Brake Modulation

[Davis]

Would someone please describe what this "brake modulation" is, or direct me to a site with a "usable" explaination? Thanks!

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ITB Rabbit GTI

[apr67]

Modulation is as simple as turing something on or off. Modulating would be how fast you turn it on and off.

So to modulate your brakes, you would bring them to the point of lockup, or sometimes just beyond , and then release some of the pressure, and then start applying pressure again.

[JLawton]

It's like manual ABS........

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Jeff L
#74 ITB GTi

[Davis]

In other words it's a fancy way of saying varying the pedal (pad) pressure.

If that is the case than it gets me to the reason for the original question. That is, I don't understand what brake pad advertisers are trying to tell me when they use phrases such as "easy to modulate and very consistent", "good modulation", "outstanding initial bite with great modulation", "pedal modulation required", etc. Does this relate primarily to ABS systems? How does the ability of a pad to "modulate" apply in a race situation?

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ITB Rabbit GTI

[John Herman]

A pad which is easy to modulate will not lock up easily because you can keep it at its limit easily with slight changes in pedal pressure. If the brakes have good initial bite, it means the brakes will reach their maximum braking limit quickly. The downside is a brake with "good initial bite" can tend to be grabby and over slow the car when you lightly (relative term) touch the brakes. However, if your tracks you run require "brute force" braking, then you probably want to go with the maximum torque availble and can trade-off some of the modulation. If tracks require just a touch of the brakes for high speed corners, you may not want a "high initial bite." So to try and break down the ads, "easy to modulate and very consistent" means the pads will be easy to keep at the limit and they'll release quickly once the pedal pressure is reduced, also they will remain that way over the designed operating temperature range, "good modulation" again means you should be able to easily hold the brakes at their limit, "outstanding initial bite with great modulation" means the pads will reach their peak effectiveness very quickly, and can easily be kept there, "pedal modulation required" this ones confusing as all brakes need modulation when working at the limit, but if used in conjuction with the other pads in the ad, probably means the pad will be harder to modulate.

[924Guy]

Agreed, "modulation required" sounds like a grabby pad to me...

I would characterize (in general) an easy-to-modulate pad as one with fairly linear response characteristics, rather than peaky, non-linear response. Of course, pad temp and other factors can aggravate these conditions.

Another important thing to be mindful of is that linear response for a pad would be a characterization of brake friction - Newtons (N) of brake force as a function of force applied to the pad. When actually installed on a car, this would be expressed as specific brake torque - N-m/bar - the amount of brake torque generated per unit line pressure going into the caliper or slave cyl. No big deal, except to keep in mind that you're putting in force at the pedal, not pressure; you're using the brake master cylinder and pedal linkage to generate the pressure, and non-linearities in that system (or elsewhere) could conceivably frustrate your attempts to achieve a brake system which is nice and linear so as to provide easy modulation.

Poor linkage geometry or odd booster behaviour could be examples of these sources of non-linearity, since the linearity you, as the driver, are looking for is one of decel vs. pedal force input.

Hope that helps rather than confuses...


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Vaughan Scott
Detroit Region #280052
'79 924 #77 ITA/GTS1
www.vaughanscott.com

[apr67]

In my experience, the more easily modulateable the pad, the less overall braking force that the pad can generate.

[CaptainWho]

Originally posted by apr67:
In my experience, the more easily modulateable the pad, the less overall braking force that the pad can generate.

That may be true (I have no idea) but a pad that's easy to modulate is easier to control, so the driver might get more out of it than one that has higher potential but is hard to operate.

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Doug "Lefty" Franklin
NutDriver Racing

[JLawton]

Davis,
If you are looking for pads for your GTi, I use the Hawke Blue. (although we don't need no stinkin brakes ) I've been very happy with them. Work just as well in the first corner of the race (which isn't always the case in pads that need to be hot to work well) as the last corner.

Again, if it's for your GTi it's not as big a deal compared to a AS Mustang for example.

Hope it helps!

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Jeff L
#74 ITB GTi

[Davis]

Thanks for the information. It looks like as a beginner who has shown a tendency to lock the brakes, I should be starting with pads that are "easy to modulate and very consistent". At this stage I have enough other things to learn and concentrate on without having "touchy" brakes.

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ITB Rabbit GTI

[ShelbyRacer]

Originally posted by 924Guy:
Poor linkage geometry or odd booster behaviour could be examples of these sources of non-linearity

Boy, you can say that again.

On my friend's ITA Shelby Charger, we get quite a bit of deflection of the booster. We're thinking of making a "girdle" for it...

Anyone ever done that move before?


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Matt Green
"Ain't nothin' improved about Improved Touring..."

[lobster]

DAVIS, On our old ITB Rabbit we uesd good old Axxis Metal master pads and braided frt hoses worked great for us at Willow Springs, Holtville, Elvis.

Shelby are you sure that you don't have firewall flex this was a problem with some older cars .I.E. old bugs (dropped spacers) and the X or J body GMs? Glenn

[924Guy]

Yeah, I'd be after firewall flex; in fact, I've heard of some with 924's with boosters or clutch pedals that'll start tearing the sheetmetal of the firewall due in part to sticky pedals/linkages. Must be that common VW Rabbit heritage we all share...

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Vaughan Scott
Detroit Region #280052
'79 924 #77 ITA/GTS1
www.vaughanscott.com