Lotus Elan and Lotus Elan Plus 2 Forums

Afternoon Lads!

My Elan coupe has Plus 2 front calipers and standard rears. I'm contemplating my master cylinder setup and there are three choices: 1) Single 2) Tandem 3) Dual.

For performance I would choose a single every time as my chum Rob's car with a .7 master and the same brake setup has killer brakes and a tolerable pedal effort. Has anyone ever heard of total M/C failure on an Elan? I will have the M/C sleeved in stainless as it's dirt cheap insurance.

My issue with tandem or dual M/Cs is the high pedal effort although that could be reduced by moving the attachment point up to increase the leverage on the M/Cs. Other than the high pedal effort I think that dual or tandem M/Cs are the way to go as they're also dirt cheap insurance.

As always, your opinions, advice, jokes etc are eagerly anticipated.

Jonny,

As you likely know, pedal effort is un related to the configuration of the M/C (single, tandem, dual) but rather is a function of the combined hydraulic and mechanical advantages. If a single .70 diameter M/C gives the right effort/feel then look for a tandem unit with a .70 bore.

Some Japanese cars of the late 70's and early 80's used tandem M/C that are configured very similar to period Girling M/Cs. I've used these to good effect on Lotus and Jaguars over the years. Can't recall if I've seen a .70 bore, but certainly .75" bores (and larger) are available.

I've attached a couple of photos of one of these tandem M/Cs on my XKE. Sorry I don't have any of a Lotus installation.

One comment on Lotus Elan fitment, is that LHD cars will likely need some clearancing of the wheel arch due to the length of the tandem M/C. On my RHD Elan, there were no issues.

Just found this thread which also has some info:

elan-racing-f12/dual-circuit-brake-master-cylinder-t17529.html

And finally, a better photo of a tandem M/C from a Datsun application:

Having experienced near-total MC failure on a tandem system a long way from home on a holiday weekend, I'm firmly in the Dual or Tandem camp. The weakest link of the MC is the bore, and if it's scored or full of crud (as mine was) can affect both circuits at once. So dual is the safest, and has the option of adjustable balance as well, with the price of getting the balance right.

I find the idea of a single system on a high-performance car on the road more than a bit frightening. Outlawed on new cars in the U.S. since 1966, and while I'm not pro-regulation, that was a good one.

Note that increasing the bore size increases the pedal effort, not the other way 'round. Think of it as a fixed amount of leverage pushing a lot more fluid down the pipe(s) at once. The smaller bore requires less effort because it isn't moving as much fluid per inch of stroke. But you need to push it further to get the same amount of pad motion.

But isnt a tandem cylinder like having a pair of side by side ones (usually with a balance bar) in terms of pedal loading?

Effectively you are pushing two pistons with double the surface area of a single one, so by my reasoning if you had a 0.7" dia bore single piston cylinder using a tandem one would require a bores size of as near as dammit 0.5" to retain the same pedal feel and travel.

The above is based on a diagonal split system because the front calipers having a larger area than the rears would I believe imbalance things.

Or have I completely misunderstood something?

I know when i built my first bias pedal box I quickly realised that I should have revised my hydraulic theory. After doing the calcs I ended up with smaller bore sizes than the original and I even had two different sizes to allow a front braking bias with the balance bar in the mid position.

Chancer wrote:But isnt a tandem cylinder like having a pair of side by side ones (usually with a balance bar) in terms of pedal loading?

Effectively you are pushing two pistons with double the surface area of a single one, so by my reasoning if you had a 0.7" dia bore single piston cylinder using a tandem one would require a bores size of as near as dammit 0.5" to retain the same pedal feel and travel.

Or have I completely misunderstood something?

One of us is completely misunderstanding it.

I belive the force from the peddle generates pressure in the first chamber and this is transmitted to brake slave cylinders and the back of the second master cylinder piston.

The second chamber acts on the other brake slave cylinders, the bores of the two master cylinders can be stepped to give a equal or off set out put, i.e. un-stepped for about 60/40 stepped to give 50/50.

You only generate one pressure the rest is transmuted by the second piston to work done, the form of more travel.

I hope this is right, or I have had the wrong end of the stick for more then forty years.

Jon the Chief

That makes complete sense Chief John, it also gives a failsafe operation if one seal fails.

Once apon a time I would have known but with the passage of time...............................

What I wrote is correct with regard to side by side cylinders and a balance bar.

There is also a thing called a PDWA (pressure differential warning activator) which is located between the pistons on many tandem cylinders, its purpose is to light up the brake warning light if there is a failure in one circuit, on my Stag it was remote unit with all the brake lines running into it, when the light came on during a long heavy braking I decided that the unit was faulty so promptly replumbed the system to do away with it, later when I started studying and understanding things better I realised what a muppet I had been and that I had probably later sold the vehicle with a faulty master cylinder and the brake warning light system disabled for all but the low fluid function.

I was only 21, at that age I thought I knew everything, now I realise just how little "everything" was!

Like your Stag, the federal plus 2's and plus 2S's have the remote PDWA along with the tandem master cylinder. Not sure why the +2's were deemed to have needed it while the two seaters went without. It was a US DOT requirement, I guess, so logic may have not been involved.

One like this?



These seemed to have a fashion in the late 70's/80's they then appear to be replaced by level float switches.

I don't know what the history of this was, but I bet it was manufactures chipping away at regs for penny pinching reasons.

Jon the Chief

Yes just like that if I can trust my memory after 35 years!

Not sure that they were replaced (in the true sense of the word) by a float level switch, I assume you mean in the reservoir, as the PDWA is a spring loaded shuttle valve that is only actuated when there is a loss of pressure in one brake circuit or a significant imbalance between them as when bleeding.

Most later tandem master cylinders that I have seen have a swich connection between the two pistons, usually underneath, which I believe to be the same function, maybe it has since been done away with.