unstable during hard braking

[dede911]

Hi,

after a long research to upgrade the brakes on my 240hp SC, I recently installed the C12 setup from Tuthill.

Last weekend I had my first trackday with this brakes in Spa Francorchamps during the Porsche days.

During hard braking (200km/k) the car was unstable, with plenty brake power left.

(With the stock setup I could brake very hard without having to pay attention to lockup)

Because the weather was unstable, I used this time street tires (normally Toyo 888)

The C12 calipers have a Hydr bias F/R 1.43, no proportioning valve.

Is this to much rear biased and could this be the reason for the unstability?

What do you think?

André

[Bill Verburg]

Originally Posted by dede911

Hi,

after a long research to upgrade the brakes on my 240hp SC, I recently installed the C12 setup from Tuthill.

Last weekend I had my first trackday with this brakes in Spa Francorchamps during the Porsche days.

During hard braking (200km/k) the car was unstable, with plenty brake power left.

(With the stock setup I could brake very hard without having to pay attention to lockup)

Because the weather was unstable, I used this time street tires (normally Toyo 888)

The C12 calipers have a Hydr bias F/R 1.43, no proportioning valve.

Is this to much rear biased and could this be the reason for the unstability?

What do you think?

André

that's a little low but should be ok, especially w/ lsd and low stiff track suspension, the rotor size also affects this

could be too small a m/c too, that reduces the pressure curve closer to on/off and is harder to modulate.

could be as simple as grease on a rotor or pad or air in a caliper too.

[johnsjmc]

I would also check corner balance

[dede911]

Hi Bill,

disc sizes OD's are stock, so no influence on torque

My LSD is in fact an ATB, not helpful during braking.
Suspension is front 21TB's and rear 27TB's, not really stiff

Modulation was different than before but controllable, pedal travel is not excessive

I can bleed once more and check rotors

In my opinion it should be possible to lock the wheels without getting unstable.
Is this a correct assumption?

André

[dede911]

bump

[Porchcar guy]

How long was the new system in the car before track day ? How did you bed the brakes in after install ?

[Matt Monson]

Replacing the ATB with a proper LSD will make a world of difference.

[Bill Verburg]

Originally Posted by dede911

Hi Bill,

disc sizes OD's are stock, so no influence on torque

......
André

So how is this an upgrade then

Originally Posted by dede911

Hi Bill,

......
My LSD is in fact an ATB, not helpful during braking.
Suspension is front 21TB's and rear 27TB's, not really stiff

Modulation was different than before but controllable, pedal travel is not excessive

I can bleed once more and check rotors

In my opinion it should be possible to lock the wheels without getting unstable.
Is this a correct assumption?

André

Originally Posted by dede911

Hi Bill,

......
My LSD is in fact an ATB, not helpful during braking.
Suspension is front 21TB's and rear 27TB's, not really stiff

.....

André

Same as before right?

Originally Posted by dede911

Hi Bill,

......

Modulation was different than before but controllable, pedal travel is not excessive

....

André

I what way, if you have the bigger caliper pistons w/ the same m/c it will be harder to modulate and there will be more pedal travel

Originally Posted by dede911

Hi Bill,

......

In my opinion it should be possible to lock the wheels without getting unstable.
Is this a correct assumption?

André

yes, stock brakes do that all the time

A blown shock is another possibilty

instability covers a lot of ground.

is it in front? in back?
is it felt in the pedal? the seat?, the steering wheel?

does the car , yaw? in front? in back?

[bivenator]

Blown shock, as mentioned, caused me similar problems. Mine pulled to the side opposite of the bad front shock.

[Plavan]

Originally Posted by bivenator

blown shock, as mentioned, caused me similar problems. Mine pulled to the side opposite of the bad front shock.

+1

[dede911]

Bill,

I see it as an upgrade for following reasons
-six pot caliper front gives more pad stability: stock calipers developed uneven disc wear, much more toward the middle of the disc
-very reliable design, no dust covers, high temp seals
-the calipers can be spaced for thicker discs
-lightwheight
-competition proven by Tuthill Porsche in uk

The ATB is a few month's old, so this is not really an option

Suspension setting are the same as before (last trackday was in September )

During the hard braking, the car feels like moving left-to right in the back, so that on the limit, I had to correct it slightly with the steering to keep it straight
I don't know what you mean with yaw (my English is lousy)

Guy,
For bleeding I used a vacuum bleeder (with compressed air). This was done a few weeks before the event.

André

[Quicksilver]

Do you have a rear spoiler? (duck, whale, or other)

200 k/h is starting to get up to the speeds where the lift with the stock decklid is going to generate enough lift that rear will dance when you get on the binders hard. If you didn't have instability under the same conditions before the brake change then you have some sort of bias problem. (By "the same" I mean same speeds, the same section of track, the same tires, the same body work, and same suspension.)

---------------
To add to Bill's questions and comments on brake mods:
Brakes are rather simple in basic operation and people make a whole bunch of hocus-pocus about them. You are simply taking the inertia of the car and turning it into heat via friction. The rotor is simply a heat sink and it only has to stay between certain operating conditions to work.
- The heat can't go high enough that the pad/rotor combination will fade.
- It must be able to shed heat fast enough that it cools enough before the next time the brakes are applied.
- Extra mass (weight) means it can absorb more energy. (Good)
- Extra mass means that you are also increasing unsprung weight and rotating mass. (Both are bad)
- Fade is when you reach the point where the increase in the pad surface temperature causes the coefficient of friction to fall. (Bad)
- Increasing caliper size increases unsprung weight. (Bad)
- Increased pad size increases the quantity of pad you have to deal with wear. (Good)
- Increased pad size reduces the quantity of rotor surface area that can radiate heat. (Bad)
- Multiple pistons simply spread the clamping force more evenly across the brake pad helping to even out pad wear. (Good except for the extra weight)
- Brake pads don't have a stability problem. When they are doing something they are being clamped to a flat surface with a ton of force. They aren't going anywhere.

A major quantity of brake upgrades don't do much. Ultimately a real upgrade will increase cooling, increase the ability to ignore added heat, or increase the size of the rotors.

[ChrisBennet]

Tail wagging under braking could be toe out in the rear. It wouldn't hurt to have the alignment checked.

When I first started tracking my car it would do the tail wagging thing so often under braking that I got used to it to the point where I corrected the wag without realizing I was doing it. One day at Watkins Glen, I had an instructor ride along with me and afterwords he complimented me on my tail wagging correction. I told him I didn't realize I was correcting. He said I should get the alignment checked. I said: "Don't they all do that?" and he "No, Chris, they don't!"

He was right of course and fixing the alignment solved the tail wag problem.

Because I hadn't driven anyone elses car and had no frame of reference, I'd just assumed that the tail wagging thing was just part of the normal "track experience"!

[Bill Verburg]

Originally Posted by ChrisBennet

Tail wagging under braking could be toe out in the rear. It wouldn't hurt to have the alignment checked.

When I first started tracking my car it would do the tail wagging thing so often under braking that I got used to it to the point where I corrected the wag without realizing I was doing it. One day at Watkins Glen, I had an instructor ride along with me and afterwords he complimented me on my tail wagging correction. I told him I didn't realize I was correcting. He said I should get the alignment checked. I said: "Don't they all do that?" and he "No, Chris, they don't!"

He was right of course and fixing the alignment solved the tail wag problem.

Because I hadn't driven anyone elses car and had no frame of reference, I'd just assumed that the tail wagging thing was just part of the normal "track experience"!

That would do it too, you have more rear brake which could be causing more toe out during braking.

[dede911]

Hi,

rear toe is a good remark.

According to my latest alignment report, total rear toe is 0°11'. For 16" rims this should be around 1.35mm.

Factory limits are between 0°10' and 0°30°

My setting is thus on the lower limit.

Any suggestions for toe?

Thanks.

André

[dede911]

Hi,

I had the car in the alignment shop today.
1) corner balance: they made some small adjustments, cross difference is now 3.5kg, close to perfect

2) alignment

Fr toe total +1.7 mm
camber 1°50'

Rear toe total +5.7 mm, left +3.1 right +2.6mm
camber left -2°33', right -1°55'

He could not get rear toe lower...

What do you think?

André

[Steve@Rennsport]

Originally Posted by Matt Monson

Replacing the ATB with a proper LSD will make a world of difference.

This one is just huge.

ATB's do not keep the rear wheels rotating at the same speed under trailing throttle and behaves just like an open diff (which is NOT good).

LSD's (clutch-pack style) keep both rear tires rotating at the same speed which really stabilizes the car and this is critical when you have more rear brake bias than stock. Thsi virtually eliminates the tail-wagging these cars have under hard braking into a corner.

My suggestion would be to sell the ATB diff to an autoX'er or street driver and get the Guard LSD which will fix your problem. Plus, its adjustable which cannot be said for an ATB unit.

[dede911]

Steve,

Thx for your reply.

What do you think about the alignment?

André

[Quicksilver]

Originally Posted by dede911

Hi,

I had the car in the alignment shop today.
1) corner balance: they made some small adjustments, cross difference is now 3.5kg, close to perfect

2) alignment

Fr toe total +1.7 mm
camber 1°50'

Rear toe total +5.7 mm, left +3.1 right +2.6mm
camber left -2°33', right -1°55'

He could not get rear toe lower...

What do you think?

André

Obviously the rear alignment is wrong and needs to be fixed. I always have Steve Alarcon do my alignment/cornerbalancing and he is a specialist in aligning cars that won't align plus he focuses on Porsche so there is a lot of knowledge there. I don't know what the tricks are to get it into spec but if you pop him a PM he should be able to give some quick pointers on how to get it in range.

Other people who generally post here that you could reach out to are Steve@Rennsport, TRE Cup, and john walker's workshop.

[Gary.H]

Originally Posted by Bill Verburg

So how is this an upgrade then

This ^^^^

I thought the main 'upgrade' for 911 brakes was calipers that allowed you to go to bigger discs (better heat dissipation) ? A different caliper on the same sized disc seems to be missing a trick ?