Very hard brake pedal after spinning tires?

[ras06LGT]

Despite how stupid the title sounds, hear me out.

 

I've noticed that occasionally when I'm on the gas and spinning all four tires, right after I let off the gas and jump on the brakes the pedal is very firm and I do not get any noticeable deceleration. A quick release and re-apply of the brakes results in normal pedal feel and deceleration. This only seems to happen when the brakes are applied immediately after coming off the gas. Any slight delay and the brakes feel fine. It's almost like it only happens if I get on the brakes while the tires are still slipping.

 

This has happened more than once, but I would not say it happens regularly. I've only found it by messing around in parking lots, since I wouldn't floor it in traffic if I had any expectation of having to stop quickly.

 

Car is a 2006 LGT with manual trans. Pads and rotors were replaced early summer of 2013. Hoses and fluid where changed late last fall.

 

Any thoughts on what might be causing this? On a car with no VDC/TC/collision pre-fill I don't see any link between spinning tires and a hard brake pedal.

[ras06LGT]

Actually, a thought just popped into my head... I'm probably feeling brakes with no power assist. Sliding around and spinning usually means liberal throttle usage. High RPMs and open throttle means the engine isn't really making any vacuum. By jumping on the brakes right away I bet there's not enough time to recover vacuum for brake assist.

 

Seeing as I got new tires this year, I wanted to see what they would do in all sorts of situations, including stomping the brakes while tuning. If I get on the brakes (deplete most of the vacuum), then stomp the gas, then jump back on the brake, I bet I can reliably repeat this behavior. That, or the booster check valve is bad.

 

Sound plausible? More testing tonight I guess...

[MrTris]

Actually, a thought just popped into my head... I'm probably feeling brakes with no power assist. Sliding around and spinning usually means liberal throttle usage. High RPMs and open throttle means the engine isn't really making any vacuum. By jumping on the brakes right away I bet there's not enough time to recover vacuum for brake assist.

 

Seeing as I got new tires this year, I wanted to see what they would do in all sorts of situations, including stomping the brakes while tuning. If I get on the brakes (deplete most of the vacuum), then stomp the gas, then jump back on the brake, I bet I can reliably repeat this behavior. That, or the booster check valve is bad.

 

Sound plausible? More testing tonight I guess...

 

You're headed in the right direction with your hypothesis, it sounds logical, anyway.

 

Couldn't think of a better 'test' to perform -- let us know how it works out.

[Soarin]

Yeah, I'm pretty interested in this too, but your hypothesis seems pretty spot on. Let us know.

[iNVAR]

I've hit the brakes with my left foot in 2nd while in full boost and not letting off the gas before. Yes, the pedal is very firm, and yes, it does very little. In fact, I can keep accelerating. Probably because of lessened assist.

[GTEASER]

I'm gonna guess that it has something to do with the VDC/TC/ABS pulsating the ABS unit to keep tires from spinning and introducing pressure into the braking system automatically. So when you hit the pedal, you are trying to push the M/C piston into a braking system that already has pressure in it, which is why it is hard and the force you put on the pedal doesn't seem to do much.

[MrTris]

I'm gonna guess that it has something to do with the VDC/TC/ABS pulsating the ABS unit to keep tires from spinning and introducing pressure into the braking system automatically. So when you hit the pedal, you are trying to push the M/C piston into a braking system that already has pressure in it, which is why it is hard and the force you put on the pedal doesn't seem to do much.

 

He's an 06, shouldn't have VDC, and, I don't think the ABS system was so advanced as to have control of the brakes/wheels when brakes were not being applied. Only just in the last few years, and, only in the new Eyesight models has Subaru implemented braking technology that primes the binders before the driver even hits the brakes with other input, from like, radar.

[GTEASER]

Tris, how does Subaru Symmetrical AWD work? It applies the brakes to the wheels that spin, that's part of the reason its 4 channel ABS and not 2 or 3. Traction Control detects the wheels spinning and applies the appropriate brake, VDC detects yaw and lateral traction loss and applies the appropriate brake, ABS detects the wheel that stops spinning and relieves the hydraulic pressure to that wheel. All of this going on has an affect on the pedal feel, firmness, and the amount of force that the driver senses he is inputting into the brake pedal versus the affect of that pedal force on the actual dynamic changes in the car that he is sensing.

[Rhitter]

Tris, how does Subaru Symmetrical AWD work? It applies the brakes to the wheels that spin, that's part of the reason its 4 channel ABS and not 2 or 3. Traction Control detects the wheels spinning and applies the appropriate brake.

 

Does it actually do this? In my experience 1 wheel just loses traction and the power is cut. Or if I turn off traction control that 1 wheel spins and spins.

I think each of Subarus AWD systems work a little differently.

 

Edit: I have VDC and pretty sure a rear LSD so my system works a little differently than the 05-06s.

 

Not the greatest source, but here you go

http://www.cnet.com/news/not-every-subaru-all-wheel-drive-system-is-created-equal/

 

In the manual cars

The standard viscous coupling

When slip on the front or rear axle is detected, a locking center differential is able to send up to 80 percent of available torque to the axle that has the best traction. The center differential uses a viscous coupling that operates without the aid of computer control and reacts to the mechanical differences in grip.

 

In automatic cars

Symmetrical AWD that uses Variable Torque Distribution (VTD) and pairs uniquely with their older five-speed auto gearbox. In this case, the nominal torque split is a rear-biased 45:55 default, ditching the viscous center diff in favor of a hydraulic multiplate clutch in combination with a planetary-type center differential.

 

When slip is detected, based on input from sensors measuring wheel slippage, throttle position, and braking force, the electronically controlled clutch can lock down at 50:50 front-to-rear split where maximum grip is required.

 

Side-to-side?

On both the front and rear axle, you'll generally find a standard open-type differential, but performance models (such as the WRX and the Legacy-based 3.6R models) will often feature a limited-slip differential on the rear axle to help with rear-end grip when cornering.

 

My experience with traction control is that it just kills power / cuts throttle input when it detects slip.

[iNVAR]

05/06 5MT's do NOTHING to the brakes unless you are applying the brakes yourself.

[Underdog]

Maybe the check valve in the hose going to the brake booster is leaking/stuck?

[MrTris]

Does it actually do this? In my experience 1 wheel just loses traction and the power is cut. Or if I turn off traction control that 1 wheel spins and spins.

I think each of Subarus AWD systems work a little differently.

 

Edit: I have VDC and pretty sure a rear LSD so my system works a little differently than the 05-06s.

 

Not the greatest source, but here you go

http://www.cnet.com/news/not-every-subaru-all-wheel-drive-system-is-created-equal/

 

In the manual cars

The standard viscous coupling

When slip on the front or rear axle is detected, a locking center differential is able to send up to 80 percent of available torque to the axle that has the best traction. The center differential uses a viscous coupling that operates without the aid of computer control and reacts to the mechanical differences in grip.

 

In automatic cars

Symmetrical AWD that uses Variable Torque Distribution (VTD) and pairs uniquely with their older five-speed auto gearbox. In this case, the nominal torque split is a rear-biased 45:55 default, ditching the viscous center diff in favor of a hydraulic multiplate clutch in combination with a planetary-type center differential.

 

When slip is detected, based on input from sensors measuring wheel slippage, throttle position, and braking force, the electronically controlled clutch can lock down at 50:50 front-to-rear split where maximum grip is required.

 

Side-to-side?

On both the front and rear axle, you'll generally find a standard open-type differential, but performance models (such as the WRX and the Legacy-based 3.6R models) will often feature a limited-slip differential on the rear axle to help with rear-end grip when cornering.

 

My experience with traction control is that it just kills power / cuts throttle input when it detects slip.

 

This.

 

Any idea what happens when brake is applied to a single wheel when it's slipping? (He asked sarcastically, not knowing exactly how Subaru's AWD system works, but knows enough that it isn't via the brakes.)

 

Not only does the technology barely exist now, to where brakes are activated without driver input (active radar assisted, ala Eyesight, and other similar technologies), but it sure as hell didn't exist ten years ago, and even today, there are too many other external factors to make this operation safe under real world conditions.

[Rhitter]

This.

 

Any idea what happens when brake is applied to a single wheel when it's slipping? (He asked sarcastically, not knowing exactly how Subaru's AWD system works, but knows enough that it isn't via the brakes.)

 

Some other cars do this (Ford Focus ST and the GTI) it's the VLSD. The open diff directs power to the wheel with least resistance, so power gets sent to the other side (hopefully). Unless I am missing something, Subaru's don't have this. Or at least I have never seen it advised / talked about. VDC might mimic that a little, but if it does, in my experience not very well.

[MrTris]

Some other cars do this (Ford Focus ST and the GTI) it's the VLSD. The open diff directs power to the wheel with least resistance, so power gets sent to the other side (hopefully). Unless I am missing something, Subaru's don't have this. Or at least I have never seen it advised / talked about. VDC might mimic that a little, but if it does, in my experience not very well.

 

Right, but, my point was that in slip scenarios, brake, ie - caliper to rotor initiating wheel slow/stop action -- is not applied. VLSD is viscous, meaning, where the fluid is, there's power. So, it's directing fluid in the diff in order to direct and control the power. Has nothing to do with brakes.

[Underdog]

There are many AWD systems from 10 years ago and prior that used the brakes to limit wheel spin in lieu of a mechanical LSD.

[Rhitter]

I think I was using the wrong term. I was using it to mean virtual limited slip differential or electronic limited slip differential. Not viscous.

 

So open diff with the computer controlling the brake (on the slipping wheel) to mimic an LSD.

 

In the Subaru - nothing happens. Power is just cut or the wheel just spins with traction control off.

 

I think we are on the same page.

 

My main point to GTeaser was - I don't think that's how Subaru's AWD works.

[ras06LGT]

Being an 05/06 with no fancy electronics, the AWD is purely mechanical.

 

We got about 4 inches of snow last night, which allowed me to confirm that it's indeed low vacuum in the brake booster causing the "issue". I only get the hard brake pedal when doing a brake --> gas --> brake maneuver. I also verified the check valve is working correctly.

[Underdog]

Can you pull vac on the booster and then pinch off the hose between the check valve and IM and then see how the pedal feels? This would indicate a leak somewhere after the check valve.

[ras06LGT]

Before I start my car every morning I press the brake pedal and clutch (out of habit) and there is still vacuum assist in the booster after sitting overnight, so I doubt it's leaking.

[MrTris]

I only get the hard brake pedal when doing a brake --> gas --> brake maneuver.

 

You're doing it wrong.

 

It's Gas --> Brake --> Dip!

[ras06LGT]

Can you pull vac on the booster and then pinch off the hose between the check valve and IM and then see how the pedal feels? This would indicate a leak somewhere after the check valve.

 

After pinching off the line the pedal still feels normal. No indications of any leak.