Joined: Aug 2017 Posts: 741 Threads: 8
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Location: N W Kent
07-09-2020, 01:18 PM
(This post was last modified: 07-09-2020, 01:22 PM by Stuart Giles.)
The top gear will be 1:1 in any A7 box as the input shaft is locked to the third motion shaft in top gear. But you might have the sports/van 5.625:1 rear axle which would explain overall low gearing. easiest way to check would be to temporarily rig a rev counter (if you don't have one fitted) and use a satnav/smartphone app to record your speed in top gear. The MPH/1000 result along with your overall wheel diameter will predict the CW & P ratio.
Edit: Reckless Rat obviously types faster than me.... If you do have a 5.625 CW&P I would think that some of the trials lads will be only too pleased to offer you a swap
Joined: Feb 2018 Posts: 367 Threads: 14
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Location: Western Brittany (France)
Hi Duncan,
I would measure your diff ratio first.
Easy to do precisely with only one wheel up because it gives twice of course.
Renaud
Joined: Aug 2017 Posts: 1,337 Threads: 34
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Location: Cheshire
Car type: Race Ulster, 1926 Special, 1927 Chummy, 1930 Box
At 60 mph the engine speed difference between a 5,25 final drive and a 5,625 one is less than 300 revs/min. Thats hardly going to put it into screaming its head off territory. Austin Sevens are in any case overgeared. Normally fitting a 5.625 final drive, results in a higher top speed, particularly if you have standard sized wheels and tyres.
Alan Fairless
Joined: Aug 2017 Posts: 1,978 Threads: 90
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Location: Ripon
Without a crank handle ( the case has been sealed off unfortunately) I can't really count the ratio of revs to the rear wheel and I have no tachometer fitted
I suppose I could take out the plugs ,engage top and push it while watching the rotor arm, unless I'm missing an obvious method...
As it is now peeing down and the car is outside I'll have to either wait for it to stop or get my workshop floor wet!
Joined: Aug 2017 Posts: 2,401 Threads: 33
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Location: Deepest Frogland 30960
Car type: 1933 RP Standard Saloon
07-09-2020, 02:44 PM
(This post was last modified: 07-09-2020, 02:48 PM by Reckless Rat.)
Put the car in neutral, jack up one wheel. Mark both the wheel and the propshaft with some chalk. Rotate the propshaft 5 times. If the diff is a 5.625 then the wheel will go round 0.88 revolutions. 0.95 turns if it's a 5.25 and 1.02 times if it's a 4.9.
If you start with the wheel mark at 12 o'clock then the 5.625 goes round to half past 10, the 5.25 stops at half eleven and the 4.9 goes to half past 12.
Joined: Aug 2017 Posts: 1,978 Threads: 90
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Location: Ripon
07-09-2020, 04:08 PM
(This post was last modified: 07-09-2020, 04:10 PM by Duncan Grimmond.)
I don't quite understand the method I'm afraid. Lying on the floor to turn the prop shaft I mark the wheel at 6 o clock 5 turns of the shaft I got just less than two revs of the wheel ending at 4 o clock
How do I interpret this information?
Joined: Aug 2017 Posts: 1,018 Threads: 53
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Location: The delightful town of Knaresborough, North Yorkshire
This needs more information.
Which wheel are we jacking up, the nearside or offside?
Which direction are we turning the prop? Usual clockwise viewed from the front of the car?
Joined: Aug 2017 Posts: 2,401 Threads: 33
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Location: Deepest Frogland 30960
Car type: 1933 RP Standard Saloon
That's a 5.25 ratio diff. The sun & planet wheels have doubled the multiplication. If it had been a 5.625 your mark would have stopped at 3 o'clock, and a 4.9 would have gone to between 6 and 7 o'clock. Sorry for the confusion.
Joined: Aug 2017 Posts: 1,018 Threads: 53
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Location: The delightful town of Knaresborough, North Yorkshire
I've just tried it on my Ruby.
Using the nearside wheel, 5 revolutions stops at about 4 oclock. So a 5.25 diff as expected in my Ruby.
Duncan shall I come and take you up and down the road in mine? Don't know how fast it will go as still running in, but happy to go towards 50 if it will. I'm free Tuesday evening or anytime Wednesday and Thursday. Then away for a week.
Joined: Aug 2017 Posts: 1,978 Threads: 90
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Location: Ripon
It seems that I have a "normal" diff?
Andrew, that's a kind offer. Wednesday in the day timewould be good for me, the kettle works and my number is 07817826796.
