[TR] use of overdrive

John Macartney macartney.john at yahoo.co.uk
Wed Aug 25 04:23:15 MDT 2010


Can't say I entirely agree with the concept of disengaging overdrive on 'up'
changes. The key selling feature of the Laycock unit was the driver could do
exactly that - i.e. up change from overdrive second to overdrive third to
overdrive fourth being entirely feasible and in no way damaging to the unit.
In 
any case, if you evaluate the overall gearing between an overdrive gear
and its 
next one up on the tree in direct - overdrive second to direct third,
the 
gearing is hardly different. So what is to be gained apart from showing
off that 
you've got seven ratios?
As for engagement and dis-engagement, the
'thump' tends to vary with individual 
overdrive units. Some engage
'violently', some do it in a more 'slurred' 
fashion. That said, I know that
Laycock also designed the unit for engagement 
under full power, so a
foot-to-the-floor change shouldn't cause overdrive 
damage. Historically on my
own cars, my overdrive technique varied according to 
the car being used. On
some I slightly depressed the clutch for a smoother 
engagement, on others I
let it change without clutch intervention.
For dis-engagement, I usually
depressed the clutch a little - and this was 
mainly in consideration of the
sudden over-run load being imposed on driveline 
splines and the Hardy Spicer
yokes in accelerating engine rpm upwards by 800 odd 
rpm in a nanosecond. I
also don't like jerks and jolts which accrue using this 
method.
Just my
preferences because there isn't a 'right' or 'wrong' way of using the 
Laycock
set-up. It's primary design purpose - not known by many, was not to use 
the
overdrive as a maximum power tool. With the long stroke / high torque 
engines
of the fifties and sixties, overdrive operation was to effect an 
up-change at
maximum engine torque level (around 2500-3250 rpm) rather than at 
maximum
engine power (5000 rpm upwards). If overdrive users try this method, I'm 
sure
they'll gain even more enjoyment from the Laycock product.

Jonmac


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