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Old 04-07-2010, 08:32 PM   #1
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Location: Lethbridge, AB, Canada
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Year: 1981
Coachwork: Bluebird
Chassis: Ford B-600
Engine: Ford 370 Propane
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Identify my rear end...

Can anyone tell me what type of rear end this is? I think the gear ratio is in the 5:1 range and I would like to change it to something a little more highway friendly. I don't even need to use first gear this thing is geared so low.


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Old 04-08-2010, 11:40 PM   #2
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To find the gears/ratio

Some place on the unit there may still be an ID tag. You can determine the ratio very easily by raising one rear set of wheels. If the brakes are air powered, the rear brakes must be released. Make a easily seen mark on the side of the drive line. The vehicle trans must be in neutral. Standing near the raised wheels, where the drive line can be seen, make a line on tire near the road. While turning the rear wheel one revolution, note the number of times the drive line turns. When the wheel is moved one turn and the drive line turn 4 and a bit, the ratio is 4:10 to 1. One of my buses had a 12:40 axle ratio and was a 2 speed gear differential. The unit was locked in low range. Large trucks and buses use a limited number of ratios. Any ratio can be changed. My old Crown rear end gears are no longer made so a very late truck rear axle will be used. Used complete units are sometimes cheaper than just the price of the gears.
Using a complete rear differential with all the componets will also yield more modern brakes and all the parts and hardware. The 10 hole Budd wheels can also be used. Last unit I got cost $800 from a 2006 Int. A hi speed rear axle can make good miles per gallon and a slow climb up a hill. Frank
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Old 04-09-2010, 11:06 AM   #3
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Re: Identify my rear end...

So true about the mileage vs. ability to climb a hill. I do a lot of camping in the mountains and I surely wouldn't want to have to get out and push! I will look closer for a tag and maybe get some rpm readings at highway speed before I concern myself much further with this. Just gotta install a tach!
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Old 04-09-2010, 12:36 PM   #4
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Re: Identify my rear end...

lol sorry but someone had to say it "It looks like a ford rear end!"
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Old 05-02-2010, 09:31 AM   #5
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Re: Identify my rear end...

John, your memory is correct. The speed of a driven tire (except with an off-road locked axle) is two times the indicated speed less the speed of the opposite tire.

If you are making a tight turn in a parking lot, the outer rear wheel is making a 40-foot circle and the axle length is seven feet, then the inner rear wheel is making a 33-foot circle in the same amount of time. If the indicated speed is 5 mph, then the outer wheel is going 5.48 mph and the inner wheel is going 4.52 mph. 5 mph time two is 10 mph. 10 minus 5.48 is 4.52.

If one tire is spinning on ice, the other is standing still, and the indicated speed is 35, then the tire on ice is spinning at 70. This is why they heat up and occasionally explode on the idiots who insist on just sitting and spinning when they are stuck.

You can see the same thing if both wheels are raised and the driveshaft doesn't move. The opposite tire turns in the reverse direction at the same speed you rotate the near tire. Zero minus 1 equals minus 1, zero minus 2 equals minus 2, etc.

So, if 4.10 revolutions of the driveshaft will rotate the tire once going in a straight line, it will turn the tire twice when the opposite wheel is stopped. A 4.10 rear axle will rotate the driveshaft only 2.05 times per tire revolution when turning an unblocked tire by hand.

To get more precision, count multiple rotations of the tire and divide the results. Five rotations of the tire with a 4.10 would show 10.25 axle rotations. The 10.25 divided by 5 tire turns times 2 for the other tire turning zero is 4.10. The difference between decimal point variations of the axle ratios will become more apparent the more turns you count, and will offset the difficulty in determining fractional rotation positions of the axle.
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