As title, is anyone prepared to share their experience of succeeding (or failing) to achieve a good setup with the servo disabled/removed, and same for the abs?
Is there a recognised amount by which the servo multiplies pedal effort? My thinking is that it must be 3 to 4 times.
I seriously would not do it.Â
The ABS on our race car was deleted and we had massive balance issues especially in the wet. It makes the system significantly front bias. I would expect a significantly heavy pedal as well, which you may or may not be happy with but it is easy to forget or underestimate just how much help they give.
Playing with a brake bias valve constantly is no fun and in my view not an effective solution and it has been quite noticeable how we are out braked by ABS equipped cars for many reasons, not least because they have extra confidence in the function.Â
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Thanks for the response. Iâm especially intrigued by your suggestion that removal of the abs unit essentially resulted in a significant front bias. So does that imply that front rear bias is actually controlled within the OE unit?
The problem weâve encountered (someone else drives the car) is that the OE abs is simply not up to the rigours of serious track use. Also, I think Iâm right in saying that Supercup cars have abs disabled, but maybe they retain the unit in the car and therefore still have the benefit of a more balanced front/rear bias setup?
In 1972 I added an in-line servo to my old bitsa Ford, suddenly the car gained brakes to match the excessive amount of go it had. After the meaty engine the servo was by far the best addition.
In 1994 I paid ÂŁ800 extra for optional ABS on my old Astra, it only kicked in on three occasions in 205,000 miles, but in each case it very much more than paid for itself.
The flickering DSC light on my NC told me the six-year-old OE tyres it came with were utter pants. New tyres from NickD and the light has not flickered since, apart from when the DSC held the car on the straight and narrow through a vast diesel spill (and the tailgating old Porsche behind me nearly fishtailed off the road before shrinking to nothing in the mirror).
The DSC on an NC is amazingly good, keep it, cherish it, cosset the wonderful brakes.
Supercup cars do have the ABS disabled but many will have a tandem brake bias valve and the retention of the ABS unit itself aids with reduction of pad knock off. The car, all MX-5âs, tend to not use the rear brakes as effectively as they could. In particular changing grip levels expose this significantly and it in particular the wet conditions that early front lock up occurs. If you are getting it in the dry however, and overworking the ABS, I would suggest potentially over braking and or lack of finess in braking. The lack of ABS exposes itself on track most in the wet when it is getting the balance right to give the best results is a juggle and even different for different corners and ABS cars just donât have that concern.
Removing the servo, I expect, will result is very significant effort, to the point of potentially not being able to apply enough in braking zones. Having raced a car with exceptionally heavy brakes to the point that I was actually twisting in the seat, you get past a point where you can apply modulation simply because you are having to press so hard.
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The ABS system includes a system called EBD (electronic brakeforce distribution), in effect brake bias.
Does EBD remain active even if electrically the abs unit has been disabled?
Unlikely, as EBD is is a function of the ABS system.
JS
I would tend to think the same, at least in any intelligent/variable form, but will there nevertheless still be some brake bias going on? I think that is the inference from a post by NickD above.
Unfortunately youâre confusing brake balance (relative braking effort front/rear), which is fixed by the physical design of the system, with EBD which is an active system which maintains maximum braking on individual wheels especially in slippery conditions. No ABS pump = No EBD.
JS
No, Iâm pretty sure I understand the point perfectly, but was trying to determine whether a âswitched offâ abs unit would nevertheless provide an element of brake proportioning.
To add, on my car, the abs unit has been removed completely, and brake balance without it is âinterestingâ.
Agree with JS46 above comments - and delete the servo at your peril .It has a boost ratio of approx. 3/1, and has a feature called âjump-inâ, which reduces the threshold force necessary to bring the brakes into contact with the discs. In other words, it not only reduces greatly the pedal effort, but improves the initial feel of the system.
It will be terrible without it, and even worse if you use harder (low - mu) racing pads.
After a couple of laps, you wonât have any effort left in either leg!
Never tried it on an MX5, but did so on a racing Sierra many moons ago, and (just) lived to regret it!
Iâm intrigued by your reference to a âjump-inâ feature. Do you have any idea just how it works?
It is in the construction of the vacuum servo. The valve which admits air to the servo when you press the brake pedal incorporates a feedback mechanism to apply force to the pedal pushrod. This consists of a block of fairly hard rubber, which apportions the forces between the servo diaphragm and the pushrod (in fact the area of the 2 surfaces is what gives the boost ration of the servo).
To get the âjump-inâ feature, there is a small gap between the push rod end and the feedback rubber block, which means that the atmospheric air is applied as soon as the pushrod is pressed (from the pedal) without applying any feedback force onto the pedal.
This means that the master cylinder pushrod begins to move, and apply enough hydraulic pressure to take up the small amount of clearance between pads and discs before you get any pedal feedback from the master cylinder.
The amount of âjump-inâ is usually tuned to the system parameters of the vehicle, and can vary according to the manufacturerâs preferences.
If you have ever driven a large, heavy car (limousine -type) then it will typically have more jump-in then a light roadster to give a very light initial pedal feel for the âluxuryâ market.
How do I know this? I used to design brake systems and componentsâŚbut not for Mazda!
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Thank you, thatâs very helpful. If Iâve understood things correctly, what youâre saying is that this is a feature on servo units installed in a very large number of cars, and certainly not a specific feature found just on Mazdas?
Also, from your design experience, by just how much would you expect a brake pad to need to move in the caliper to apply maximum braking force? And Iâm assuming here that pad knockback, even if present, if essentially ignored for the purpose of this discussion.
Yes, it is fairly common, and by no means unique to Mazda.
The question of pad movement is really difficult to be specific - rather like âHow long is a piece of string?â. It depends on the size and type of caliper, and how much load (pressure) being put through it. Once in contact with the disc, the pad itself hardly moves at all - its is the deflection of the caliper (and a small amount of pad compressibility) and system expansion which you are feeling. The initial pad knockback is frequently what you feel at low (road use) loads.
OK?
Yes, I get all that. Not specifically in an MX5 context, but Iâm trying to work out what is the smallest diameter master cylinder I can use in conjunction with a given multi piston caliper. The particular master cylinder range (by AP) has a stroke length of 25mm and clearly I need to allow for a safe amount of caliper piston movement. Does that make sense?
Letâs assume for a moment that you retain the servo, otherwise you will get unacceptably high pedal loads, (or excessive pedal travel with a smaller master) and it is fairly simple to work out the necessary stroke you will need to ensure you donât bottom out the master cylinder, once you know the fluid consumption/deflection of the calipers under. typically, 10 bar pressure. Usually brake pressures are lower than this, but that is a useful maximum for most manual systems.
AP should be ale to supply that data.
Are the calipers 4pot AP Racing types, or single-side reaction calipers? They will have different stiffness. Without that data, you will be guessingâŚ
I assume you will retain the split system (to be legal on the road, and highly recommended even off it!), so you need the stiffness of both front and rear calipers.
A typical master cylinder diameter is 22mm, and the stroke will be of the order of 25mm. Typically, the knockback per piston is of the order of 0.1mm max, but that is itself variable depending on the stiffness (or otherwise) of the stub axles and caliper mountings, and also the amount of lateral âgâ which will be applied. It CAN be a great deal more when used âin angerââŚand the fade will increase the pressures necessary to get the same deceleration.
Add to that the system deflections in the hoses, valves, mountings etc to be safe.
Ideally you donât want to use more than about half/ two-thirds the cylinder travel, since the deflection at the pedal gets uncomfortably high. The extra travel is mainly used for half system failure cases.
It starts to get a bit complex, but you need some data to get started.
Sorry if I ramble onâŚ
Just my free advice - money back if not happy!
Aldi
Correction - in the above reply the 10bar should have read 100bar! Typo⌠: 
Sorry!