Rear Axle: Rear Axle Ratio 1947 Champion convertible

[avantibngrant]

I have a memory of my father's 48 Champion where the right front tire would rub on the floor/floor supports during a hard right turn with 6.70x15 tires - I think they were that. I am restoring a 47 champion, and I noticed they came with 5.50x15 tires which are quite a bit smaller than the 6.70's. What size are other people using on the 47-49 Champions? If I am limited to the 5.50x15 range, than i think the OD transmission will be better mated to the 4.11 rear axle. With the 6.70x15 size, the 3.56 ration would be better. Just curious to see if anyone else has issues with tires of the 6.70x15 diameter range.
Regards

Neil

[RadioRoy]

The standard rear axle ratio for a 47-52 Champion is 4.56 for overdrive and 4.10 for non overdrive. In either case, one is an optional replacement for the other. My 51 Champion Custom came with a 4.10 and no overdrive. When I added overdrive, I left the 4.10 in there, but experience says that I would have been better off finding a 4.56.

Also be aware that tire sizes changed in the 60's. Even with the same number, they were a smaller diameter in later years. If you look at a Coker catalog, the diameters of the tires are listed.

[avantibngrant]

The standard rear axle ratio for a 47-52 Champion is 4.56 for overdrive and 4.10 for non overdrive. In either case, one is an optional replacement for the other. My 51 Champion Custom came with a 4.10 and no overdrive. When I added overdrive, I left the 4.10 in there, but experience says that I would have been better off finding a 4.56.

Also be aware that tire sizes changed in the 60's. Even with the same number, they were a smaller diameter in later years. If you look at a Coker catalog, the diameters of the tires are listed.

Thanks Roy. I am wondering what size of tires you are using to find the ratio 4.1 too high? I am wondering if the 6.70x15 equivalent would even fit on the 47 if my foggy memory serves me well, I think it will rub on the body on turns. In 49 the 6.40 was offered instead on the 5.50. If I go with the 5.50 equivalent diameter, the ratio of revs per miles per hour is about the same for the 4.1 as with the 6.70 with the 4.56.

Neil

[RadioRoy]

I have had two or three sets of tires on the car and tire size has little to do with the feeling of relative power. Currently it has BF Goodrich 6.40 X 15 tires on it from Coker. I bought those because they were the skinniest tires, with the biggest whitewall and the largest diameter. Before that, I had some 7.35 X 15 bias ply tires from teh 1970's on the car and they were smaller diameter than the 6.40 tires. Tire size in general is a smaller percentage of change than axle ratio - assuming we are talking about vintage tires, not modern wheels and tires.

What is the ratio in the car now? The Studebaker engineers knew what they were doing. Look at the diameter of the size or sizes that were recommended for your year and find tires in the Coker catalog with those diameters. Even if you don't buy tires from Coker, the catalog is valuable a source of information. 4.56 with overdrive is the best alla around choice, with 4.10 being optional.

Why not drive the car first and see how it feels?

If you are insistent on changing the rear axle ratio, maybe you will change your mind when you try to find one. The ones that fit are 47-50, and Champion only, so that is not an extremely large number from which to choose.

[avantibngrant]

Sorry to take so long to reply, but I wanted to get a better understanding of the limitations of the tire sizes with the body. What I am trying to accomplish is to rebuilt a very badly rusted 1947 Champion convertible by setting the body on a 4 door frame with an engine and drive train already there. The convertible has a stuck engine, but an overdrive drive train witht the 4.56 rear end. The frame has no overdrive with the 4.1 ratio. I am wondering if I should chane the rear end when I add the overdrive. Per a road test I have from 1947, the Champion in overdrive would go 23.2 mph per 1000 rpm. This translates to a tire diameter of about 24.9 inches for the 5.5x15 tires. Fairly small compared to the later Champions. The Coker tire recommended for the 47 Champion is a 5.5x15 listed as a diameter of 25.9 or about 4% bigger than the original. The tires in 1949 were up to 6.40x15 as standard which has a diameter of 26.9 or 8% larger than the 47 road test model had. When I look under the front fender of the convertible, I can see where a tire was rubbing at the pedal splash protector on the left hand side. This makes me believe that the tire size is probably limited to about the 27 inch mark or so or the 6.40x15 like you are using. Base on your experience, if i go with that size, I probably should change the rear ends. I will be al lot easier to do before it is all put together. I can have all the bolts loose on the frame ready to change when the body is lifted off.
Engine speed at 60 with od will be 2393 rpm and without od will be 3418 rpm with the 6.40x15 from Coker or with the 5.50x15 from Coker it will be 2485 and 3550.
If I leave the 4.1 axle 60 mph numbers would be 2152 and 3074 with the Coker 6.40x14 and 2237 and 3196 with the 5.50x15 Coker tires.

Thanks very much for your input. I have had a 52 Champion 4 door without the overdrive and the 4.1 axle. Even with the 6.40x15 tires, the engine ran very fast at even 50 miles per hour. By comparison, it would be turning 3074 at 60 mph.

Regards
Neil

PS. sound like I know the go to guy if the radio does not work!!

[RadioRoy]

It looks like you have done your homework and are no stranger to calculations. That's a good thing, and also a rare thing. I will tell you that I initially came to the same conclusion based on the fact that Studebaker offered a 4.10 with overdrive as an option, plus the fact that I did not have access to a 4.56 at the time. Subsequent to building and driving the car for many years, I decided that it would have been better to install the 4.56. The car is underpowered at best and the engine seems like it's power curve is at slightly higher RPM. Plus, the clutch is touchier coming off the line than it would be with a slightly better exle ratio. In addition, you can get scared at 70 if you have to attempt to stop with those miniscule brakes. You might find yourself driving closer to 60 or 65.


My suggestion is still that the 4.56 is the better choice, based on: 1) my experience on both flat land and hills with my 51 and 2) what the factory chose as standard equipment for thousands of models.

Remember that the difference in axle ratios is around 7% while the overdrive drops RPM a full 33%.

I cannot offer an experienced opinion based on the differences in the earlier models, but the weights and horse power rating are pretty close.

The best thing is to do what you want, and keep an open mind and your options open. You might also save both differential assemblies until you can do an extensive road test.

Good luck.

[avantibngrant]

Thanks Roy. I will keep both rear ends for sure. Actually I have 3, but it is a long story. Number one is on my fathers 48 4 door which the 47 convertible will sort of become. My story is in the Canadian Avanti newsletter.
Regards
Neil