17 replies to this topic

[mariner]

I am sorry to ask general questions on this forum but there are people here who cover Europe, Asia and North America and have specialised knowledge.

I have to buy two new tyes for my 2006 Cadillac CTS which was sold in the UK but obviously made in the USA. The Goodyear RS-A OEM tyres are no longer supplied in the Uk by Goodyear - no problem you often have to find an alternative tyre after 2-3 years.


I have noticed however that GM have developed a special code , the tpc code for replacement tyres see this link

http://www.gm.ca/gm/...basics-markings

I have never noticed this on UK tyres, is this a US only thing or does anybody know if GM does this process world wide?

If GM really do check replacement tyre brands for all the data listed on the link then it would make sense I think to get such a tyre, otherwise I am just guessing or reading tyre reveiews which are mostly subjective.

It is our daily drivier so ride, wet weather grip etc. are more important than ultimate sideways G force which is ofen the focus of most tyre comparative tests in magazines

[McGuire]

That is a North America-only thing as far as I know. Quality, name-brand tires will meet or exceed all the important GM standards... the main area you can potentially run into problems is with TUG (force variation etc) which could possibly produce some NVH issues in a few of the more sensitive vehicles, CTS not being one that I am aware. If you buy quality tires of the correct size and rating and replace in axle pairs I'm sure (99%) you will be fine. A non-TPC tire causing an actual problem is exceedingly rare. You may well even see an improvement as I have when installing Michelins on Cadillac E-body and DTS. I am finally over the 2005 USGP enough to say something nice about them again.

[Greg Locock]

That is a North America-only thing as far as I know. Quality, name-brand tires will meet or exceed all the important GM standards

Force & Moment
Residual Aligning Torque (RAT)
Rolling Resistance
Braking Traction
Driving Traction
Uniformity
Noise

Groove Wander
Ride & Handling

Disgaree on the above factors in particular. Different tires of the same size from different manufacturers (or even the same manufacturer, or even the same name tire built in a different factory) could easily vary by 20% for the first 3. They are controlled by the ply layup and fillers, not just the external shape of the tire. These numbers aren't simply 'higher is better (or whatever)' for instance RAT is ideally one particular value for a given vehicle, same with the cornering stiffness curve.

[desmo]

On some levels though wouldn't engineering a car so that it only performs acceptably on one very specific tire or is extremely sensitive to the tires fitted be poor engineering? Modern cars can easily last 20+ years, what are the realistic odds that one tire the car was engineered around will even be available by the time is even halfway through its lifespan?

[McGuire]

Disgaree on the above factors in particular. Different tires of the same size from different manufacturers (or even the same manufacturer, or even the same name tire built in a different factory) could easily vary by 20% for the first 3. They are controlled by the ply layup and fillers, not just the external shape of the tire. These numbers aren't simply 'higher is better (or whatever)' for instance RAT is ideally one particular value for a given vehicle, same with the cornering stiffness curve.

I hear what you are saying and I have been there myself with a few cases, but if what you are saying were true in a practical sense, it would be impossible to walk into a tire store and successfully purchase a set of suitable tires. In fact, it's not even difficult.

I am giving mariner practical, useful advice and I am very confident it will work out for him. This is based not only on my direct knowledge of the TPC requirements from my years in the biz chasing such issues, but from replacing the TPC tires on my own vehicles, probably dozens of sets over the years since TPC codes were introduced -- around 1975 if I recall properly. Cadillacs, Buicks, Corvettes, Pontiacs. Quite true that values are often not "higher is better" especially in terms of tertiary characteristics, but also not terribly critical with quality replacement tires in the great majority of applications. They'll be in the window. (Though there have been some weird TUG issues in past years.) Actually, in practice it is not incredibly difficult to obtain a better all-around tire than the TPC OE equipment. Ask any Corvette owner. A couple of reasons for this, not the least unit cost; also the dreaded RR requirements.

The only additional note I would add in all this is to avoid the outlandish tread patterns and stay conservative; stick to those that most resemble the original fitment. Especially in this application, a luxury sedan where the owner has stated he is more interested in ride, comfort, and silence than in ultimate grip. Lots of performance tire tread designs look to me like they were designed to create problems.

[Fat Boy]

I am finally over the 2005 USGP enough to say something nice about them again.

Ya, well, my CTS has Goodyear's on them which I've been perfectly happy with the ride and performance. Go figure. I bought them because they were on close-out at The Tire Rack, but they're actually reasonable tires.

[Greg Locock]

On some levels though wouldn't engineering a car so that it only performs acceptably on one very specific tire or is extremely sensitive to the tires fitted be poor engineering? Modern cars can easily last 20+ years, what are the realistic odds that one tire the car was engineered around will even be available by the time is even halfway through its lifespan?

The car is optimised for one or two tires. It'll probably be OK on other tires, but it is /optimised/ for just those.

To give a lslightyl off topic example, we set the police cars up for them. They bring in their (various makes of ) cars, and the tires they want to use (cheap, robust), and then our STiG fangs around the track and tells them what tire pressures to use for each car with each tire and still get safe limit handling. I was a bit of a non believer in some of this until I went out with him and saw the effect of 2 psi on the rear tires for brake-in-turn stability. Went from benign to off the road in 2 psi. Made me think.

In a similar episode I was able to model the exact problem that a car had in a couple of maneuvers, so was able to suggest a suspension tune that would allow the fleet buyer to fit his nasty preferred tire without killing his drivers.

It wouldn't exactly amaze me if it becomes a requirement to only fit tires off a certain list for each car model. One more Firestone case that goes the wrong way would do it.

[desmo]

I'm sure the OEM suppliers would love that. It would probably put every aftermarket tire maker that didn't have a special relationship with at least one of the big automakers out of business. What automaker is going to pay to cert tires other than OEM fitment? Particularly if they have no prior business relationship with the other tiremaker? Then the OEM tire makers can charge whatever they please for replacement rubber without having to worry about a competitive marketplace to constrain prices.

Sorry guy, we can only fit these $750 each tires to your 5 year old Escort, it's the law. I guess it's probably essentially totaled then even if it only has 40K on the odometer. Real shame.

[Greg Locock]

desmo-

That's possible, but pessimistic. More likely would be that the aftermarket tire manufacturer would pay somebody to assess the tire as being acceptable. Incidentally total cost of ownership includes cost of tires (that's why cops and fleets want cheap tires) so a car manufacturer that offered cheaper approved tires would have a competitive advantage. However I don't think it is worth a huge debate of hypotheticals it'll be driven by case law, not public demand, obviously.

In theory that is the situation with exhausts in the EEC and Oz. If your exhaust system is noisier than the OEM part as ascertained by a completely useless roadside test then your can get flagged as unroadworthy. Bit like emissions stuff in the USA. The trouble is there is no simple test for tires.

[McGuire]

Ya, well, my CTS has Goodyear's on them which I've been perfectly happy with the ride and performance. Go figure. I bought them because they were on close-out at The Tire Rack, but they're actually reasonable tires.

Goodyear F-1 and RSA are both TPC tires for the CTS so if they are one of those you fell in the box. CTS is one of the shorter lists. One of my Pop's cars ('05 Buick) might need tires and I am headed out there tomorrow to tinker on our Model T, so I looked it up. Including winter treads and optional wheel sizes, GMSPO lists 111 recommended tires for the car.

Our Ford. No recommended tires. Guess we can't drive it.

[desmo]

Ford? I thought you were partial to the General.

[McGuire]

Ford? I thought you were partial to the General.

noooo, the Model T is fascinating technology. For example, the ignition fires the spark plug constantly through 90 degrees of crank rotation. Depending where you set the spark lever on the steering mast, from ~15 ATDC at full ****** to ~60 degrees BTDC at full advance. Battery/magneto ignition; four ignition coils, one for each cylinder. On battery there is 6V through the primary circuit, on the low-tension mag ~4 to ~30 V AC to the coils varying with engine speed. One of the reasons they can run on anything resembling gasoline. Very clever car, the perfect machine for its time.

[McGuire]

Force & Moment
Residual Aligning Torque (RAT)
Rolling Resistance
Braking Traction
Driving Traction
Uniformity
Noise

Groove Wander
Ride & Handling

Disgaree on the above factors in particular. Different tires of the same size from different manufacturers (or even the same manufacturer, or even the same name tire built in a different factory) could easily vary by 20% for the first 3. They are controlled by the ply layup and fillers, not just the external shape of the tire. These numbers aren't simply 'higher is better (or whatever)' for instance RAT is ideally one particular value for a given vehicle, same with the cornering stiffness curve.

Getting back to tires... I think we need to distinguish what each of these metrics represent. For example, if a tire has the same UTQG classifications (the three T's: tread wear, traction, and temp) and speed and load ratings, the chances are slim to none it will not also match or exceed the TPC braking and driving traction values. The manufacturer could indeed build such a tire but it would be a freak, they'd have to be trying. Here TPC is providing validation so much as qualification. Individual variances in manufacture (listed as "uniformity" here) are always a concern, but that's true within or without the TPC system.

In practice, tramlining/groove wander and related issues are the usual problem with replacement tires, along with a snarl of NVH issues, and are invariably caused by goofy tread patterns and reduction in aspect ratio. I advise people to stay away from that approach if they still want to enjoy driving the car. If they go that route, they are signing up for that misery. Stick with a conventional replacement tire on a stock size, stock offset wheel and they are very unlikely to have any problems in these areas. The same for noise -- primarily a function of tread design.

Now for the dirty little secret: a non-TPC replacement tire does not have to meet TPC's rolling resistance standard, obviously, which is one of the more difficult aspects of the TPC yardstick, forced upward by the pressure of EPA/CAFE standards. You can see how slipping that requirement allows a better tire in most of the other metrics, tread life aside. So in practice it is pretty easy to arrive at an overall better tire outside the TPC standard. The downside, of course, is an increase in rolling resistance, but the average consumer will have a hard time even quantifying the incremental increase in fuel consumption. The building where they do all the TPC stuff is at Milford Proving Grounds, where I imagine a good part of their time these days is spent on RR.

[mariner]

Thank you very, very much guys for the comments. I guess I have only just learnt what the major car maker's engineers knew long ago, carefully choosing an ordinary car tyre is harder than choosing a track tyre as there are more potential conflicts!

I think I am going to try the Bridgestone Potenza RE50A as they appear to have been a CTS OEM fitment and they seem to be more readily available in the UK than the Goodyear tyre which repleced the RS -A OEM tyre.

[Fat Boy]

Goodyear F-1 and RSA are both TPC tires for the CTS so if they are one of those you fell in the box. CTS is one of the shorter lists.

Sweet. My car won't explode. They're Eagle F-1's. I had to go outside and look, though, I didn't know off the top of my head. They're really not the right tire, because the car came with run-flats, which were just awful. I got what I got and a can of fix-a-flat in the truck should fate rear it's ugly head. Ya, I know it'll take out the tire pressure sensor. It's a small price to pay to not have to drive on the run-flats. Economically, it makes sense as well because the run-flats are so expensive and so short lived. The only problem will be if I have some sort of catastrophic tire failure. That's what the cell phone and Triple A is for.

[McGuire]

Thank you very, very much guys for the comments. I guess I have only just learnt what the major car maker's engineers knew long ago, carefully choosing an ordinary car tyre is harder than choosing a track tyre as there are more potential conflicts!

I think I am going to try the Bridgestone Potenza RE50A as they appear to have been a CTS OEM fitment and they seem to be more readily available in the UK than the Goodyear tyre which repleced the RS -A OEM tyre.

If the Potenzas match the UTQG and load/speed ratings of your OE ties, I am very confident they will be fine. However, the tread design looks like it potentially could be slightly noisier than the RS-A... I might be splitting hairs.

Edited by McGuire, 15 November 2009 - 20:38.

[Lee Nicolle]

Goodyear F-1 and RSA are both TPC tires for the CTS so if they are one of those you fell in the box. CTS is one of the shorter lists. One of my Pop's cars ('05 Buick) might need tires and I am headed out there tomorrow to tinker on our Model T, so I looked it up. Including winter treads and optional wheel sizes, GMSPO lists 111 recommended tires for the car.

Our Ford. No recommended tires. Guess we can't drive it.

Lots of cars around that you cannot buy tyres for!!Or at least roadworthy legal ones.
A lot of OEM tyres are almost not advailable aftermarket, or are so prohibithivly expensive to a non starter, and that is is the vehicles model run.And most cars these days have spacesaver spares which are generally fairly poor, and are often unadvailable new either.As for runflat they are so bloody dear and heavy as to be uneconomical.
Most major manufacturers make a range of tyres, some are more performance orientated, some are for ride and comfort and some are purely for mileage.And then the miriad of aftermarket brands from third world countries. While most are servicable do not expect long lasting or a decent ride.
And as for older cars, even 10 y/o ones a lot of the sizes are obsolete so you are forced to use one size fits all tyres which are generally pretty draedful and often are to small/ large for the rims and do not have the speed or load rating that should be on the car.
My 71 Galaxie the only tyre advailable with the original rolling diameter is far to wide for the rims, has too much load and not enough speed rating. They fit all 'full size' US cars and commercials which originally had several different sizes. And ofcourse a 1700kilo 60s car does not need the same load rating as a 2400kilo late 70s Caddy etc
Plus I have seen the rims split, both through old age and the heavier load rating because of this.
I actully now use a wider 16" rim with a tyre that is easy to obtain and has the correct ratings though the thought Police do not like it as it detracts from originality! But is safe and roadworthy unlike the so called replacements.
And this is so common, some cars really are unsafe to be driven as the 'replacement tyres are too large or small for the original rims.And this is for popular models of their days, not obscure cars.
I would suggest that sooner than later this will cause an accident and law suit. Which is still cheaper for the manufacturer than building sizes that are no longer big sellers.

[McGuire]

Lots of cars around that you cannot buy tyres for!!Or at least roadworthy legal ones.

Yep, that is a problem. If you are really going to drive the car you may very well have to fit non-standard tires. As car collectors we are used to this.

Even replica/reproduction tires are not what people think they are. For example, if you have exact replica 1955 tires (made from the original molds etc.) you have fairly crappy tires by modern standards. If you set out on a cross-country trip in '55, you were probably going to suffer at least one tire failure. That's just how it was. The overall design, material, and build quality are nothing like current tires. You will find poor concentricity, significant variations in loaded diameter, massive imbalances, etc and so forth. That's how the originals were. For one thing, bias tires never were that great at dimensional control (which made stagger possible). Reproduction tires are fine for show cars and around-town use but they are not a match for modern tires on the highway, especially if you expect something approaching modern ride, handling, and tire life. Which is kind of a disappointment because there are many collector cars that make nice road cars even today if not for their tires (well, and the brakes too, to be honest about it.)

The only solution I know is to study each case and select carefully, pay attention to what other owners are doing, and employ whatever ingenuity you can. Like many enthusiasts, I've run two different sets of wheels and tires on some cars, one for show and another for the road. Kind of a pain but what can you do? OTOH, modern tires on a vintage car can make a nice combination when you get it right. Modern radials on a '50s-'60s American boat are a sweet package with suitable wheel alignment settings.