Brake fade - how good are XC70 brakes?

[PJ810]

I don't like doing it to my car, but from time to time I like to test and experience how it performs an ABS stop under various conditions.

During a very spirited drive of about half an hour on a twisty road I had an impression that the brakes were fading just a little. My Volvo 240 never suffered this on that road (although the speeds would have been somewhat lower and the 240 is much lighter). However, with similar speeds my Peugeot won't have a hint of brake fade.

So I found a safe spot to do a 100km/h (60miles/hour) full stop on a straight, dry road. Upon flooring the brake pedal my first observation was that ABS didn't cut in. Only until the car slowed to about 70km/h did ABS activate. Is this normal? I would have expected more initial braking power - easily enough to ensure ABS activation.

Immediately after stopping I accelerated to 100km/h and tried it again. ABS kicked in even later.

Third time I only went up to about 60 or 70km/h before commencing my 'emergency' stop. No ABS at all. The brakes struggled to pull the car up despite my foot being flat to the floor. Definite fade.

Note that I have NEW discs and NEW pads. The car has just been in for a full service and check up at a reputed Volvo specialist mechanic. During normal relaxed driving the brakes feel fine, very progressive and pleasant.

So my question is: are XC70 brakes not up to the task of slowing the car or do I have a problem? I would hate to be loaded with passengers and gear and have to do a 150km/h to zero stop. Surely this can't be right?!

[Willy]

Hello,
You mention that ABS kicked in late, but does that mean that the wheels were already skidding at the time ABS did activate?

The fact that the discs and pads are new is not the best circumstance for testing. I assume your car has brake assist,
meaning that the response of the braking system depends on the way you hit the pedal.
Could it be that the fading you felt was in fact the brake assist that intervened, regulating the pressure on the brakes, trying to avoid ABS from activating?
I think it may be difficult to reproduce the exact way you hit the brakes in a previous test and that these tests can easily be
misleading, certainly if modern brake assist systems are active.
That said, I haven't experienced anything wrong with the brake power of my car, but I don't remember having done an emergency stop either :o
Willy

[PJ810]

Sorry, no, the car did not skid. Didn't have the power to skid... and hence not have the power to activate ABS until it slowed down.

I don't think the car has brake assist. Low-spec 2001 model. I've experienced brake assist in a 1999 Mercedes C280 and it was quite clear what it was doing.

Essentially it felt like I pushed the pedal to the floor, brakes applied heavily but just didn't have the power to pull the car up quickly enough.

Just to recap, I slammed on the brake very very quickly. Off the accelerator and straight to the floor on the brake with full force to emulate a panic stop.

Will also add: pads and discs recently replaced but bedded in. About 1000-1500km of easy driving.

[budrichard]

If your brake pedal went "to the floor", you have a lack of hydraulic fluid or adequate pressure buildup. Since this happened right after work on your brakes, I suspect the work was not done correctly. A "reputed Volvo specalist" requires no Factory training.
In any event, ABS will not function unless the system detects skidding. I believe the system is tested each time the vehicle is turned on and an alarm would be displayed if not functioning.
From you description of pedal movement and fade, the symptoms are of a brake system problem rather than ABS. Have the system checked ASAP and don't drive until it is checked by a Volvo Service Technician at a Volvo Authorised Dealer.-Dick

[Willy]

Yes, Follow Dicks advice!
Willy

[ifnt420]

MY01 & MY02 V70XC did not come with the Electrical Brake Vacuum pump as found in later MY03+ XC70s.
A few impressions of before and after my Electrical Brake Vacuum pump retrofit:

-Initial brake bite is better with the pump installed.
-Less Brake force is required.
-Occastional no brakes at Cold start is no longer present.

On another note, a set of Stainless Steel Brake lines is a good investment if you are doing repeated full stops from high speed.
I have the Takespeed SS brake lines and they work great. [thumbup]

[PJ810]

ABS is fine. When there's enough braking power to "skid", the ABS cuts in and does a perfect job. I've tested it during a tight turn and all works very well.

The brake pedal doesn't "sink" to the floor. It's when I PUSH it to the floor that I'd wish for more force back and more braking power. I am talking 100km/h. From 60km/h I wouldn't need to push it to the floor to activate enough braking pressure to have the ABS kick in.

Anyway, considering the speed, the concensus is still that there's a problem and the car should have the power to activate ABS at any speed?

[skibo]

On another note, a set of Stainless Steel Brake lines is a good investment if you are doing repeated full stops from high speed.
I have the Takespeed SS brake lines and they work great. [thumbup]

And how exactly do these SS lines improve braking? Is it just because they are 'stiffer' than the 12 inch rubber hoses from the frame to caliper?

[TiredGeek]

Well on my '07 D5 the brakes are superb :D
I live in northern Scotland and the roads here are very winding, I regularly drive hundreds of miles on them at high speed (70+mph), braking VERY hard for the bends and the occasional oncoming traffic, I get the brakes stinking and smoking but haven't suffered fade the way you describe it. I do get some fade if the bends are close together, but never enough to cause me to worry, the brakes bite well and it slows very quickly.

Bear in mind I'm not talking about "normal" driving here, the roads are so quiet, the distances to things so great (145 miles round trip to the nearest superstore), that everybody drives like rally drivers up here! I mean braking from 90mph 100yds from the bend and powering away in second gear....
I very regularly see the DSTC light coming on, and I can report that with light load in the boot, the XC 70 D5 flies flat and lands quite well ;)

I note the disks are bigger on the '08, I wonder if they'll fit the older models....

[Willy]

And how exactly do these SS lines improve braking? Is it just because they are 'stiffer' than the 12 inch rubber hoses from the frame to caliper?

If it is the same as with motorcycles, and I suppose it is, the advantage of steel brake lines is indeed that they do not
expand under pressure, so more pressure gets where it should get.
However, although teh advantage may not be marginaly, drivers who don't use the brakes at the limits won't feel much improvement, if any.
Willy

[ifnt420]

If it is the same as with motorcycles, and I suppose it is, the advantage of steel brake lines is indeed that they do not
expand under pressure, so more pressure gets where it should get.
However, although teh advantage may not be marginaly, drivers who don't use the brakes at the limits won't feel much improvement, if any.
Willy

Exactly! Brake fluid will start to boil when you are making full stops repeatedly from high speed. Hence factory rubber hoses will expand; SS brake lines will help reduce the brake fade due to hose expansion.

I made the suggestions because PJ810 was making repeated full stops from 100kmh, which is very hard on brake fluids.

[PJ810]

I made the suggestions because PJ810 was making repeated full stops from 100kmh, which is very hard on brake fluids.

There's still the issue that it didn't have enough power to activate ABS at the start of the initial stop.

[ifnt420]

Just curious, which brand brake pads and rotors did you use?

[Jeremy]

Old brake fluid can also cause this issue too.

[jdj2]

I agree that brake fluid contamination or poor quality brake fluid may be the culprit.

I would recommend a complete flush and bleed with good quality brake fluid (I think the spec is Dot 4) and then repeat your experiment to determine if it's still an issue.

Brake fluid absorbs moisture from the air, so it can become contaminated with water just sitting there. Then when the brakes get hot, the water molecules start to boil and intoduce vapor into the system causing poor performance. Of course when the brakes cool, the vapor turns back into liquid, and everything seems fine.

[Puff The Magic Wagon!]

There's still the issue that it didn't have enough power to activate ABS at the start of the initial stop.


You're not doing it right - put your foot 6 inches above the brake pedal and jam it down with as much force as you can. If that doesn't work, then start weight training exercises for your legs ;)




Seriously, getting ABS to come on is not what it is all about. You need to brake smoothly and firmly & REMAIN IN CONTROL - ABS is not in control.

I had a friend who spent £1500 ($3001) on a set of 6 pot AP Racing callipers/discs/pads for his Subaru Impreza. He was convinced that because he could get ABS to come on when he stomped the brakes at 60mph (Impreza is lighter) he would stop quicker than anyone else - completely untrue as what ABS does is release the brake slightly if it feels the wheel lock, hence making the stopping distance increase. Although you have steering ability, you are still skidding momentarily.

At the end of the day, the XC is a heavy car and if you want to get it to stop better, then spend money on uprating the braking components. I have AP Racing 4 pots on my Impreza and I can get that to stop in 1/2 the distance (without skidding as my car doesn't have ABS) than I can my XC which I have put uprated pads/discs only on (Brembo/DS2500s). With APs then it might only be 2/3s but it is still 1 heavy car (not taking into account tyres/grip either.

Braided SS lines offer resistance to expansion under pressure (ie foot on pedal) and this allows more pressure to go to where it is needed - the callipers - to increase pedal pressure/braking force. The biggest improvement that people will notice is "feel".

For brake fluid, replace the standard stuff with Dot 5.1 or a synthetic, both of which offer a higher resistance to boiling (increased boiling point) when under hard use. No increase in braking ability though.

Try your braking stuff at a marker post, with ABS activating and without it activating and see the difference in the stopping distance. Work at stopping without ABS to make your stopping distance shorter.

[PJ810]

You're not doing it right - put your foot 6 inches above the brake pedal and jam it down with as much force as you can. If that doesn't work, then start weight training exercises for your legs ;)




Seriously, getting ABS to come on is not what it is all about. You need to brake smoothly and firmly & REMAIN IN CONTROL - ABS is not in control.

Yes, I totally agree. However, my exercise was not one of an emergency stop, but of seeing how the car behaves and what power it has, as I had a feeling that there was fade during a spirited drive. Hence I wanted to see if the car had the power to activate ABS from 100km/h during consecutive attempts. And I know how to activate ABS having done it experimentally many times before in many different vehicles. I applied the brake pedal hard and pushed it all the way to the floor very very quickly. Rest assured about that!

I know my Volvo 240 could lock up easily at 80km/h, and so any car should.

Threshold braking is naturally the quickest way to stop. However, that said the general recommendation is to activate ABS. I never was much of a believer of ABS until I used it a couple of times. There's no way I could outbrake it by threshold braking. I find the road surface and grip levels are always different. I'll either skid or not brake hard enough. Add to that our terrible Australian roads where the surface under each wheel is going to be inconsistent and you're much better off programming your mind to leave it up to the ABS computer to sort out.

Agreed though, if I were to execute an emergency high speed stop, I'd apply more gradually, and then press into ABS mode after transferring the weight fully onto the front wheels for maximum grip.

And yes, there is a problem with my car that will be looked at shortly. Thanks for the tips re brake fluids, s.s. brake lines and auxilary valves.