1970s Radial tire adjustment

[34837]

My 73 Deville came to me with radials, first available as an option that year. The spare is also apparently original. So- from what I've read tweaks to the front suspension were necessary for radials, can I assume that this would have been done in the factory? The car was bought of off a dealership that had ordered most available options, don't have a build sheet but original bill of sale denotes a CB radio as only add on. Thanks!

[Big Fins]

How are the tires wearing? Nice and even? Any lateral feathering of the tread? This issue would mostly indicate a toe issue. Toe-in should always be 1/16" in. When radials replaced bias ply tires, we were always to increase the right side caster by a half a degree positive. Camber was generally set at 1/2 positive. The camber setting also depends on the size of the persons riding in the vehicle.

The originals were labeled as LR78-15. Any modern replacement should be 235/75R-15.

If you're getting good wear patterns, you may not need to make any adjustments.

[klinebau]

Typically, a radial tire will want more caster than a bias ply.  The caster specs for bias ply are usually negative and a radial tire will want positive caster.  The suspension will probably not allow much more caster than zero, but the most positive caster you can get evenly on both sides will be best.

[James Landi]

What does your owner's manual and door tag say is the recommended tire inflation?  Makes a world of difference if your tires are over inflated.

[35-709]

Radial tires need 7 to 10 psi (32 to 35 psi) more pressure than the old bias-plys.  Yes, the ride may be a little  stiffer, but that is the trade-off for better handling and increased tire mileage.  Use any pressure you want, but the fact remains radials need more pressure to perform as they were intended.
And yes, I wholeheartedly agree with "klinebau"  that cars with radials need positive caster.

[TJ Hopland]

With a 73 being more or less the same car through 76 and radials being standard later you can just compare specs including inflation for the later radial cars.

And I agree the caster is the biggest change and caster is the self centering feeling you get. Radials seem to need help with that where bias don't.  If you have the alignment set for bias they will tend to have pretty squirrely steering and it will require constant quick corrections to try and keep it going straight.  You add that issue in with other worn stuff it can make the car almost undrivable on the highway.   

[Big Fins]

You can find all the settings in the factory service manual. Setting the proper ride height through the torsion bar adjustments is the main starting point.

[TJ Hopland]

You can find all the settings in the factory service manual. Setting the proper ride height through the torsion bar adjustments is the main starting point.

Torsion bar on a 73 Deville?   And I just flipped through the sections of my 73 factory service manual that talk about tires and alignment and I don't see any mention of radial tires anywhere so if there was any such info and data for 73 that must have been printed in a supplement?    I was pretty sure it wasn't in there because I remember back 30 years ago when I got my car I was trying to figure out what the specs should be for radials and I know I had the manual back then. 

I'm too lazy to go look for a 75 or 78 manual.  I would assume the 78 just gives radial specs and would be a different car in the case of a Deville but I would think its possible the 75 could give both specs.

[The Tassie Devil(le)]

The December 1972 Serviceman makes note regarding the Optional Radial Tyres.

It advises that the cars fitted with these optional tyres are to have a warranty document inserted into the Gloveboxes of new cars by the selling Dealer as they are not installed in the vinyl Owners Manual holder as these are prefilled prior to the vehicles being made.

I would have thought that any 1973 Cadillac fitted with the optional Radials should have some reference to the necessity for different alignment specifications.

Bruce.

[Big Fins]

Torsion bar on a 73 Deville?   And I just flipped through the sections of my 73 factory service manual that talk about tires and alignment and I don't see any mention of radial tires anywhere so if there was any such info and data for 73 that must have been printed in a supplement?    I was pretty sure it wasn't in there because I remember back 30 years ago when I got my car I was trying to figure out what the specs should be for radials and I know I had the manual back then. 

I'm too lazy to go look for a 75 or 78 manual.  I would assume the 78 just gives radial specs and would be a different car in the case of a Deville but I would think its possible the 75 could give both specs.

My bad there. I had Eldorado on the brain for some reason. No torsion bars, just coil springs.

[34837]

Thanks all, the car rides and handles well but I'm always searching for perfection (that's affordable!) Replacing rubber bushings wherever, and I'm still set on ORIGINAl shock replacement down the road although the what I believe to be the factory shocks on there  now  show no leakage etc. Pretty sure I've got all maintenance records going all back. Great info for when I take vehicle I again for maintenance. A.

[cadillacmike68]

Typically, a radial tire will want more caster than a bias ply.  The caster specs for bias ply are usually negative and a radial tire will want positive caster.  The suspension will probably not allow much more caster than zero, but the most positive caster you can get evenly on both sides will be best.

If the caster is negative, self centering of the steering will be reduced. To help get a little more positive caster, you can use the camber offsets / eccentrics in the upper arm ball joint socket to put the top of the wheel to the back instead of to the front while keeping the same degree of camber.

[The Tassie Devil(le)]

I have just checked the tyre pressures and wheel alignment specifications in the 1972 1974 Shop Manual and I was surprised to see that the figures for both the Bias Ply and Radial Tyres were exactly the same.

How could Cadillac have got this so wrong?

I have been using Radials since the '60's, and the tyre companies always stress that the inflation pressures should be a minimum of 4 psi higher, and the alignment, especially the Toe-in to be reduced to 1/16".

Bruce.

[TJ Hopland]

What section(s) are you seeing mention of radials?   I kinda flipped through the areas I thought it would be in the 73 and didn't see anything but if 72 had it you would think 73 would too.

[Big Fins]

especially the Toe-in to be reduced to 1/16".

Bruce.

What are you saying that it was reduced from? An 1/8"? That used to be the norm with bias ply tires. In the advent of radials, the spec called for 1/16" toe in. That was GM Tech training schools answer. "0" would make the car wander all over the road.

[Roger Zimmermann]

I have just checked the tyre pressures and wheel alignment specifications in the 1972 Shop Manual and I was surprised to see that the figures for both the Bias Ply and Radial Tyres were exactly the same.

Bruce, where did you see a reference for radial tires in the '72 shop manual?
On page 10-1, there is a description of the tires used on different series. They are all L-78-15. It's my understanding that those tires are bias-ply. Or am I wrong (it would not be the first time!)?

[Big Fins]

You are correct Roger. The term L78-15 refers to a bias ply tire. The term LR78-15 would refer to a radial ply tire.

[The Tassie Devil(le)]

Sorry everyone, I meant the '74 Shop Manual.   I have chastised my typing finger for hitting the wrong key.

Bruce.

[The Tassie Devil(le)]

What are you saying that it was reduced from? An 1/8"? That used to be the norm with bias ply tires. In the advent of radials, the spec called for 1/16" toe in. That was GM Tech training schools answer. "0" would make the car wander all over the road.

Where did I say "0" ?

Bruce.

[Big Fins]

No where. I had many people ask me back then, why not set the toe at 0. And I had to explain to them the reasoning and the physics of it.

You said you reduced the toe setting from an 1/8". I was simply asking what you set it to and explained what the factory setting was to be with radials.