Frt / Rear Brake Bias (and the Role of ABS)

[Cdn17Sport6MT]

General question for my learned sisters and brothers: As some of us try to get every last mile out of our respective cars or trucks the eventuality of very $pendy or unavailable ABS units starts to be evident (at least in my experience). So the question becomes: will an ABS based brake system still typically be balanced by way of frt /rear balance? Now I should qualify it...Depends on the load in the vehicle, tires, dry/wet conditions, etc...However, generally, will there be balance by virtue of frt versus brake size, type, etc?

Is a no-ABS-functional car a safe car still?

This leads to my question related to folks who upgrade brake sizes, say, on one "axle" only???

 

[KGMtech]

Most cars and CUV's in the last 20 years have a rear brake bias, this keeps the front end from diving down so much in stop & go city driving. So now your rear pads wear faster than the fronts. If you are an aggressive driver, you might not see this occur.

Doubt that many would even try to upgrade their brakes to a larger rotor / caliper as the cost is so high and the market so small.

 

[Pew]

General question for my learned sisters and brothers: As some of us try to get every last mile out of our respective cars or trucks the eventuality of very $pendy or unavailable ABS units starts to be evident (at least in my experience). So the question becomes: will an ABS based brake system still typically be balanced by way of frt /rear balance? Now I should qualify it...Depends on the load in the vehicle, tires, dry/wet conditions, etc...However, generally, will there be balance by virtue of frt versus brake size, type, etc?

Is a no-ABS-functional car a safe car still?

This leads to my question related to folks who upgrade brake sizes, say, on one "axle" only???

ABS won't alter the F/R distribution

During a drive on public roads, having one side stronger than the other without careful planning of the suspension and tires can drastically alter the brake bias and lead to very wanted consequences, ie: having too large of rear brakes will cause the rear to lock up sooner than the fronts, causing ABS to intervene.

Track-speaking, upgrading one side but not the other will not be balanced like from the factory but it may be better suited to what the driver will want. In a way ABS will interfere but at that point, the car shouldn't have ABS. This is all tied into the aero and suspension of the car so this is a very per-case basis.

An easy way for manufactures to change the balance is to install a larger rotor on the side they want, to accommodate extra weight and roles.

 

[Jetronic]

If the ABS is switched off completely, it can be like pulling the handbrake. Not all cars, but some definitely lock up the rears. Especially earlier EBD systems. The real modern stuff can limit front brake pressure aswell if needed, so the rear brake bias isn't necessary.

 

[Jetronic]

ABS won't alter the F/R distribution

During a drive on public roads, having one side stronger than the other without careful planning of the suspension and tires can drastically alter the brake bias and lead to very wanted consequences, ie: having too large of rear brakes will cause the rear to lock up sooner than the fronts, causing ABS to intervene.

Track-speaking, upgrading one side but not the other will not be balanced like from the factory but it may be better suited to what the driver will want. In a way ABS will interfere but at that point, the car shouldn't have ABS. This is all tied into the aero and suspension of the car so this is a very per-case basis.

ABS incorporates a function called EBD ( Electronic Brake force Distribution) which replaces the old proportioning valves, and can limit or even reduce brake pressures on an axle. So it does alter the bias like a driver does with a balance bar, except on the fly...

 

[Skippy722]

ABS won't alter the F/R distribution

During a drive on public roads, having one side stronger than the other without careful planning of the suspension and tires can drastically alter the brake bias and lead to very wanted consequences, ie: having too large of rear brakes will cause the rear to lock up sooner than the fronts, causing ABS to intervene.

Track-speaking, upgrading one side but not the other will not be balanced like from the factory but it may be better suited to what the driver will want. In a way ABS will interfere but at that point, the car shouldn't have ABS. This is all tied into the aero and suspension of the car so this is a very per-case basis.

An easy way for manufactures to change the balance is to install a larger rotor on the side they want, to accommodate extra weight and roles.

My 2016 Chrysler 300 had “brake force distribution” that would reduce brake pressure to the rear.

 

[Cdn17Sport6MT]

So, I'm hearing that the ABS system, other than the obvious about it being a fairly mission-critical element, essentially MUST be in place and functional? Scrap the car otherwise?

 

[Jetronic]

some cars won't even start without ABS present and responding

 

[Cdn17Sport6MT]

Comments aside re "engineering your own brake system" seems to me that the only solution with a dud abs system is plumbing-in a rear brake system pressure reducing valve... possibly not strictly legal...?

 

[Jetronic]

calculate the brake bias you have as is, easy to do if you know the piston and brake disc sizes. There will be calculators online. Then compare that to the weight distribution (at 1G braking, the centre of gravity moves forward by the height of the cg, so 2ft high means 2ft forward).

you can put lower mu brake pads at the rear/higher at the front etc to correct bias some.

 

[Cdn17Sport6MT]

Most cars and CUV's in the last 20 years have a rear brake bias, this keeps the front end from diving down so much in stop & go city driving. So now your rear pads wear faster than the fronts. If you are an aggressive driver, you might not see this occur.

Doubt that many would even try to upgrade their brakes to a larger rotor / caliper as the cost is so high and the market so small.

Learned something, here.

Re folks "upgrading" frt brakes, say.... yeah they DO contemplate that (not for me though; I'm not an automotive engineer!) Car forums, make-model-specific ones, folks do do that. Like fit (bigger) CX9 Mazda frt brks on a Mazda6...

 

[Skippy722]

So, I'm hearing that the ABS system, other than the obvious about it being a fairly mission-critical element, essentially MUST be in place and functional? Scrap the car otherwise?

I can confirm you can “disable” by removing the ABS fuse and it’ll still drive just fine, but you lose traction control, stability control, EBD, and the brake based differential on the 300. It’s hard to do snow donuts when you’re only spinning one wheel It seemed to also default to a more front heavy braking feel (I can’t prove it, just felt a bit different) and will set a check engine light.

Or you can buy a “Tazer” which is an OBD module that allows you to do various things, like “line lock” to roast the rear tires without smoking your brakes.

 

[Jetronic]

Learned something, here.

Re folks "upgrading" frt brakes, say.... yeah they DO contemplate that (not for me though; I'm not an automotive engineer!) Car forums, make-model-specific ones, folks do do that. Like fit (bigger) CX9 Mazda frt brks on a Mazda6...

If you have over heating issues that could help, but you're likely racing the car in that case.

 

[Pew]

Learned something, here.

Re folks "upgrading" frt brakes, say.... yeah they DO contemplate that (not for me though; I'm not an automotive engineer!) Car forums, make-model-specific ones, folks do do that. Like fit (bigger) CX9 Mazda frt brks on a Mazda6...

Yea regular focus owners liked to put ST front brakes on their car and leave the rears stock. Made no sense to me, especially for a street car since you can just stick a higher mu pads at the front without the extra rotational mass.

 

[meep]

In general most cars I’ve touched have larger brake systems in the front than the rear. That itself biases towards the front. In addition, some add brake proportioning valves, which basically require a certain pressure already in the system before any starts to make it to the rear, while also giving the rear the ability to drain all pressure back upon disengagement. It’s a simple system which people tend to think does more than it really does. some suvs and trucks go one step further by adding a lever to the proportioning valve to vary its affect based on axle position, compensating for weight over the rear axle. I’m not sure what EBD actually does, and when it starts to matter, but ideally all of these systems are designed to be in an appropriately balanced state prior to intervention, and then intervention brings in the last (whatever) percent it can squeeze out of the system based on conditions, loads, etc.. to that end, 80% (guesstimate) should easily be available even if the abs and all is down.

HOWEVER, the generations of folks who’ve not driven cars without these systems have a real disadvantage now, in that the safety systems work so well, I’ll bet many haven’t learned what it’s like to not have them. AND, todays cars have a lot more power and capability than pre-90s models did, so it’s easier to get into trouble that’s harder to get out of.

 

[Cdn17Sport6MT]

In general most cars I’ve touched have larger brake systems in the front than the rear. That itself biases towards the front. In addition, some add brake proportioning valves, which basically require a certain pressure already in the system before any starts to make it to the rear, while also giving the rear the ability to drain all pressure back upon disengagement. It’s a simple system which people tend to think does more than it really does. some suvs and trucks go one step further by adding a lever to the proportioning valve to vary its affect based on axle position, compensating for weight over the rear axle. I’m not sure what EBD actually does, and when it starts to matter, but ideally all of these systems are designed to be in an appropriately balanced state prior to intervention, and then intervention brings in the last (whatever) percent it can squeeze out of the system based on conditions, loads, etc.. to that end, 80% (guesstimate) should easily be available even if the abs and all is down.

HOWEVER, the generations of folks who’ve not driven cars without these systems have a real disadvantage now, in that the safety systems work so well, I’ll bet many haven’t learned what it’s like to not have them. AND, todays cars have a lot more power and capability than pre-90s models did, so it’s easier to get into trouble that’s harder to get out of.

Thoughtful reply; thank you.

I suspect you are right about the OEM brake system being 80% right without reliance on the ABS system (and in particular, it's EBFR function). Think about it... if you were designing the car and knowing that with NO ABS MIL evident (and yet a gummed-up / non functioning EBFR system in many cars beyond a certain age, I figure) - if the brake system were reliant to any great degree on this system, there'd be numerous crashes (I think) and a potential liability issue for the Mfr.

So, I conclude that a knackered ABS system does not condemn the car to the wreckers.