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BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5478 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

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calculating axle ratios
Posted by: BlownMGB-V8
Date: May 02, 2018 03:03PM

Never hurts to double check.

So basically a math problem, for selecting tires for the mower, which is 4wd. First step, count tire rotations. I jacked up one side and left the others on the ground, put a big tape marker on both tires and idled to see when the tapes matched up. This should give the same ratio as when all four tires are rotating, right? (That is the primary question BTW) I know it is but someone please reassure me, I've been known to miss sometimes. I know, I know. Hard to believe, right? But yeah, it does happen.

So a ratio of 19 turns on the main axle to 25 on the steering axle. (checked it over 4 series or 100 rotations) Should be a 1.316 or 0.76 ratio depending on which way you are going. So if I have 20" diameter steering tires I'd want 26-1/2" mains, 27 should work. Or if I had 25" mains I'd want 19" which aren't available. Or with 18" I'd want 24".

Which is very curious as the standard equipment is 18 and 26, meaning the steering tires (at the rear) are dragging. Except when turning.

But, my last mower was set up similarly and when I matched the tires for straight travel it took a lot of strain off the entire machine and it mowed better without working as hard. Didn't seem to be any worse for cutting up the yard either. Anybody got a clue about why they'd do this? (Guess that's the second question)

Jim


DiDueColpi
Fred Key
West coast - Canada
(1254 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
What if the Hokey Pokey really is what it's all about?

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Re: calculating axle ratios
Posted by: DiDueColpi
Date: May 02, 2018 04:39PM

The math seems right to me Jim. Even without a Scotchy scotch on board.
The over driven front wheels make sense from a liability viewpoint.
I would think that the last thing the OEM wants is for the rear steering to overcome the front drive wheels.
Probably a tough deal on pavement but no biggie on grass.

Cheers
Fred


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5478 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: calculating axle ratios
Posted by: BlownMGB-V8
Date: May 03, 2018 10:43AM

You are likely to be right, I didn't consider the liability issue. Makes me feel more confident about matching the ratios though. I never use the transport gear. I think I'll go with slightly oversized tires so I can run less air pressure and get a softer ride. Thanks as always for your valuable insight.

Jim


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