The cheapest rear-brake upgrade you’ll ever see.
Well, this is something I have been thinking about for a long time (and even put some of my thoughts into other threads where I thought them to be relevant)…..
…but now I’ve actually done something about it, and time will tell if I consider it to be worthwhile (from an ‘improvement’ point of view) – or at least an alternative to Lotus parts which are proportionally quite expensive.
The desire for doing this was based on a number of factors, mainly to get the brakes as good as possible (with standard callipers), whilst still keeping the ‘correct’ front/rear balance.
I’ll start with the usual disclaimer – I AM NOT AN ENGINEER. But I am a fairly competent home mechanic, with qualifications in Physics, Metallurgy, Mathematics, Vehicle Examination, Forensic Examination (and others which are irrelevant to this thread) – and I can read / research things on t’internet!
Next, the usual theoretical statements (which are actually facts, not theory)…
1) Brakes are working at their optimum retardation just before the wheels lock up (so we don’t want the wheels to lock, just nearly lock).
2) On a dry road, the stopping distance is actually minimised if you DO lock the wheels – but there is no steering/directional control (Blame Isaac Newton, not me!).
3) Braking efficiency is not directly correspondent to retardation (you need to factor in tyre/road friction, aerodynamics, weight, energy dissipation and all sorts of other things) – as such, there is no such thing as a single optimum brake set-up for all situations.
4) Braking at high speed throws more weight onto the front axle than braking at slower speed.
It has long been my opinion that the brake callipers as fitted to the elan when it left the factory are still good enough for the job – as long as the whole brake system is maintained properly, and good tyres are fitted to the car.
That said, a little ‘tweaking’ of other components can be beneficial to the whole thing….
I have already swapped the master cylinder for a slightly smaller bore to give a longer pedal travel, with more ‘feel’ – this (theoretically) doesn’t produce more pressure in the brake system for any given movement of the pedal, but Mechanical Advantage of the lever should make the same pressure achievable with slightly less effort – ie, in an emergency brake situation…. And as such, more total pressure can be generated in the system with just this one ‘swap’ (see thread here).... a singular improvement for the whole system!
On a stationary car, it makes perfect sense to have 100% of the brake pressure acting on all 4 wheels at the same time – and fluid dynamics dictate that liquid (brake fluid) is non-compressible, so the pressure will be the same in the master cylinder as it is at the front callipers, as it is at the rear ones.
However, once the car starts to move (forwards), weight is thrown to the rear axle, and once you slow down, weight is transferred to the front – the harder you brake, the more weight is transferred to the front wheels – and the better the front brakes are, the more the weight will shift forwards..
Lotus (as well as other manufacturers) counter the physics with a variety of systems – starting with the obvious ones, fitting bigger brakes at the front than the rear, then fitting limiting/proportioning valves to the rear wheels.
During the production of the elan, 3 various combinations were fitted, all of which were intended to stop the rear wheels from locking up under heavy braking (and thus stopping you from spinning out of control if travelling in anything other than a perfectly straight line) – the system was gradually simplified until we got to the 2/15 proportioning valves fitted in line to each rear calliper.
Now, this system isn’t perfect, but it’s perfectly acceptable for 99.9% of the time – but if the valves go wrong, they are not cheap to replace (from Lotus), and there is no obvious simple swap available from any other manufacturer (with the same specifications) – unless YOU can find one, but a few of us have been looking for a while, both here and abroad…. The obvious limitation to US sourced ones is the brake pipe fitting – but that isn’t insurmountable, if the values work out right.
Also, the pressure to the calliper doesn’t equally relate to rate of retardation, and an ‘improvement’ to the brakes (bigger discs, different callipers, different pads), either front, rear or both, will have an effect on the desired split between front/rear pressures – as will temperature of the pads, and grip between tyre/road.
On a wet road, you should have more pressure to the front than on a dry road – but for the 99.9% situation we are considering, then the OE stuff is okay.
Remember, we are trying to get all 4 wheels nearly locking up, but not quite, under all circumstances – it can’t be done without very expensive kit like you’d get on a top-of-the-range supercar.
Anyway, all of the above has been well documented elsewhere on the forum – together with various theory on how to maintain the correct 70/30 split when upgrading the front brakes.
Most of the ‘solutions’ seem to involve increasing the size of the rear brakes to compensate – but I’m looking at it from a different point of view – and if my theory is proven to be correct, then no-one will ever have to purchase a rear brake (calliper/disc) ‘upgrade’ again.
Now, as many of you are aware, I have been doing a bit of work on the suspension & brakes for the last 9 months or so, and last week I took the car in for its first MOT test in over 2 years – bearing in mind that everything at the front was ‘new’, I just stuck a set of new pads in the rear callipers, new discs, and freed off the top slider in each calliper that had seized (not unexpected, and part of my usual routine anyway).
The car failed the MOT on ‘service brake efficiency’ – it registered 48%, not the minimum 50% required – I put it down to a combination of factors (air still in the system from the master cylinder swap/new pads not bedded in at the rear/front pads not warmed up) – until I saw the brake test figures which showed the RNS brake at 53 kgf, ROS at 49kgf (low by many standards – but fairly equal!). The front was 233 & 252 (a tad lower than I would like, but as the car had only been 2 miles in a year, not disastrous – a bit of a clean and it’ll improve)…..
….what surprised me (and prompted this whole exercise) was the handbrake figures – 143/135 respectively – and with the weight registered on the rollers, it came in at 21%!!!!!
The MOT pass standard is 16%, and I’ve only ever managed to get 18% at best, and usually 17% in the past – so much for my theory that it may be the new pads in the rear – they obviously work well enough to get 3 times the grip on the handbrake than the footbrake, with all other variables removed, before hitting the brake roller ‘slip point’.
Looking back at old MOT sheets, my rear brakes aren’t that far off what they’ve ever been, but the front were a bit lower, so the overall efficiency failed to meet the 50% requirement….
To pass the test, I had 3 options –
1) Reduce the weight of the vehicle by 192Kgs (emptying the boot and getting the lighter mechanic to operate the brakes would give me about 60Kgs!).
2) Clean/bleed the brakes to where they ought to be.
3) Increase the hydraulic braking at the rear wheels to nearer the figures that the handbrake say can be achieved with the current discs/pads.
Then the tester said something which made me think – “The back brakes are shite, get rid of the prop valve and it’d pass easily” – Now, I have known Ian for over 30 years, and we have either been rivals or collaborators in numerous Competition Car building/racing events over that time, and I knew that whilst he wasn’t serious about getting rid of the only thing that keeps the car pointing the right way under heavy braking, he did have a point……
….if you remove the proportioning valves, and replace them with a bit of copper pipe, there will be 100% pressure to the rear callipers and they’d then give a roller reading at/above the handbrake figures – giving an easy pass!
Additionally, the brakes would be a tad ‘sharper’ as there would be no restriction in the pipe diameter where it passes through the valves – fluid dynamics dictate that although the pressure in the system will always be constant, the flow rate (speed) of the fluid moving will vary with the diameter of the pipe.
This is all great on a static car with no weight shift (ie, for the MOT test) – but would be downright dangerous out on the road (or track!).
However, by changing the proportioning valve to another one, with suitable characteristics, we could have the best of everything – which brought me back to where I started – with nowt available off the shelf....... or so I thought.
As we were discussing it (and agreed that the only way to do it properly is to get the brakes working efficiently by bleeding, bleeding and bleeding again!), the idea of an adjustable ‘bias valve’ was brought up.
These are a simple system which is fitted to completion cars with a single master cylinder – they just reduce the pressure to the rear brakes (usually a single line which then separates to the 2 rear callipers/drums) to keep it all in a straight line – however, they are adjustable via a knob or lever to give the driver control over the bias as the race goes on and brakes get hot, tyres wear, and weather changes – as such, they are plumbed into the system inside the car in reach of the driver.
If I could fit one in each side (to replace the proportioning valves), it could work…..
A bit of Googling showed that many are available – with a variety of ‘values’ to choose from – the ones I went for were from my old friends at McGill Motorsports, as it was the closest to the OE values, and uses an M10 thread. (these ones)
It has a 0.3 pressure reduction from an adjustable ‘knee point’ of anything between 180psi and full pressure (so, the lowest point at which it can be set corresponds to 180psi, or ~12.4bar, and then works at 0.3 x inlet pressure at anything above that point).
There is a Wilwood version which adjusts down to 100psi (~7bar), but as an American company, they are usually an imperial thread, which wouldn’t be such an easy swap in the UK – they do list a version with M10 threads, but I couldn’t find them easily available over here.
I plotted the range on a graph, and compared it to the OE values of all 3 systems used by Lotus:-
1 – 1st system used by Lotus, with 0.3:1 proportioning valve & 65bar limiter.
2 – 2nd system used by Lotus, with 0.3:1 proportioning valve & 70bar proportioning (0.1:1) valve.
3 – 3rd system used by Lotus, with 0.2:1 proportioning valve.
4 – 1st system used by me, 0.3:1 proportioning valve at lowest setting of 180psi.
5 – theoretical system for someone with upgraded front brakes - 0.3:1 proportioning valve at higher setting of 265psi.
The black box is the area where maximum pressure is exerted during harsh (emergency) braking (~1000-1200psi, or 68-80bar) – as you can see, all of the systems are in the same ball park at this point.
There are a number of advantages to the setup, and also a couple of possible ‘issues’ which I have hopefully overcome…..
First thing to consider is that this ONLY adjusts the pressure in the pipes, and has NO bearing on the friction between pads and discs.
However, if you have upgraded the front brakes, and want to restore maximum braking effect to the rear wheels, this will allow you to do just that – and then adjust it again for track use if it is raining (but unless you’re doing serious track stuff, it probably will never be touched).
It will allow you to easily pass the MOT test – just wind up the pressure for the test, and put it back down again afterwards (I DO NOT RECOMMEND DOING THIS JUST TO GET A ‘PASS’ - - FIX YOUR BLOODY BRAKES PROPERLY!).
It can allow ‘fine tuning’ OS/NS brakes to give the same braking effort – again, it is no substitute for having them working properly to start with, and shouldn’t be a way to overcome a ‘sticky’ calliper/slider.
Possible ‘negatives’ are that the units will ‘self-adjust’ with vibration during normal use, maybe unnoticed until you need to do an emergency stop. This is unlikely, due to the friction on the threaded knob, but once I’m totally happy with the setting, I’ll probably use some locking wire to stop it moving.
The only way to see how it all works with my setup (only ‘upgrade’ being better front pads) is to try it out….
… so this week, it’s off to the local airfield with a stopwatch and measuring tape - nowt to hit when it slides!!
Once I have it working at the setting I want for maximum braking, it’s off to the MOT station to see what values I get at the rear wheels! – then future adjustments can be done on the rollers using those values as a ‘start point’.
All-in cost, under £70 for the 2 valves, and 2 short brake pipes to fit them (didn’t actually cost me anything, I had a roll in stock).
My car failed on weak OS rear brake this morning, I'm trying to decide between cleaning out my valve or the upgrade 
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Japs nick most ideas but they do have a very nasty habit of getting them to work - reliably too! One of the best attributes of the M100 is the relative reliability of that big lumpy thing under the bonnet which was sourced from the Japanese wing of GM, damn it!
