Lotus Elan and Lotus Elan Plus 2 Forums

I bought this servo a while back and is supposed to be rebuilt for my S4. Now i'm second guessing if its the correct type.

It's is 7 inches across (22.5 ish measured around the outside of the metal cylinder) It has 5/8 and the Name Girling stamped on it. See pics. I cannot find any numbers on it.

Is this suitable for an S4 elan? (2:1) or is it for a +2 (3:1)

Thanks

James

Hi James,
My S4 servo is 5/8" hydraulic cylinder with 5 1/2" diameter booster can - yours looks like the 3:1 version for the +2. From the replacement bolted on clamping ring it does look like it has been rebuilt at some time.

Regards,

Roger (James)

Interesting "puzzle"... Always something to learn.

Is it the 5/8" & 5 1/2" relationship the clue to what the boost ratio is?

I (mis?) understood that it's set by the areas of ends of the internal control piston... No?

ardee_selby wrote:Roger (James)

Interesting "puzzle"... Always something to learn.

Is it the 5/8" & 5 1/2" relationship the clue to what the boost ratio is?

I (mis?) understood that it's set by the areas of ends of the internal control piston... No?

You are quite correct Richard it is the control piston which determines the boost ratio - nothing to do with the diameter of the vacuum piston as I implied. I just noticed that James's vacuum can was a larger diameter than mine and posted without thinking...... Senior moment Please ignore my previous post, sorry to mislead. Thanks for putting me right.

Regards,

oldelanman wrote:it is the control piston which determines the boost ratio

Yes, but a higher boost ratio, or servo gain, requires a larger diameter diaphragm to provide the additional force.

CBUEB1771 wrote:

oldelanman wrote:it is the control piston which determines the boost ratio

Yes, but a higher boost ratio, or servo gain, requires a larger diameter diaphragm to provide the additional force.

So my initial assumption was correct - the vacuum chamber diameter is a clue to the boost ratio and it's likely that James has a 3:1 ratio.

Regards,

"This drawing from the workshop manual shows the servo in simplified form. The servo on the 110 gives a boost ratio of 2.78:1 and is determined by the ratio of areas of either end of a control piston (H)"

(Ref: http://www.head2head.free-online.co.uk/Rover/servo.htm)

Guys, Sorry about this but...

...isn't the 2:1, or 3:1, (or whatever) ratio a measure of the hydraulic multiplication factor, rather than a measure of max pressure available (which would be in proportion to the diaphragm diameter)?

My brain hurts now...

Still Confused.... How do I know which i have. Is there an easy way to identify or test?

James

At the opposite end to the can there should be a nymber stamped into the metal - I believe for the elan it should say '2' - this probably has a correlation to the 2:1 boost (whether it says 3 or 1.5 on others I am not sure but I believe different boosts apply to Elan and Euoropa ones though externally they look the same).

HTH

holywood3645 wrote:Still Confused.... How do I know which i have. Is there an easy way to identify or test?

James

We have gotten off track a bit. If the vacuum canister has a 7" OD then it is the 3:1 gain unit for the +2. That is the quick means of identification and not an explanation of how it works.

CBUEB1771 wrote:

holywood3645 wrote:Still Confused.... How do I know which i have. Is there an easy way to identify or test?

James

We have gotten off track a bit. If the vacuum canister has a 7" OD then it is the 3:1 gain unit for the +2. That is the quick means of identification and not an explanation of how it works.

So does that mean the Girling Mk2B servos fitted to later Europas & Sprints, for example, had smaller canisters, or were they also 3:1?

"Could be a later fitting by Lotus as my 72 Sprint has it and my 72 Europa Special also had it, both were fitted with Girling MK2B servos as standard"
Ref: elan-f14/spax-problems-t20852-15.html

ardee_selby wrote:Could be a later fitting by Lotus as my 72 Sprint has it and my 72 Europa Special also had it, both were fitted with Girling MK2B servos as standard"
Ref: elan-f14/spax-problems-t20852-15.html

Richard,
That quote was refering to the servo mounting support bracket in the wheelarch, not the servo itself.

Regards,

oldelanman wrote: Richard,
That quote was refering to the servo mounting support bracket in the wheelarch, not the servo itself.

Regards,

Eh? Yes, brackets were being referred to, but I thought Brians' reference to having MK2B's on both a Europa and Sprint as OEM fitment was talking about servos