer alloy to help absorb road shock and therefore contribute to a softer ride.

Obviously, it would not be the kind of absorption that one could see, like a tire flexing. But after thinking about his idea, it almost seems like he could be right. It seems to me that softer alloy would have some ability to absorb road vibration, etc. on some micro level.

What do you think? Is the softer alloy possibly an intentional design feature, or is it more of a manufacturing flaw?

 

Manufacturing flaw as a softer alloy on purpose would also have lateral flex, be expensive to engineer and manufacture, and anywhere but on a race track with relatively controlled conditions, would present one hell of a liability if they were ever to start failing in the real world.

 

The wheels get buckled because people run low tyre pressures to make the ride softer
I run 40 psi in all my low profile tyres
Happens to all rims from most brands of cars when running less than 50 profile tyres also poor driving skills and crap roads dont help

 

I would have to say that what I might perceive as a design flaw might not technically be one. The fact that the outer facing edge of the wheel in my trunk/boot has obviously been dinged and dented by what appears to be a tool of some sort does not in itself indicate a flaw in the strictest technical sense, although it is definitely something that I wish was different.

Assuming that the alloy used in Jag wheels is a safe alloy, a better way to put my original question would be, is it possible that the particular alloy used in Jaguar wheels is intentional to contribute to a possibly softer ride?

My car has 50 profile 18" tires. I recently had them balanced and saw that the tire store ran them up to 33 psi. The inside of the fuel door indicates that mine should be at 28 for the front and 30 for the rear at speeds below 100 mph. I lowered them to that spec to see how it affected the ride. I have no desire to run any wheel larger than 18" and have even considered checking into 17" after reading that it was an allowed size for the X350 XJ8.

I agree that some of our highways can be deadly to almost any wheel, regardless of the alloy used.

 

"My car has 50 profile 18" tires. I recently had them balanced and saw that the tire store ran them up to 33 psi. The inside of the fuel door indicates that mine should be at 28 for the front and 30 for the rear at speeds below 100 mph. I lowered them to that spec to see how it affected the ride. I have no desire to run any wheel larger than 18" and have even considered checking into 17" after reading that it was an allowed size for the X350 XJ8."


Bookman, if you were to every see how far under your front tires fold when cornering even mildly at 28 psi you'd immediately find an air pump and go up to at least 32 psi. Some tire guys when first mounting a new tire will run it up to 50 psi to make sure it's fully seated and then back it off to 30-35.

 

I thought about that when I read the stated pressures in the fuel door. I have never run tires that low before. I've almost always run them at 30 to 32. I'm thinking I will try all four at 30. The 28 psi idea made no difference in the ride at all.

Somewhere, I think possibly in the owners manual, I read that the recommended tire pressures only apply to the specific original equipment tires and may not apply to other brands.

I don't know that I could deal with 40 psi, though. It seems one would feel even the tiniest imperfection in the road at that speed.

 

Some references in Jag manuals actually refer to the lower pressure as "comfort setting". It did make a difference with the original Continental tires. Most newer performance tires have stronger sidewalks, so reducing the tire pressure has less of an affect on the ride. But it still does soften the ride somewhat.

If I use the "comfort" pressures, I find I roll the sidewalks with my aggressive driving habits, so the edges of the tires wear very prematurely. I run 40-45psi in my Jag. I put 32 in the wife's Jag, as she prefers the softer ride...and doesn't drive as hard as I do! I find the 26psi recommendation is too low for anyone younger than my Granma. Of course...the Queen does ride in an XJ8L...so lower pressures do have their use...

As for the idea of "softer" rims. The original wheels are cast aluminum alloy. Many aftermarket wheels are forged and machined. A forged wheel will always be stronger than a cast wheel. Not really a "flaw", but rather just the nature of the metal. Casting in large numbers is cheaper, so my guess is that money is at the root of the difference.

 

Hi bookman,

I've been pondering your mechanic friend's hypothesis about a softer alloy being used in Jaguar wheels in order to contribute to ride dampening, and in my considered opinion I believe it is highly unlikely, for the reasons I give below. Others have already given some of these reasons, and I agree with them.

The Jaguar dealer training and new-model introduction literature mentions numerous areas where engineers sought to reduce Noise, Vibration and Harshness (NVH), so theoretically it seems possible the wheel engineers might have selected a softer aluminum alloy if it possesses better internal dampening characteristics compared to harder alloys, as long as the softer alloy met all the requirements for strength, strength-to-weight ratio, hardness, malleability, fluidity/castability, machinability, paintability/polishability, resistance to deformation, wear and corrosion, etc.

But I personally doubt that any flexing would be designed into the wheel design for at least three reasons:

1. Any flexure in the wheels, no matter how small, could have an effect on steering geometry and alignment, undermining handling and potentially leading to uneven tire wear (think of how tires wear when a wheel bearing develops play);

2. Any flexure in the wheels would inevitably lead to metal fatigue, which has contributed to numerous disasters in aviation; flexure of cast wheels would almost certainly be undesirable due to the crystalline structure of cast metal that makes it more susceptible to fracturing than rolled or forged metal of the same metallurgy; and

3. From a manufacturing standpoint, metal flexure tolerances are unpredictable. My late father was an automotive engineer who worked for OEMs. I recall him having to diagnose a problem with car horns that relied on the vibration of diaphragms punched from sheetmetal plate. The engineers who had designed the horn had not allowed for a wide enough tolerance in the vibrational properties of different batches of sheetmetal. Dad tracked down the problem to some batches of metal sheet that did not vibrate properly after being cut to shape, resulting in horns that wouldn't honk. He ultimately discovered that no metal supplier could guarantee consistent sheets that would always vibrate properly, so the horn design was scrapped (of course, the Detroit engineers who designed the horn never admitted their error).

One other thought. At least some of the OE Jaguar wheels are made by the German companies BBS and ATS, who supply many of the European and Japanese automakers:

BBS Kraftfahrzeugtechnik - Wikipedia, the free encyclopedia
http://www.ats-wheels.com/ats_wheels...id=143&lang=en

Given how small Jaguar's orders would be relative to those companies' total sales volume, I would be surprised if BBS or ATS would agree to use a unique metallurgy in Jaguar wheels without a significant upcharge, and I would be equally surprised if Jaguar (Ford/Tata) would be willing to absorb an unnecessary increase in cost-per-wheel by doing so.

For me, the bottom line is that, from an engineering standpoint, it is far easier to design suspension dampening and compliance into the suspension component geometry, rubber bushings, shock absorber damping rates and coil/air spring rates than to experiment with wheel metallurgy. It might make sense to choose an alloy based on its internal damping qualities compared to other options, but I think it is far more likely that such decisions would be made by the wheel suppliers than by Jaguar engineers.

I suspect the common damage to later-model Jaguar wheels may have more to do with the large rim size / low aspect ratio tires and corresponding reduced volume of air in the tire and resulting lower compliance / cushioning of the wheel. I purchased a set of X358 wheels for my X350 and discovered after I had them installed that two were bent. While I was having them straightened, two other guys came to the shop to have wheels straightened and a third arrived to pick his up. Their cars? A Corvette, a BMW, and a Mercedes, all with large diameter rims and very low profile tires....

On the subject of tire pressure, for our '04 XJR with 19-inch wheels, Jaguar recommends tire pressures of 28-32 psi front and 30-34 psi rear. For highway trips, I tend to add about 2 psi to the high end of those specs, but for around town, I've settled on 30 psi front and 32 psi rear for the best compromize of handling and comfort. The highest pressures are just too harsh around town.

Finally, regarding the auto manufacturer's tire pressure specs, they obviously apply to the OE tires, and most tire manufacturers seem to recommend running their tires at the automaker's recommended pressures. In my experience, the automaker's specs do serve as a good reference point for tires from other manufacturers with the same speed rating and designed for similar vehicles: e.g. any Z-rated Ultra-High-Performance All-Season tire designed for High-Performance Sedans. It's rare that I deviate much from the manufacturer's specs when I replace the original tires with another brand. The one exception in recent memory was when I installed General Altimax HP tires on our BMW, the manufacturer's pressures were just too low and handling was suffering. I increased the pressure gradually and watched the treadwear and settled on 33/40 front/rear compared to the 30/35 specified by BMW which had worked well with the OE Pirellis and the Bridgestones I'd replaced those with.

Cheers,

Don

 

My car has the Falken Y rated tires that were on it when I bought it. I've seen that a lot of people who own sports or high performance cars happily tolerate (or even expect) noise and rough rides in exchange for the exhilaration they feel when driving their cars. Personally, I enjoy the spunkiness of my car, but I want to do so in a gentler, quieter, softer way if I can find a way to do it.