Has anyone tried?......

[gav]

Before I changed the rear calipers to Sierra sliders, I ran a tandem master cylinder (the one from London taxis) and a dual circuit servo. The front calipers are Caterham 4 pots (1 1/2 inch bores).This was a great set up. I made the change because I live on a hill and the Chapman hadbrake set up was hopeless.
The discs and pads are recent (and well bedded in) so I dont think there is a problem with there.
I think that I need to go down a master cylinder size at the front from 0.7 to 0.625 (which does surprise me - but going larger makes the pedal feel even more wooden.) I am looking for more pedal feel and bite and suspect that I may need to sacrifice some pedal travel to increase the feel which is why I suspect that I need separate cylinders.
I was toying with adding a servo to the fronts only but this doesn't feel right to me and i suspect that it will add a tier of confusion that I don't need.
My thoughts are to add a unit that links to the existing pedal output : - a belcrank links the existing pedal to a balance bar in the engine bay. This is an additional unit that is essentailly independant ofthe existing pedal arrangement. Wilwood do a balance bar assembly (340 4630 ) which looks promising. I had a quick measure up and with short AP calipers looks like it should fit OK.
Interested in thoughts
Thanks
Gavin

[jeff jackson]

Hi Gavin,
This may be a bit off topic, but you mentioned Sierra rear sliders?
How did you do this?
I am keen to try a different set up for the rear brakes on my +2.
Kind regards
Jeff

[gav]

Jeff

I am running Spyder dual wishbones on the rear instead of the Chapman strut (their RSC kit) and I asked them to weld on the brackets for the Sierra conversion that they do.
It uses the original brake disc, hub etc and I'm still on spinners.
They did a great job and the only problem I have now is the balance and hydraulics (which is of my making - as usual)

Hope this helps
Gavin
I tried to upload a photo. If it doesnt come out, drop me an email address and I'll send it to you

[gav]

[JonnyPlus2]

Hi Gav;

The basic problem with the Elan's brakes is that the front brakes are not really up to the job which in turn upsets the brake balance and causes the rear wheels to lock up. Instead of altering the brake balance, I suggest you fit the bigger front brakes off an Elan Plus 2.

I was just driving my mate Robert's Elan on Sunday (very tasty with a dry sump Cosworth built Twink) for the purpose of experiencing his brakes first foot. They really were brilliant ! They really felt linear. Push twice as hard and it stopped twice as fast (or so it seemed). There was absolutely NO rear lockup no matter how hard I stopped and it really stopped. Like a modern car with *good* brakes. The pedal was firm but not heavy. The car is running 205/60 Toyo 888s on 6" minilites so I was unable to lock the fronts.

Robert's set up uses standard Elan calipers on the rear and Plus 2 calipers on the front and a standard single .7" master cylinder. Sourcing the bits over there should be a piece of cake (unlike this side of the pond). I would think that the big caliper conversion is a lot easier to do than installing the bias doohickey and it's all a bolt on job. And you'll have better brakes than the standard setup. Seems like a no brainer to this cowboy.

Regards,
Jon

[gav]

Hi Jon

Thanks for the thoughts.

I have already followed this route but may have gone a litttle far - I am running Caterham 4 pots at the front which have 1 1/2 " pistons in each pot.

Even with these, the rears are more powerful than the fronts - surprised me. I was hoping that fitting a rear limiter would push the bias towards the front. It seems to make no difference to the front - it merely reduces the pressure to the rear (which is what it is supposed to do). The net effect is to reduce the overall braking capacity.

I have one circuit at present being controlled by an AP racing 0.7 master. The 0.75 master that I tried gave a really wooden pedal and I am wondering what would happen if I reduced it to 0.625.

I suspect that I ultimately need a 0.625 on the front and a 0.7 on the rear but need to do more homework on how split systems with a balance bar are calculated.

I think I have worked out a way of creating a bias system without hacking the pedals to pieces.

I'd be interested in thoughts on this approach.

Thanks
Gavin

[JonnyPlus2]

Hi Gavin,

Your result is counter to my understanding of how calipers work. Given the fat increase in piston area you should be able to lock those front wheels no problemo. So either my understanding of the theory is wrong (I was wrong once . . . I thought I had made a mistake ) or something is not right in your braking system.

In general, for any pad material, braking power has everything to do with the force generated by the caliper and *not* the pad size. The force produced at the caliper is proportional to the piston area and the piston area is proportional to the square of the diameter. So compared to Rob's setup (+2 calipers have 2" pistons) you should have something like 4 * (1.5) squared divided by 2 * 2 squared the force at the front which works out to 4*2.25/2*4=1.125 the force at the front calipers. So you have a 12.5 % increase in the force on the front calipers compared to +2 front calipers and a pretty huge increase compared to standard Elan fronts. This should enable you to stand an Elan on it's nose or lock the fronts depending on how grippy your front tires are.

If I'm wrong about how calipers work, please, kindly correct me. I'm not addicted to being right, I am pretty addicted to finding the right answer.

I'm trying to decide what could make your car behave this way. Do you have really hard pads on the front and pads with great initial bite on the rear ?

Sorry for gassing on. I hope this helps. Please keep updating, I'm very interested in this problem.

Best,
Jon

[gav]

Jon

I'm as flummoxed as you - hence my diatribe on the tech. section.

The pad material is the same front and back - DS 2500 which is anti fade but doesn't need warming up. It came highly recommended but it could be that it is a little hard.

I hate having to stand on the pedal to get full braking and what I really want is a softer but more effective push without too much pedal travel.

When I took the car for its MOT, we checked the braking and noted that the rears had considerably more effect than the fronts and deduced that it was this that was causing the back end to go light on heavy braking in a straight line. This is why I fitted the limiter valve (and not at all having something else in the cockpit to fiddle with!)

I don't really want to run a servo and would prefer to get this sorted out properly (and vaguely scientifically). The system is pretty new and holding fluid well so I don't think it is a problem with installation but rather the combination of components working well together.

I do like a brain teaser but am getting a little weary on this one.

Thanks for the interest
Gavin

[AHM]

Gav,

You need to do the maths as Jon has indicated.... but don't forget the rears!

What is the piston area of your sierra rears? if it is increased over standard you have moved the bias to the rear.

[gav]

The Sierra calipers are hard to measure on the car but it looks like around 45mm. They only have one piston, the other side is actuated off sliders.

I was told that one way to calculate it is comparing the relative surface areas - but on one side of the calipers only.
This would mean that using good old pi r squared, the rears are 1590 and the fronts are 2x (37.5/2 squared x pi) which I get to 2209.

This would explain why the rears are so snatchy - there is too much capacity there. So with the limiting valve, I can tune the rears out but I still can't lock up the brakes.

One thought I had was that I might need more pedal effort (by changing the fulcrum point of the pedal to the master cylinder via actuating rod) but with the standard set up, there isn't the opportunity to move the pivot point without surgery and this would result in the master cylinder being actuated by a rod whose effort is being applied from an angle rather than flat. This will wear out the master cylinder and I can't believe is good engineering.
The other solution to this is to extend the pedal but the height is perfect for me.

The other solution that I was toying with is to reduce the bore of the master cylinder but this seems counter intuitive.

More thoughts gratefully received.

Thanks
Gavin

[AHM]

Gav,

I understood from your earlier post that the fronts are 4 pot.

Don't forget the disk diameter.

You should also compare to standard so that you know what ballpark you are aiming for.

Do you have a limiting valve or a proportioning valve?

I suggest that you go through it a bit more logically - It could result in you hurting yourself or someone else.

[rgh0]

Hi Gavin

The braking torque applied by a disk brake is proportional to

1. The calliper piston area ( bigger gives more )
2. The hydraulic pressure ( higher gives more braking) this is in turn governed by pedal effort ( higher push by your foot gives more hydraulic pressure), pedal leverage ( larger lever gives high pressure for same effort) and master cylinder diameter ( smaller giver higher pressure for same effort)
3. The coefficient of friction of the pads on the disks ( higher gives more)
4. The effective radius of pads on the disk (larger gives more)

Ignoring what is going on at the back of the car for the moment you appear to be unable to get the fronts to lock with what you consider an acceptable pedal effort.

You have a larger than normal piston area and probably cant go larger in practice so cant change that

You have good quality and high coefficient pads in DS2500. You need to ensure they are bedded in properly and that the disks and pads are not glazed or contaminated in any way. I suspect this may be your main problem as with the big brakes you will have only be very lightly loaded on the pads and if you never use them heavily they may never have bedded in properly and they will be glazed and hard.
You could explore other compounds but I don't think they will help to much but it may be worthwhile talking to a brake shop that specialises in racing brakes and has decent knowledge to see if they have any alternative suggestions and get them to have a look at your current disks and pads condition.

You cant change the effective radius without a totally new disk and wheel and pad combination so you probably don't want to change that.

This leaves you with the only remaining option and that is to increase the hydraulic pressure without increasing your pedal effort.

Three ways to do that
1. increase the lever arm on the brake pedal to apply more force on the master cylinder piston for the same effort on the pedal and this increase the hydraulic pressure in the system. Not a lot of room to do that effectively in the Elan and it also increases the pedal travel.
2. Install a booster on the system. relatively easy to do and was standard fitment of plus 2s and later Elans so it has an element of originality if that matters. Remote boosters also typically increase pedal travel and can make the pedal feel and response to brake pressure modulation harder to do.
3. Install a SMALLER master cylinder. The smaller piston area means that the same force on the pedal will generate a higher hydraulic pressure in the system and thus increase braking torque. The downside is that you will more pedal travel as further master cylinder linear movement is required to displace the amount of fluid needed to move the calliper pistons into contact with the disks.

regards
Rohan

[JonnyPlus2]

Hi Gav,

I think that the problem is that the Sierra's sliding caliper design puts out twice the clamping force that you would think, due to Newton's 3rd Law. (The equal and opposite one) Thus the Sierra calipers clamp like a fixed caliper that has two 45 mm pistons. This will give the car (45/35) squared = 65 % more or, way to much rear brake bias. Which is just what you experienced. I think you have to chuck the Sierra calipers and get some with the standard Elan piston size of 35mm.

HiSpec makes a nice caliper with the same size pistons as the standard Lotus rear calipers: http://www.hispecbrake.co.uk/ULSIESVA.html
According to a poster on the lotuselan.net forums, HiSpec will supply them with the handbrake bits mounted the same way as the Elan's. Or you could go to a hydraulic handbrake like the rally boys and use your standard rear calipers.

The good news is that I had sliding rear calipers on my Impala SS and they were a pain in the posterior. Couldn't even get through 1 Canadian winter without the piston or the sliding dohickey or both seizing up. So you won't miss them when they're gone.

Hope this helps.

Best,
Jon