996 Turbo Brakes

996 Turbo Brakes

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ScienceTeacher

Original Poster:

335 posts

131 months

Wednesday 30th August 2017
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Sorry, roged the upload thing on my last post. A couple more snaps.

ScienceTeacher

Original Poster:

335 posts

131 months

Wednesday 30th August 2017
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ScienceTeacher

Original Poster:

335 posts

131 months

Wednesday 30th August 2017
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The Wookie

11,723 posts

174 months

Wednesday 30th August 2017
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If your free travel is a bit long OP it's not an adjustment, it's probably just new calipers with lively seals, it will likely settle down and give you a shorter pedal with a few miles and heat cycles

If it persists then I'm sure JZM wont mind giving it another bleed which usually improves things. It's often difficult to bleed a new set of calipers perfectly from dry on the car and it's not uncommon to have to give it another go after a few hundred miles.

Digga

27,818 posts

229 months

Thursday 31st August 2017
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ScienceTeacher said:
In this picture, that colour combination (IMHO) really looks superb.

Would hope the dead travel on the pedal is resolved, because that's not something you need.

ScienceTeacher

Original Poster:

335 posts

131 months

Thursday 31st August 2017
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Thank you for your kind and informative words - appreciated. So on Tuesday my lad stood next to the GT as it fired up and today at Blenheim he was next to the Big Mac driving off. Splendid.

NBTBRV8

1,955 posts

154 months

Friday 1st September 2017
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When I fitted the same brakes to my old tt I had the same issue. The pedal travel is longer than you'd expect, that is why I recommended standard brakes with upgraded pads.

Digga

27,818 posts

229 months

Friday 1st September 2017
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I have always wondered, given the GT3 has a 6 pot set up, whether it needs a bigger volume of fluid to move the pads the same distance and, therefore, whether it runs the same master and slave cylinders?

996TT02

2,973 posts

86 months

Friday 1st September 2017
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The issue of braking force is divided in two parts.

One is the ratio of the surface areas of the piston in the master cylinder and the pistons in the callipers.

There is a gearing effect.

If (say) the MC's piston is 1 square inch and the combined piston area of the callipers is 50 square inches, then one pound of pressure at the MC translates into 50 pounds (not psi - but total force) at the callipers. This is why some cars get by without power brakes, especially older ones which were lighter. The downside is that the travel of the MC piston, hence brake pedal, is correspondingly longer than the travel of the calliper piston, so you can not keep on upping the ratio, as the pedal would hit the floor in short order.

In effect, if you fit larger callipers with pistons that have larger surface area than the old ones, thus upping the ratio, you do get more braking force, but you also get more pedal travel. If you had to replace the MC with one with a larger diameter piston to maintain the old ratio, you don't get any added braking force at all.

The second part is the assistance, the "power" in power brakes. To keep things simple, this is directly related to the design of the servo unit. Assuming everything is in the best possible condition / best performing selection, if one wanted more braking force with less pedal effort and without added pedal travel, replacing or perhaps if feasible modifying the servo unit is the only way to go. Excluding increasing disc size, where you gain some added leverage as the callipers force will be acting on a slightly larger radius, but for practical purposes, such as limits on the size of the discs one could possibly fit, the gains will be small.


ScienceTeacher

Original Poster:

335 posts

131 months

Friday 1st September 2017
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NBTBRV8 said:
When I fitted the same brakes to my old tt I had the same issue. The pedal travel is longer than you'd expect, that is why I recommended standard brakes with upgraded pads.
I do respect this. The point is that it seems to be dead travel, to use Digga's words, ie no discernible resistance or retardation for 3 cm then sensible, progressive, resistance and retardation, and an impressively improved halt. How do I rid myself of dead travel? The earlier suggestion is to wait for seals to bed in (I don't understand this) or have a further bleed, which seems sensible. I do also believe a push rod adjustment would be of help.

The Wookie

11,723 posts

174 months

Friday 1st September 2017
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ScienceTeacher said:
I do respect this. The point is that it seems to be dead travel, to use Digga's words, ie no discernible resistance or retardation for 3 cm then sensible, progressive, resistance and retardation, and an impressively improved halt. How do I rid myself of dead travel? The earlier suggestion is to wait for seals to bed in (I don't understand this) or have a further bleed, which seems sensible. I do also believe a push rod adjustment would be of help.
The seals are in grooves that are designed to pull the pistons back (and thus pads away from the disc), this is called fallback or rollback. When the seals are new they often are more active and pull the pistons back more than they do once they have settled and bedded.

This gap between the pad and the disc has to be taken up when the pedal is pressed before the brakes engage and is felt as dead travel.

It is worth saying that with bigger piston area this does mean more volume consumption for the same amount of fallback, so it could be this that you're experiencing

Digga

27,818 posts

229 months

Saturday 2nd September 2017
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Just to say, some great posts on here, Slippydiff earlier and the last couple by 996GT02 and The Wookie.

Slippydiff

10,565 posts

169 months

Saturday 2nd September 2017
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The Wookie said:
If your free travel is a bit long OP it's not an adjustment, it's probably just new calipers with lively seals, it will likely settle down and give you a shorter pedal with a few miles and heat cycles

If it persists then I'm sure JZM wont mind giving it another bleed which usually improves things. It's often difficult to bleed a new set of calipers perfectly from dry on the car and it's not uncommon to have to give it another go after a few hundred miles.
Yep, as the (massive) pads bed in, the pedal should improve, and would agree, they'll be no harm in doing another bleed after a month.

Tommie38

231 posts

140 months

Sunday 3rd September 2017
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Don't forget that fitting calipers with a larger piston area will also affect brake balance. From the perspective of piston size, you are shifting the brake balance rearward. The larger diameter disc will help a little here, as will the larger pad, but brake torque calculations are something I have never got my head around. One of the good things about purpose built kits is that they can take all of that into account.

Years ago my friend and and I fitted the GT3 caliper upgrade to his 996 Turbo. He was very happy with that and did not mention pedal travel, but we did fit RS29s too. I honestly don't recall if I drove it after the swaps.

Although not as good looking, I have often wondered about something like the following:

https://www.design911.co.uk/mobile/#part;id=12556

You could be certain of an increase in braking at the front (I'm less worried about moving bias forward), and obviously quite affordable. In the past I have seen issues with mild pad overhang, so would check that, but not the hardest thing to machine away in a good workshop.



ScienceTeacher

Original Poster:

335 posts

131 months

Tuesday 12th February
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Just returned from Ken’s. New rear discs and RS29 pads to match the bigger fronts. Should have had it done to start with. More sure footed and definitely feels that the braking isn’t all being done at the front and the car seems flatter under moderate braking. Pedal feels a little firmer. All good. Excellent experience at Ken’s, as always.
Once again, my price projection for sensible preventative work is correct. Going into my fifth year of ownership and it’s still circa £4K pa on 2000 miles. Main coolant hose over engine needs replacing. Putting it off ‘til later in the year. Engine out...

catsey

115 posts

24 months

Thursday 14th February
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to late but put it out there I have full set alcon discs for 996 GT@ with pads only done 2k miles for sale
pm me