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Simon Johnston
United Kingdom
6145 Posts |
 Posted - 13/10/2018 : 14:13:24
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The J2/PB ratios seem pretty good to me. This is from the J2 Instruction Manual.
Simon J
J3437 |
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Oz34
United Kingdom
2545 Posts |
Posted - 13/10/2018 : 15:02:22
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I must say Simon, that also looks fairly good to me. For some reason I always had it in my mind that there were still gaps but that they weren't quite as big as on the PA.
Dave |
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Simon Johnston
United Kingdom
6145 Posts |
Posted - 13/10/2018 : 16:55:28
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Dave,
Second could be a bit higher and the change from third to second in the J2 still needs a lot of care.
I hadn't really looked carefully at that chart before and it's interesting that the axle ratio was carefull chosen to give a - theoretical - 80 mph at 5,500 rpm.
Simon J
J3437 |
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NeilF
United Kingdom
66 Posts |
Posted - 13/10/2018 : 18:54:19
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My PA gearbox screams in 1st and 2nd gear and I try keep the revs low in both which makes driving difficult around the hills of Cornwall. I know in the past S&V manufactured replacement gears and I spoke to them in 2017 about making another batch but it appears the cost of gear cutting is prohibitive. A similar discussion in this forum in 2014 ended without a positive conclusion although several owners expressed an interest in buying replacement gears.
The herringbone gears are extremely difficult to manufacture requiring very specialised machinery, cutters and tooling. Both sets of herringbone gears in my box look to be in good order and are quiet on the road so it appears to me that my box would be better if the spur gears were replaced. I know that spur gears will still be noisy but if they were new and undamaged by years of crashing I would have more confidence in using higher revs.
I have studied the drawing of the gearbox, peered into mine through the top and making changes to the spur gears to get better ratios is virtually impossible because of the need to connect with the 3rd gear herringbone set and the reverse pinion.
I am very impressed with the gears Brooklands EngineCraft have produced and would be interested in purchasing a set of replacement spur gears to suit my PA box. While this will not match the better ratios of the J/PB box it will be an upgrade of the 84 year old box.
Neil |
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Simon Johnston
United Kingdom
6145 Posts |
Posted - 13/10/2018 : 20:52:52
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There's surprisingly little difference between the two sets of ratios as this picture shows.
Simon J
J3437 |
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Oz34
United Kingdom
2545 Posts |
Posted - 13/10/2018 : 21:41:00
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Thank you Simon. I was going to do that and you've saved me the trouble!
Dave |
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Simon Johnston
United Kingdom
6145 Posts |
Posted - 13/10/2018 : 22:08:45
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Simon J
J3437
P.S. As an aside, if you have a rev counter it acts pretty much as a speedometer when in third gear, i.e. 6000 revs is just over 60 mph. Oops, did I say 6000 revs?  |
Edited by - Simon Johnston on 13/10/2018 22:12:17 |
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Oz34
United Kingdom
2545 Posts |
Posted - 13/10/2018 : 23:30:05
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Actually Simon, the PA rev counter is marked in mph in 3rd & 4th just as your speedo is marked in revs in 3rd & 4th.
Dave |
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Simon Johnston
United Kingdom
6145 Posts |
Posted - 14/10/2018 : 01:19:38
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Yes, I know that Dave. It was just a general comment. My speedo is in front of the passenger and there is a large rev counter in front of the driver. It acts as a speedo, more or less, if pressing on in third gear on country roads. Just sayin'
Simon J
J3437 |
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Simon Johnston
United Kingdom
6145 Posts |
Posted - 18/10/2018 : 08:49:40
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Dave,
Mike Allison did an excellent article in the 1998 Yearbook on gearing in which he explains why the PA gearing, whilst apparently not all that different to the J2, does cause problems in certain circumstances. Well worth a read.
Simon J
J3437 |
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Tim Sharp
United Kingdom
303 Posts |
Posted - 18/10/2018 : 09:36:25
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Guys
Associated but not quite on topic. Many of us have tweaked our engines to give more power throughout the power band (my blown PB is allegedly now in the mid 80's BHP) but I have searched the Forum but cannot find any suggestions as to altering diff' ratios to give a longer back axle ratio to move the lower gears up the speed bands.
I would have thought that even if the replacement crown wheel and pinion set was made bespoke it must be cheaper than the cost of replacement gearbox gears?
I acknowledge and understand that the difficulty is in calculating a replacement ratio that the car will pull and not bog down at the start but similar would apply in designing replacement gearbox gears.
I will be interested in your thoughts.
Regards
Tim
Tim Sharp
PB0685 - Nuneaton |
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Colin Butchers
United Kingdom
1487 Posts |
Posted - 18/10/2018 : 09:54:15
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Tim. The standard rear axle ratio for the PA/PB is 5.375:1 using an 8 tooth pinion with a 43 tooth crown wheel. Higher ratio gears were offered by Abingdon of 5.125:1 (8/41). With a power output of 80 bhp your car should easily pull a ratio of 4.875:1 (8/39). 8/39 crown wheel and pinion sets are available from the usual stockists (including Roger Furneaux's "Mad Metrics" business) designed for use by MMM cars.
Colin B. |
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Tim Sharp
United Kingdom
303 Posts |
Posted - 18/10/2018 : 10:50:09
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Colin
Thanks for that. I am currently running a 7/36 4 star diff (5.143) so could definitely consider the 8/39. I wonder if there is anything longer easily available?
Regards
Tim
Tim Sharp
PB0685 - Nuneaton |
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Colin Butchers
United Kingdom
1487 Posts |
Posted - 19/10/2018 : 10:43:33
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The standard rear axle ratio for a Q Type was 8:36 (4.5:1) with an ever higher one available at 8:33 (4.125:1). I am sure that there are gear cutters around who could make you one with any ratio you want - but the Qs ran with 28-30 psi boost and lots of wide tracks available where you could work the car up to 130 mph. Better to stick to 8:39 until you have everything sorted out before going any higher. Incidentally Tim, where did your present diff come from ? Is it from a YA ?
Colin B. |
Edited by - Colin Butchers on 19/10/2018 10:47:50 |
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george
United Kingdom
863 Posts |
Posted - 19/10/2018 : 11:26:16
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| Tim , with a ratio of 7/36 you are probably talking about a YA diff [ I think my old one] If it is the only ratios I have seen are 7/36 MG or 7/37 Morris 10. The 8/39 Colin refers to will not fit . The pinion bearings in a YA diff are imperial [1in] the pinion in an N or P are metric 25mm. Geoff |
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