Changing rear end gears

[Guest daytona]

I have a 1930 Cadillac with 5 to 1 gears, is it better to change to 4 to something gears or install an overdrive. Does anyone have any suggestions who does this, with 5 to 1, 45mph feels like top end. Also is installing new gears difficult, I am guessing yes but would like any feedback.

Thanks

[Guest De Soto Frank]

You might want to talk with Matt Harwood on this.... he recently acquired a '30 Cadillac and decided to add an overdrive... he thought carefully about it before making his decision.

[NTX5467]

Usually, when you make a big change in the rear axle ratio, on anything, the differential case itself will most certainly need to be changed, too . . . NOT just the gearset which attaches to it. Reason is that there are two "hard" architectural points in the rear axle housing--where the pinion gear's front-to-rear centerline is and where the side-to-side vertical centerline of the differential case's ring gear flange is in the rear axle housing. In other words, as the ring gear thickness might vary with the particular number of teeth on it and the accompanying change in diameter of the pinioin gear's teeth, the total build thickness of the ring gear and the mounting flange it attaches to will need to vary somewhat with different ratio spreads. This is why many aftermarket performance "deep ratio" rear axle gearsets come with a spacer to go between the new ring gear and the mounting flange . . . to compensate for the different ring gear thickness. In production, the thickness of the mounting flange itself is what the OEM's use to not need the spacer. This restricts the particular differential case to a particular range of possible gear ratios.

Not knowing what gear ratios from newer Cadillacs might also fit the existing rear axle housing (in the case of some Buicks, the rear axle "guts" were common for many years, with the later years having the more "high speed" ratios than the earlier model years did), plus the potential availability (and cost!) issues of gearsets for the earlier axles, finding an overdrive unit can be a much better option. It might also be that a complete transmission/OD unit would end up being what is swapped.

Other than adding the various switching mechanisms to make the OD unit function, it might also be necessary to modify the length of the driveshaft. Balance will be more critical, too, as the driveshaft is still spinning at the 5.00 to 1 speed for a given road speed, whereas the engine will be running in the OD ratio of .7 (approx), which will make the engine think it's now got a 3.50 rear axle ratio behind it.

As for installation, I greatly suspect it'd be easier to find somebody to do the installation of the OD unit than tackle a rear axle ratio change . . . in a newer vehicle much less a much more vintage one (with many items very possibly being "unobtainium" or on the other side of the world).

To me, that's how the dynamics of the situation would play out.

Just some thoughts,

NTX5467

[jscheib]

The problem might be finding gears to replace what you have for the better touring ratio. Some of us with Buicks have a source that is about to make some new gears to fit the existing rear end. I am surprised that the Cadillac would be at top revs at 45. With the similar original ratio in my small '32, I can get it to 55 easy and even 60 without too much strain on the engine. I do not want to go the fast very often, but also would like to have less strain on the engine.

Send me a PM or e-mail if you want to communicate more. I am nearby and we may have crossed paths around here. at some time. I am trying to remember when I last saw a '30 Cady.

John

John

[nzcarnerd]

Looking at the Standard Catalog, the standard ratio is indeed 5.08:1, but there were optional ratios of 4.75 (which became standard in mid season) and 4.39. I guess the 4.39 was only suiitable for roadsters in lowland areas. Perhaps a 4.39 is hard to find. The 1932s used either 4.60:1 or 4.36:1 which would explain the extra speed obtainable with a '32. The 1930 car with a 5.08 rear end and a tyre size of 7.00x19 would be doing just over 3100rpm at 60 mph (which is past peak power and probably as fast as you would want to rev it). A 4.39 diff would bring the the 60 mph revs down to around 2700.

It depends whether you want to keep the car looking original underneath or have it as a driver. There are many ways of constructing a rear end using a mix of a modern internals with earlier style drums and hubs.

I recall reading a story somewhere (from Popular Mechanics maybe?) by Jay Leno where he detailed getting a rear end gear set built for his 1932(?) Packard.

Edited September 18, 2011 by nzcarnerd (see edit history)

[Ivan Saxton]

Correct me if I am wrong. My understanding is that V8, V12, and V16 had effectively the same rear axle, but with some differences in ratios. (This was production economy in making just one car, but with three different engines and differences in chassis length.) And this was the same as the first 341 in 1928. There was a 341 in Melbourne which had a very high ratio, and the fellows who owned it used to entertain themselves in the late 1950s by tail-gateing British sports cars at weekends until they ran out of breath, and then run away from them.

It is not so straightforward to fit an overdrive with a torquetube as with an open tailshaft.