Curbside reader cadiman67 writes; Hello: I have a 1969 Cadillac Deville (Convertible) and am not happy with the ride – it feels very harsh when I go over any potholes or similar imperfections in the road. I have replaced the shocks and springs and rebuilt the front end. When I look at the underside of the vehicle I can’t help but think that the 14 rubber body mounting pads may be causing this problem since the rubber has obviously hardened over the years but no mechanic is in agreement with me. However, none of them admit that they have ever replaced all of the body mounting pads on a vehicle so they can not say for sure. Have you ever replaced all of the body mounting pads on a vehicle? If you have what was the reason for doing so and what were the results on the ride? A few years ago a coworker claimed he had that done to his late 70’s model Ford station wagon and was amazed at the improvement in the ride but he is the only person who has ever said that. Everyone else insists the body mounting pads have nothing to do with the softness of the ride. I would hate to spend the money to have them replaced and then still have a harsh ride. What do you think?
First, thanks for your inquiry. Yes, body mounting bushings are part of the system that isolates you from the bumps in the road. There is a reason that they did not bolt the body directly to the frame. However, they are only part of the isolation system, so let’s start from the ground up.
Tires are the first part of the system. When the tire hits an imperfection in the road, it flexes to absorb part of the harshness. The key to this working properly is a tire with the proper load rating. This is not to be confused with the “maximum load” stamped on the tire. The inflation pressure is what ultimately determines the tire’s load capacity. Back in the day, some tire retailers provided a Load and Inflation Chart with the purchase. Unfortunately, those charts are not included with the purchase anymore. Because that info isn’t as available as it once was, few mechanics and tire installers calculate the proper pressure when the tire size is changed. Worse yet, there are those that just put 32psi or some other value they consider “right”, or inflate every tire to the max pressure on the sidewall without even consulting the tire placard.
However, thanks to the internet, load and inflation charts can still be found. A little Googling turned up this, Tires 101 from Toyo, which includes a guide to using Load and Application tables in determining the proper inflation pressure of a replacement tire. Of course it does not include the old 9.00-15 tire size so we can’t follow the directions in guide letter for letter, instead, we’ll have to do some calculations.
It used to be common to list pressures for average and full rated load, but even in the 60’s we were starting to see the idea that the average consumer couldn’t be trusted. Looking online, I found the curb weight of the 1969 Cadillac Convertible is 4590lbs, add to that the max load and we need tires than can carry about 5700lbs. Looking up the available replacement springs showed that the fronts are rated for 2810lbs per pair while the “Heavy Duty” rears are rated at 2802lbs per pair. Since the stock tire pressures and spring ratings are equal front and rear we need 1400~1425lb load capacity per tire at full rated load. In cadiman67’s follow up E-mail he noted his Caddy is wearing Michelin 225/75r15 tires, so we’ll take that info and look at a Load and Inflation Chart. Since I can’t find a data on a current Michelin tire in that size we’ll compare the various types of 225/75r15 tires as found in the Toyo publication.
Load [ Standard Load ] [Extra Load]
Index Size 26 29 32 35 38 41 psi
102 P225/75 R15 1631 1720 1797 1874 1951 2028 lbs
ISO Metric (no P prefix)
Index 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 psi
102 1246 1312 1348 1378 1444 1481 1499 1565 1631 1654 1687 1753 1781 1819 1874lbs
Light Truck [ Load Range C ] [ Load Range D ]
Size 35 40 45 50 55 60 65 psi
LT225/75R15 Single 1445 1585 1720 1875 1980 2100 2205
Also note this warning from the Toyo Guide:
WARNING! Please note that size for size, LT-metric tires require higher air pressures
to carry equivalent loads of P-Metric tires and that any failure to adjust air pressure to
achieve the vehicle’s load requirements will result in tire fatigue and eventual tire failure
due to excessive heat buildup. Due to the higher PSI requirements of LT-Metric tires they
may not be suitable for replacing O.E. P-Metric tires because of the ride harshness that
results from higher PSI.
I’ve included the LT tire specifications because I’ve seen many times where tire stores sell LT tires for these Cadillacs citing that they are such heavy vehicles, that “they need a truck tire”, fact is the curb weight of a 1969 Cadillac convertible is only 75lbs more than a 2012 Chrysler 300 AWD.
So, recommendation #1, after reviewing the links above and making the calculations yourself, is to check and adjust your tire pressure accordingly. Note the minimum inflation pressures in the above chart, they differ based on the type of tire. The other thing to consider is that the original tires, being of the late numeric system, were an “82” series tire while your current tires are a “75” series tire, combined with the fact that your current tires are smaller and you’ve got much less sidewall to deflect and absorb the bump. The 9.00×15 has a 7.3″ sidewall while the 225/75 is only 6.6″ high, a loss of about 10%. Also, consider when selecting your inflation pressure that the lower the inflation pressure, the less responsive the steering will be and the lower cornering ability will be. It’s up to you to determine the best trade off for you.
The front suspension has been rebuilt with the springs and shocks replaced, but it isn’t clear if the problem existed before the rebuild or started after. One thing that is often done improperly when rebuilding a suspension involves the tightening of the bolts that secure the control arms. If you look at the picture above, the sleeve that the bolt goes through is serrated. With the stock bushings it is important that the through bolts are not tightened until the full weight of the vehicle is on the wheels and the suspension is allowed to properly settle. If an alignment rack with slip plates is not available that means driving the vehicle back and forth a few times. If the bushings are tightened before the weight of the car is applied to the suspension, the bushings will be preloaded when the weight of the vehicle is placed on the suspension. This will cause them to excessively resist suspension compression and to loose their ability to effectively isolate road irregularities from the chassis.
The shock absorber also plays a significant role in how the vehicle rides. Shock absorbers are often very model specific but the aftermarket only offers a one size fits all. Typically for many cars today the only shocks available are of the Heavy Duty variety which are inherently stiffer than the original standard duty versions. The shock’s main purpose is to keep the tires on the road. They do this by resisting, or slowing down, the movement of the suspension. Valving that is too restrictive will cause a harsh ride. With the stud style mount as used on the top of the front shock if the bushing is not properly installed it can create a situation where it will transmit impacts to the frame. What about the rear suspension? All the same rules as to shocks, bushings and proper installation apply there too.
Suggestion #2 would be to verify that the bushings and shocks were properly installed. If the bushings were replaced with polyurethane instead of the factory rubber style, they will transmit more of the impact to the chassis.
Now on to the focus of your question, the body mount bushings. Yes, I have replaced body mount bushings on a vehicle before. In that instance it was a case of project creep, (seen here) I had the body off of the frame for other repairs and I figured might as well replace the body bushings while they were accessible. However the only bushings available for that vehicle were polyurethane so they are stiffer than factory rubber bushings. However I did not notice any significant change in ride, but then again it was a Scout II so I don’t expect it to ride like well…… a Cadillac.
I would only consider replacing the body mount bushings as a last resort, and even then only if you are really committed to keeping the car for the long run.
So, readers what do you say, have you ever had the body bushings replaced on one of your vehicles? If so, what was your reason and did you notice any difference in the ride? Have you ever had a car whose ride you considered harsh and if so what did you find, if anything, that fixed the issue to your satisfaction?
Do you, like cadiman67, have an issue with your Curbside Classic that you would like our readers to weigh in on? If so send your questions to CurbsideClueless@Gmail.com.