AC Ace Bristol gear box?

[bertalanffy]

I wanted to ask, if different gearboxes were built into the AC Ace Bristol?
I own BEX449 and my gearbox is stamped BW82/CR5. Is this the/a correct gearbox? Were they also sold with overdrive gearboxes?
Another question I have: I have a Kph speedometer which shows the wrong speed, were there different speedo drive mechanisms for Mph and Kph and where could I get them? The foto is the one I have.
Thanks for any help

[AC Ace Bristol]

.
Bertalanffy

Making reference to my Workshop Manual for the  2 litre Bristol Engine type 100c- 100D - 100D2 and Gearbox
BWCR9  and BWC12CR  which was supplied by AC Cars Thames Ditton,
I assume that  the Standard Bristol  gearbox fitted to Ace Bristol's was   BWCR9  & BWCR12.
Maybe Fraser Nash or Cooper Bristol's etc  came with the Bristol  BW82CR5  gearbox?  often  with code suffice TC  for
"Turret Control".

Hopefully a Club member or Ace Bristol enthusiast can elaborate.............
I believe BEX449 was rebuilt  and raced by Nigel Winchester, suggest you give him a call, as he might have used different
gear ratios.    ;)

Regards Over drive , AC supplied The Laycock  "A"type  overdrive unit in period, and many members used the Laycock Type
as it is lighter and smaller and doesn't require Chassis modification.

Regards KPH &  MPH speedo drive, I would have thought they would be different gear ratios, please also check tyre size as this will also effect speedo accuracy.

Hopefully someone with more knowledge and experience can help... ;)

Keith

[bertalanffy]

Thank you for the information Keith. Does anybody know who might have a BWCR 9 or BWCR 12 gearbox?

[James Eastwood]

Bristol gearboxes do come up on eBay occasionally.

[TTM]

What are the differences between a BWCR9 and BWCR12?

A careful read of the Bristol manual has shown that both types are only mentioned on the front page, or that I am an idiot for not finding out where the differences are clearly mentioned.

[ACOCArch]

Although far from being expert in the AC Ace-Bristol I have read the original handbook for that model, and the AC Cars Ltd published workshop manual for the Bristol engines and gearboxes, both of which are held in the Archive. As far as I can tell:

a. Throughout, the handbook defines only the 100D2 engine, and includes no designation for the gearbox type;

b. The workshop manual cover defines the 100C, 100D and 100D2 engines, and the BWCR9 and BWCR12 gearboxes. Internally, the workshop manual defines the 100 D2 engine, but not the gearbox type. Nor is there any differentiation in the instructions therein between each type of engine and/or gearbox.

c. For completeness, the AC Cars handbook for the Greyhound does show different gearboxes for the 100D2 and 110 Bristol engines but, again, does not show the gearbox type.

Re the speedometer,  it is not clear whether the inaccuracy you mention is simply down to kmph v mph or something more fundamental. At great risk of stating the obvious:

a. The optional final drive/rear axle ratios  offered by AC Cars will  make a pro-rata difference to speedo accuracy and I would expect each optional rear axle ratio  to have a specific speedo drive gear ratio. No data have been found about this in either the handbook or the manual.

b. Any change in rear tyre section from the 1950s standard section could affect speedo accuracy. It is the rolling radius of the wheel/tyre combination which is of interest.

c. For any one axle ratio, for converting from a kmph speedo to one displaying mph is it not the case that only the speedo dial face is different?

Sorry, not a great deal of help!!

Good luck

John

[TTM]

Some drawings of the internals are available here, see pages 4 and 5.
Parts 14, 15, 16 and 18 seem responsible for the speedo input.

Bertalanffy, how bad is the inaccuracy of your speedo?
For what it's worth the needle on my speedo "bounces" a bit around the actual traveling speed, and it gets worse the faster the car goes, mostly above 100 km/h (=62 mph), but I guess this is simply due to old age. If yours is worse then perhaps it just needs a closer inspection and overhaul?

Comparing both speedos side by side suggests to me that the needle seems to cover exactly the same range between 0 and the highest figure, suggesting that only the facia is different between both. I could be wrong but I doubt that Borg Warner would have gone through the hassle of designing different drives just to account for speedo calibration. It makes sense to me that Smiths had to design their speedometers in function of any input "signal" a car manufacturer would ask them to work with. Perhaps things were even simpler than this and there were already some norms on this topic and it was just a case of choosing which off-the-shelf speedo would work, with no further adaptation, with the ratio of the gearbox output.

[ACOCArch]

Dear TTM and Bertalanffy,
Re speedo calibration, not sure if this is the case with the AC Ace but on many cars there is a  number in small print somewhere on the speedo face. This relates to the speedo drive gear ratio. If our theories are correct then, for cars with the same axle ratio the same number should be on both mph and kmph speedo faces.

Speedo needle bounce is quite common with cable-driven analogue instruments. As the cable rotates it alternately binds and flips causing the speedo needle to bounce. Ultimately, the cable will break.
First check is to ensure the cable follows a smooth path with no kinks.
Beyond that I find that an occasional very sparse lubrication of the cable with a light grease  helps. To prevent the outer cable liner from swelling, ensure any lubricant is suitable for plastics. Make sure too the instrument stays lubricant-free! 
John

[AC Ace Bristol]

John

You make a some  good points and observations, The inner speedo cable can catch the outer case as it wears and often  requires lubricant often due to heat from close proximity of  the gearbox and exhaust system,  another cause of speedo  fluctuation  can be if the cable routing is too tight a radius thus causing binding. where it  can wind up and release.
The Ace has different speedo cable lengths pending LHD or RHD. another factor  the cable length and  the connection to gearbox differs if you have overdrive.
My Ace is running a 3.9 differential and running on 165 by 16 inch Avon Turbo Steels and as a rule of thumb the speedo reads approximately  20 mph per 1000rpm in 4th gear. ;)
The tyre size and differential gearing will obviously have major effect on accuracy of your speedo.
Hopefully the above along with Johns blog  helps analyse and cure your problem.

Keith