I am getting a lot of ABS pulsing when breaking very mildly on ridged roads.
On a series of sharp rebounds , the brakes can feel like they are working on ice.
Roads are dry but cold which doesn't helps things at the moment - this nearly resulted in a rear end prang yesterday.
The brakes are superb 95% of the time, but the trouble is when pulling up to junctions with bad surfacing - a scenario I guess Prodrive didn't road test perhaps?
I'm glad I didn't choose 18" wheel options as the skipping would be worse with lower profile tyres.

I understand that the WRX STi has upgraded "sport" ABS which makes allowance for higher braking force with Brembos.
Is there anything I can do on my car to de-sensitise my ABS, perhaps by swapping to the STi ABS module (pricey?) or is there a tweak that can be done by a specialised company?


Nick
MY04 WRX Wagon PPP
Prodrive brakes

 

What you are experiencing is one the negative effects of a brake upgrade with too much front torque. In super-short, over simplified summary, excess torque does not interact well with ABS. ABS is getting confusing signals, is always one step behind and the result is that you actually worsen brake performance. Unfortunately, there are several big-brake systems available today which pay no regard to the original pressure-torque or pressure-volume relationships of the original vehicle…and in fact many make it a point to affect drastic changes in these relationships in order to give the consumer that feeling of 'increased bite.' While the upside is certainly a firmer pedal and higher partial-braking deceleration for the same pedal force, the trade-off can be ABS confusion.

The whole story is available here if you are interested: http://www.stoptech.com/whitepapers/...ake_122701.htm

I'm afraid that this information does not solve your problem, but at least can see exactly what is going on with your brakes.

Good luck.

 

Thanks for the info..
so If I read it right, the WRX ABS software is getting mixed up ONLY in the scenario of when I touch my brakes where I have just crested several ridges.
The g force pulled on the unloaded wheel is such that the ABS isn't programmed for and intervenes, thinking I'm about to lock up?

Normally loaded up wheels under braking isn't causing the ABS to trigger - so I get instant braking action.

I'll have to find some info on the STi Sport ABS version..

Do people with Prodrive brakes on Classics get this problem?

Nick

 

StopTech

I think that you may find that our brake kit is one of the few that have actually been developed to suit the application rather than just using a std caliper with piston sizes that are too large. As you correctly state, some kits give better pedal feel for just this reason and 6 pot kits are generally guilty of this as it's almost impossible to get small enough pistons to keep the hydraulic ratios right. Think you are barking up the wrong tree in this respect with our kit though

Nick

Your car already has the same basic ABS system as the STi and has EBD (even Forester has it as well) so you would not necessarily see any benefits. The scenario you describe is very common and it is something that we have tested at length but as the std car suffers in the same way we haven't been able to make it significantly better on the std tyres. Changing to 18" wheels actually helps as the tyre dynamic is better at damping out the undulations that are causing the oscillations that the ABS can't cope with. There's also more grip which has to help.
A simple way (but not very easy to do the first time !) to recover from it is to lift off the brakes and try again. Does defeat the idea of having ABS though but it's no different to the std car in this respect.

Regards

Mike

 

Hey... at least we agree that having to lift defeats the purpose of ABS!

 

Damn,
I had the chance of getting 18" wheels on the lease but thought better of it coz of tyre costs - do a fair amount of miles.
Guess I'll have to keep closer attention to road surfaces and change my style.
Thanks all.

Nick

 

I'm not going to argue with the like of StopTech and Prodrive, but surely the problem is not the brakes, but the wheels skipping over the uneven surface and confusing the ABS. If so, wouldn't suspension/wheel/tyre/pressure changes help? Mike has already hinted at that, but doesn't elaborate.

I would also argue (or ****, here I go ... ) that 18in wheels would make the problem worse in that they are likely to be heavier and increase unsprung weight which is where I'd be looking for a fix. You've already got Prodrive ally calipers which will help a lot but the discs are heavy, so discs with alloy bells would be a good move. Then maybe some spring/damper changes?

Richard.

 

I see the Prodrive site recommends uprated rear pads with the package to retained balance, yet IM Subaru brochures recommend 18" wheels.

I'm still curious about the "sport" ABS that the STi is meant to have. What different is it meant to do compared with std WRX ABS?
I take it that if a tweaked ABS module is programmed to expect a higher deceleration force then my problem won't happen?

Now I'm a bit puzzled why IM sell a product that causes less efficient braking (in particular but common UK road conditions) than a std brake set up?
I haven't seen any comments from Classic shape Imprezas with the same problem - does this mean it doesn't exist - why not?

With these thoughts in mind, I think that I should be talking to IM about this from now on, rather than be on a public soapbox.
If I get any useful replies from IM then I'll post them up.

Nick

 

Have now done another 1000 miles since this showed up and its hardly happening anymore.
My dealer can't find anything wrong with the ABS.
I must have either changed my braking style or perhaps the Bridgestones have scrubbed a bit more?

Would still like to know what is different about the STi Supersport ABS anyone?

Nick