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Tim Sharp
United Kingdom
303 Posts |
 Posted - 19/10/2018 : 11:37:35
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Geoff
Yes, thanks for that it is your old 7/36 diff I am using. So what I now need is an N or P diff to get an 8/39 fitted to it.
Colin
Please see Geoff's posting.
Regards
Tim
Tim Sharp
PB0685 - Nuneaton |
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george
United Kingdom
863 Posts |
Posted - 19/10/2018 : 11:44:45
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| Tim ,The YA diff is better engineered by far than the N or P ,it is also massively less expensive .It may well be a better solution to get a quote for a one off to suit the diff you have. As an aside when I used the 7/36 I did not really notice the change to my current 8/39. Geoff |
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coracle
United Kingdom
1940 Posts |
Posted - 19/10/2018 : 12:45:03
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Following this tangential "torque" of varying the diff ratio as an alternative to moving from PA to PB ratios, I beg to differ.
Although the graph comparing the two boxes apparently show little difference, in practice it is quite dramatic. My first experience was of a PB box which was fine; I then rebuilt a PA and was shocked by the difference. This provoked me to change that PA to PB ratios within 3 days (fortunately I had a spare box handy.)
Tim's suggestion of a "longer" diff ratio is perfectly reasonable, provided you never get to third gear. If you do, however then that gap between 2nd and 3rd is still there, just at a higher speed. That gap, particularly if you have a heavy flywheel, causes a significant delay in gear change and thus a consequential loss of acceleration. This is greatly reduced using PB ratios.
Where the PA box really manifests itself is on a hill that does not allow you to get out of second because by the time the cogs have synchronised allowing selection of third, your speed has dropped to preclude using third. You are thus stuck screaming away in second but not able to select third.
A longer diff on a PA box will not sort this out but the closer second/third ratios of a PB box and a lighter flywheel will mitigate the problem. |
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Tim Sharp
United Kingdom
303 Posts |
Posted - 19/10/2018 : 20:47:17
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Geoff
Thanks for the aside. I am concerned that a change of 0.268 will not make alot of difference.
Nigel
I currently have a Wolsey gearbox with PB ratios (so Andy King tells me) so am starting from the best base I think.
My problem is hillclimbs where I am quickly running up to 6,000 rpm but find that there is not enough time / track for me to go up from 2nd to 3rd and then back down to make the change worthwhile. I am currently recording very quick times off the line and having to change from 1st to 2nd very quickly. Therefore I think I can pull a longer ratio. So I am thinking of making the change to a longer diff ratio so that I am not running out of revs where I could go quicker but do not have time to make the up and down change. The flywheel is a lighter unit already.
(I'm never sure if a move from 5.14 to 4.875 is "higher" or "lower" but I do recognise that the lower number is a "longer" gearing)
So what I think I need to do is change the diff ratio but possibly to longer than 8/39.
Regards
Tim
Tim Sharp
PB0685 - Nuneaton |
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coracle
United Kingdom
1940 Posts |
Posted - 20/10/2018 : 00:59:14
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Tim,
I was only addressing your gearbox verses diff ratio point:
"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 see the logic in your particular case when you already have PB ratios and a lightened flywheel.
How about trying bigger tyres on the back with a greater rolling radius as a simple first step to "lengthening" the gearing?
Nigel.
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