Brake System design and IVA

[magpies]

what does the IVA say about their testing of the braking system?

I presume they take into consideration the front/rear weight and then brake test? then as long as the results fit within a designated range all will pass (rears never to lock before fronts etc)- but is that true?

I have seen the same type of car with very differing brake disc / caliper /servo / master cylinder / rear brake limiter set ups.

[CrutyRammers]

Read all about it here:
https://www.gov.uk/government/publications/iva-man...

But basically, your assumption is correct, yes. It must function within the specified ranges, and be well constructed and safe. How it achieves that isn't specified.

[Steve_D]

The tester will inspect what you have and do the brake test on the rollers.
The test is not simply stamping on the brakes it involve barke forces measured at different pedal pressures etc. The tester will fill out a whole A4 page of data which is then added to measurements of CofG relative to axles etc.
He will then go away and punch all of that into a spreadsheet.

If the results are marginal for front/rear balance you may well have to do a brake test outside to confirm the backs do not lock first.

If you have a brake balance bar you can make adjustments but you will then have to wire lock the bar before it will pass.

Steve

[magpies]

Hi Steve

thanks for the info...all much as I thought except how do they measure C of G?

Presume this will be about the weight transfer during braking?

[cerberatony]

It seems that my car might not get in the rollers as it is 62mm sump to floor. The IVA inspector says that if it does not go in then he will have to use a deselerometer probably spelt wrong

[Steve_D]

magpies said:

Hi Steve

thanks for the info...all much as I thought except how do they measure C of G?

Presume this will be about the weight transfer during braking?

I have no idea how the spreadsheet works but do know they measure front of car to centre of axle then they measure front of, and height of the engine crankshaft.

Steve

[AcC8braman]

My car failed twice due to rear locking before fronts.

Fitted a proportional valve to balance off but needed to be permanently locked off before passing

[magpies]

How do you work out which servo to use?

as an example the TVRs built in the 2000's use the Ford Ka and the TVR S in the late 80s/early 90's used the Ford XR2i. neither of those can be directly related to each other, except for probably weight!

[Frankthered]

magpies said:

Hi Steve

thanks for the info...all much as I thought except how do they measure C of G?

Presume this will be about the weight transfer during braking?

My Minari failed IVA on brake balance a couple of times, so this is a sensitive subject. I have the fail sheet - I'll upload it if I can figure how!

From looking at the sheet, it seems that the machine has a means to measure the axle weights both statically and dynamically during the test. (They don't give you any dynamic figures though.)

The static weights give the location of the C of G fore and aft and the dynamic values will allow calculation of its height.

During the test itself, they measure the braking forces and weight distribution at each wheel and express these both as ratios of the combined rears divided by the combined fronts.

The "Weight distribution ratio" must always be greater than the "Brake distribution ratio".

The test is carried out including the weight of the driver. My car failed because the numpty doing the test added a proportion of his weight to the front axle, but failed to add any of his weight to the rear axle.

Oops, but guess who still had to pay for the retest!!!

[anonymous-user]

How does all that^^^^ work out if you have ABS on the car??

[CrutyRammers]

It has to comply with all the above, plus:

IVA Manual said:

". If the vehicle is fitted with an anti-lock braking system, all
components must be present, undamaged, secure, connected and
operational so that the system is likely to function as intended."

...and a note to say that the tester needs to drive it and confirm that it works.

[Steve_D]

CrutyRammers said:

It has to comply with all the above, plus:

IVA Manual said:

". If the vehicle is fitted with an anti-lock braking system, all
components must be present, undamaged, secure, connected and
operational so that the system is likely to function as intended."

...and a note to say that the tester needs to drive it and confirm that it works.

Also says he does not need to drive it if the ABS light comes on during the speedo test indicating it has detected the mismatch of wheel speed.

Steve

[anonymous-user]

I was just wondering, as most brake test rollers are 2wd only, a lot of ABS systems will go into suspend mode when brake tested (because only one pair of wheels will be turning) and hence the "static" brake bias of the system will be prevalent. On modern ABS systems, the static rear bias is generally very high, to maximise stopping distances, relying on the ABS system operating to modulate that bias dynamically. That is why modern cars haven't got mechanical bias valves etc, and also why generally speaking, one should be a bit careful driving one if the ABS system warning lamp is illuminated!

[magpies]

so.....how do I size the servo for my car?

950kg, 51/49 front to rear weight, front engine rear drive, 295mm discs front with AP4 pot calipers, 260 discs and Sierra rear calipers (basically the TVR Griff 400 system minus the -not available- servo and master cylinder.

[Alan Whitaker]

Hi Mick
I have that to face at mot time, I went for the LS400 brake booster as I have the pedal set I thought it fit without any messing about (straight bolt on and go), I have wilwood Integers on the front and super lights on the rears with VX220 callipers just for the hand brake, I hope it all works, I have a wilwood balance valve and a hydraulic handbrake in just for the track days, I will have a brake test done at a friends garage and go from there,
With the turbo's I have put in a Jag X type electric vacuum valve, runs all the time the ignition is on.
I think you will have to get one from the ford range and see how it works out

Alan

[magpies]

Hi Alan - nice to see you on a different forum
I've gone for the Ford Ka one as the later TVRs - including the correct bore master cylinder (same as the original TVR)
BUT - I was wondering how TVR worked out that a servo / master cylinder set up from a small front wheel drive car is correct for a rear wheel drive sports car!

Mick

[Steve_D]

magpies said:

....BUT - I was wondering how TVR worked out that a servo / master cylinder set up from a small front wheel drive car is correct for a rear wheel drive sports car! Mick

The braking effort is determined by the size of the master cylinder and the calipers.
The feel of the brakes is determined by the servo...on that basis I presume TVR experimented until they arrived at the right pedal effort.

In your case you first need to determine if the master cylinder size is correct for the calipers you are using. Only then can you start to play with servo size.

An Ultima, as an example, has no servo. The braking effort required is higher than that of your normal tintop but the feel is better.

Steve

[Alan Whitaker]

Hi Mick.
Knowing TVR I bet they got something from a car of similar weight, or they just went on price.
You could give Rally Design a call, Allways give good advise.

Alan

Edited by Alan Whitaker on Sunday 2nd February 18:43