My old gal is bouncing her back end around, and I’m not talking about my “Big old wife this time”!!!

I’m in need of some advice from some of you tar experts out thar. My old girl has a bounce goin for her in her rear end and it’s not to appealing on her.

I’m addressing my 1950, 3604 and last weekend was the first I’ve had her on the road since I bought her last September. This is really a two part question.

Part one; I’m running stock slit rims all round (good ones, no rust) with brand new, zero miles (Chin made) Power Master 7.00-15 LT, Bias 4 ply rated 6 ply tires on the front.

On the back are two Mohawk, Motrac Chief 7.00-15 M/S, Bias 4 ply rated 6 ply. These were on the truck and I’m not sure how old they are but they’re in good shape, no checking or cracking of the side walls, tread is like new and the rubber is not hard but still supple. I attribute this to the fact that it was kept in the fire shed most of her life up until a couple of years ago.
Before putting her on the road, I balanced all four tires myself here at home with a good bubble balancer, then to verify my findings took then to a tire shop just up the street and spun them up on their dynamic balancer. I was real close, within a 1/2 oz on all four.

She a 3/4 ton so I expected her to be kind of a rough rider, but at around 45mph a real bad bounce in the rear develops, bout shakes your teeth out, it gets a little better at around 50 (that’s top end) and diminishes when you let up on the loud peddle (figure of speech, shes a 216) and drift below 45mph. My first thought was the M/S tires were out of round, I checked the rim with an indicator and only have around .050 runout and the tread outside runs out roughly an 1/8”, I’ve seen worse and not noticed this bad of a bounce.
So yesterday I took the rears up to my favorite tire shop that I have been using for 25 years. They’re located out in the middle of the Illinois farming community and service a about every type of commercial vehicle there is. We spun them up on their machine (a Coats) we came up with about the same weight and location (around 7oz. on both). Problem was we worked for 30min trying to get them to 0.00 but ended up putting 1oz. opposite the 7oz. to get them in balance, not the best practice but it was the only way. On the machine the tread run out was about the same as what I saw at home and they didn’t seem concerned about it.
Well back on the road and at 45mph the rear starts into harmonics again, no change. My conclusion, get those da&$ snow tires off the back end!!! Unless of course someone has another idea.

Second part is; I have a set of 16.5 rims from an early 70’s 3/4 ton 4/4 that my nephew scraped out. They fit the rear OK but of course hit the stock tie rods on the front. So, my plan is to mount up some tubeless radials on the back and leave the 7.00-15 Bias plies on the front.

I’ve heard rumors against mixing radials and was wondering if it was urban legend, just a plot to sell more tires or if there is any science to it.
Also can any one suggest a 16.5 tire that would look similar to the 7.00-15 Bias plies that were originally on the ’50, 3/4 ton trucks? The China made Power Masters that are on the front look very original.

Oh one other thing, new gas charged heavy duty shocks all round, all the springs take grease and they and the shackles appear to be in good shape, no play. All new (rebuilt) steering linkage and all new bearings front and rear, drive line all new bushings, center bearing and rubber, three new U-joints and drive shaft and yokes striped to bare metal and re-sprayed with Rustolium.

Denny Graham
Sandwich, IL

Denny G
Sandwich, IL