Vacuum for brake system.

Competition engines and ancillaries - general discussion
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cantfindausername
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Vacuum for brake system.

Post by cantfindausername » September 22nd, 2006, 8:53 am

Hello all!

Eventually I will be swapping out to a larger inlet manifold, and I have heard things about the vacuum feed for the brake system being affected by this, in that if the brakes are used often, and hard, say for a hill climb or tight track, then you can run out of vacuum and suffer bad loss of braking abilities.

Is this something or similar too anything that you may have heard of before?

Thanks,
Ant

Guy Croft
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Post by Guy Croft » September 22nd, 2006, 9:40 am

Yes it can happen, also affected by cam timing, and anyhow you only get good vacuum when the throttle is closed.

I wouldn't use vacuum assisted brakes front on a competition car, use twin master cylinders and adjustable pedal box from Tilton or AP Racing.

GC

cantfindausername
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Post by cantfindausername » September 22nd, 2006, 9:56 am

How do the twin cylinder systems work compared to a single cylinder system?

I'd love to rip out the existing pedals and install a new pedal box, I always look at those parts in the catalogues. The brakes are something that I will be upgrading later on when the build is nearly complete.

Just need more information on what is a good upgrade.

Most of the high powered MR2 owners still feel that the stock brakes with uprated discs/pads & fluid are good enough. They say if you can still lock your brakes, then they are fine as you can still brake hard enough to break the traction.

Julian
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Post by Julian » September 22nd, 2006, 11:29 am

In order to understand the need for a different braking system you need to understand what you can and cannot do

First and foremost, you can not increase the braking power if you can already lock the wheels up on your car. The limitation on braking power is the traction of the tyres, maximum braking ability occurs just as the wheels start to lose grip but are still turning.

You can fit bigger discs or bigger calipers but you will not improve upon that braking power unless you can increase the grip from the tyres. For competition use this means using slick tyres. Fitting wider tyres will not give you any assistance, if you want a graphic demonstration drive at 60mph in your car and then lock the brakes up (assuming it doesnt have ABS). Try the same in an old classic (example being an old morris minor) and you will find that the stopping distance is pretty much identical. Now try the same in 10 ton truck and once again almost identical results. Variations are largely down to the road surface and the tyre condition.

What you can do is increase the delicacy of the brakes, the longevity of the brakes and the balance of the brakes. All three factors have a large impact on how effective your brakes are.

Longevity is limited by the cooling capacity and the effective head range of the brake pads and hydraulic fluid. Use high quality brake hydaulic fluid changed regularly to avoid boiling problems. The brake pads you use can make a big difference but provided you are buying acceptable quality pads you should be able to at least brake to a halt once without fade. Competition and track day pads are less vulnerable to fade but may require the brakes to be warmed before they work properly, on the road it might take two or three stabs at the pedal to see any real braking capacity which is of course rather dangerous. Larger, ventilated discs can help to prevent overheating which is of course the opposite end of the spectrum.

The feedback and control (delicacy) you have over braking ability is down to a large number of factors, balance is one of them and I will return to that matter later. The others are the size of the disc, number of "pots" in the caliper, relative bores of the master and slave cylinders and the length of travel on the pedal. Another factor worth noting is the steering geometry of the car - normal road settings are designed to make control of the car easy at the cost of other factors one of which is braking ability.

Balance is a personal matter although one can make quite sweeping generalisations about what is good and what is bad. Most production cars have a limiter fitted in the system to prevent the rear wheels locking up - in effect this is a crude balance control. A more advanced solution is to fit an adjustable balance valve which provides control over what the limit is to the back.

In general you move balance to the rear to increase stability under braking so that the rear tries to pull the front. Moving the balance to the front can increase the effectiveness of the brakes but at the cost of stability, this lack of stability can be very useful under racing conditions to increase turn-in ability especially on understeer happy front wheel drive cars. An extreme example of this is the rather lively Fiat Racing Challenge Unos which have a tendency to quickly swap ends at the slightest dab of the brakes but a skilled driver can take advantage of this and achieve seemingly impossible cornering capacity (more so given that the series is limited to road legal tyres).

The most complex solution (that you are likely to see in motoring) to controlling balance is to have dual master cylinders with a balance bar. The arm that actuates the brakes works pretty much as normal the force is distributed by the balance bar which is little more than a lever and a fulcrum. Balance is controlled by moving the fulcrum along the length of the balance bar and hence changing the distribution of force to each of the master cylinders.

You can simply buy a pedal box and fit it as a replacement for your OEM master cylinders but be prepared for some hard graft. The big difficulty is usually the steering column...

Guy Croft
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Post by Guy Croft » September 22nd, 2006, 11:48 am

Model Post!

Well done Julian, a superb appraisal.

GC

cantfindausername
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Post by cantfindausername » September 22nd, 2006, 11:50 am

Great post, and very informative!!

When I do address the brakes I want it to be done properly. I will be weighing the car, getting the distribution figures etc to try and get as accurate balance as possible.

Alot of people have gone for the off the shelf big brake kits, and said they are no good basically. So I would want a functional upgrade, rather than looks.

Julian
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Post by Julian » September 22nd, 2006, 1:27 pm

Big brake kits are primarily for show - lets face it, when you've put huge 19 inch rims on instead of the weedy 14 inch rims the car started with those brake discs look rather lost in the middle.

There are unacceptable reasons for those big discs

1) Even if you use the original calipers you are increasing the swept area which helps to give more control

2) Using big calipers as well increases the swept area further - yet more control and consistency

3) The cooling capacity of the larger discs is better when compared to a smaller disc - this is a double edged sword though. More cooling is great for ordinary road pads but can be very detrimental if you are using "hot" pads

The downsides are also interesting:

1) as already mentioned you can over cool the brakes, most brake upgrades actually reduce the performance of the brakes because the selection of discs, pads and calipers are poorly matched to the environment

2) the alignment of the discs is even more critical, a small error in alignment is amplified as disc size is increased - any knock-back on the pads can make the pedal response very poor indeed

Warping of discs is another problem area and far more complex than most would give credit. Contrary to popular opinion the vast majority of disc warp is not down to simple overheating - most manufacturers treat the discs to prevent this sort of thing.

The most common cause is contamination of the metal due to a reaction between the brake pads and the discs. When hot the pads can cause a change in the crystalline structure of the metal in the disc making it much, much harder than normal. The immediate effect is not noticible but the brake pads quickly scrub the softer metal away leaving very hard high points.

You can have the discs skimmed but the hard points remain so the "warping" remains.

The solution is to do as advised and avoid leaving the brakes on when the discs and pads are hot.

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