How much G force should a call pull under braking?

[Jasmine1]

I've been testing the brakes on my 911.1 this afternoon, as I have my first track day next weekend.

When I bought the car last summer, it would show 1.30G under braking on its onboard computer.

This afternoon I've been experiencing 0.60-0.70G, which is bloody terrible. At times I got it up to 0.90G and even 1.03G from 50mph, but it felt shockingly bad most of the time.

Admittedly it is 1.5°C, wet but no standing water, and I have MPS4S with 3mm tread. These aren't ideal conditions, but 0.60G felt dangerous.

I'm now wondering if there's something wrong with my tyres and if I should replace them before my track day. Of course, I don't want to spend £700 if tyres aren't the issue, and it's just the fact it's a bit cold.

I'd be interested and very appreciative if other PHers could share their experiences and maybe report back what their car will do (if it has a G force meter!).

[Equus]

Jasmine1 said:

Admittedly it is 1.5°C, wet but no standing water

Under those conditions, 0.6 -0.7G is doing fine.

If you're running 'trackday' tyres, expect them to be absolutely apalling in the cold and damp.

[andygo]

I woud imagine it's the cold and possibly salt covered roads.

Summer tyres aren't great at temps under 9C and the fact your tyres have only 3mm tread might suggest that they are slightly age hardened as well. I don't think the tread depth is a particular factor in the dry however.

I'd possibly be popping new PS4S's on as if its wet on your trackday you might be grateful for the improved wet performance gained by having better tread depth.

[coppice]

Hope it isn't a wet track day with 3mm tread left

[paradigital]

coppice said:

Hope it isn't a wet track day with 3mm tread left

Or any track day at all tbh. I’ve easily scrubbed a mm or so off the fronts on a trackday, which would leave you with little to no legal tread for the return journey.

[Jasmine1]

coppice said:

Hope it isn't a wet track day with 3mm tread left

Normally I’d agree but this is just a cheap day to test the car out, and isn’t to be taken too seriously. I’ll probably go home after a couple of hours.

Plus 911s wear their shoulders very easily due to poor camber, it’d be very easy to kill fa fresh pair of tyres if you are a little ham fisted.

[Haltamer]

Hmm, I usually log between 1.2G - 1.4G on the Civic depending on tyre compound / conditions.

Given the lower reading, Unless you were into ABS I'd have thought the loss in deceleration is down to pad / discs / hydraulics rather than tyres?

Temperature is a big aspect, but by the sounds of it your pads would be up to temp after a few runs I'd hope.

[Equus]

Haltamer said:

Hmm, I usually log between 1.2G - 1.4G on the Civic depending on tyre compound / conditions.

On cold and damp roads?

It depends mainly on the coefficient of friction between the tyre and tarmac. Your figures are what you'd expect for dry tarmac, but the coefficient of friction for wet tarmac is roughly half that of dry, which puts the OP's figures exactly where you'd expect them to be.

[Jasmine1]

Haltamer said:

Hmm, I usually log between 1.2G - 1.4G on the Civic depending on tyre compound / conditions.

Given the lower reading, Unless you were into ABS I'd have thought the loss in deceleration is down to pad / discs / hydraulics rather than tyres?

Temperature is a big aspect, but by the sounds of it your pads would be up to temp after a few runs I'd hope.

It’s definitely the tyres. The ABS was coming in with what felt like anything more than 1/3rd pedal pressure.

The steering wheel also wiggles as the front wheels lock and unlock rapidly

[Haltamer]

Equus said:

On cold and damp roads?

It depends mainly on the coefficient of friction between the tyre and tarmac. Your figures are what you'd expect for dry tarmac, but the coefficient of friction for wet tarmac is roughly half that of dry, which puts the OP's figures exactly where you'd expect them to be.

Ah, those quoted figures are from patchy-moist / dry tarmac to be fair, so does sound reasonably square on further thought.

Other than the wet, biggest next step is temperature - Have a look at vredestein Quatrac Pro's for cold wet handling - Laws of physics still apply, and I don't think I'd bother with them on track (Much more opportunity to warm summers through, drainage) - They do feel nice in the cold.

[Caddyshack]

Jasmine1 said:

Haltamer said:

Hmm, I usually log between 1.2G - 1.4G on the Civic depending on tyre compound / conditions.

Given the lower reading, Unless you were into ABS I'd have thought the loss in deceleration is down to pad / discs / hydraulics rather than tyres?

Temperature is a big aspect, but by the sounds of it your pads would be up to temp after a few runs I'd hope.

It’s definitely the tyres. The ABS was coming in with what felt like anything more than 1/3rd pedal pressure.

The steering wheel also wiggles as the front wheels lock and unlock rapidly

If ABS is triggering then you won’t get as much g under braking as it isn’t braking as much….the whole point of ABS is to brake less to hold the tyre at its limit of adhesion. You shouldn’t be triggering ABS too often on the track day if you brake properly.

[Jasmine1]

It's a bit warmer today, and the car is producing higher G's.

The tyres may be worn, but I don't think they are faulty. Panic over.

[GreenV8S]

Jasmine1 said:

Admittedly it is 1.5°C, wet but no standing water

It should come as no surprise that tyres produce much less grip on cold wet surfaces. If that isn't obvious, you probably shouldn't be driving a high performance car.

[stevieturbo]

Jasmine1 said:

I'm now wondering if there's something wrong with my tyres

yes, it's winter.

[996TT02]

GreenV8S said:

Jasmine1 said:

Admittedly it is 1.5°C, wet but no standing water

It should come as no surprise that tyres produce much less grip on cold wet surfaces. If that isn't obvious, you probably shouldn't be driving a high performance car.

I am only shocked that this is not effing obvious to absolutely everyone behind the wheel. 911 driver expecting same levels of grip in the wet as dry? Thank Porsche for fitting traction control, ABS and PSM to these cars.

[E-bmw]

996TT02 said:

GreenV8S said:

Jasmine1 said:

Admittedly it is 1.5°C, wet but no standing water

It should come as no surprise that tyres produce much less grip on cold wet surfaces. If that isn't obvious, you probably shouldn't be driving a high performance car.

I am only shocked that this is not effing obvious to absolutely everyone behind the wheel. 911 driver expecting same levels of grip in the wet as dry? Thank Porsche for fitting traction control, ABS and PSM to these cars.

^^^^ Wot 'e said.

[Jasmine1]

996TT02 said:

GreenV8S said:

Jasmine1 said:

Admittedly it is 1.5°C, wet but no standing water

It should come as no surprise that tyres produce much less grip on cold wet surfaces. If that isn't obvious, you probably shouldn't be driving a high performance car.

I am only shocked that this is not effing obvious to absolutely everyone behind the wheel. 911 driver expecting same levels of grip in the wet as dry? Thank Porsche for fitting traction control, ABS and PSM to these cars.

If only both of your reading skills were as good as your inferred driving skills.

I've highlighted in bold the keyword which you missed (not to mention the second sentence, which you deliberately did not quote). Of course, the friction of the tyres is reduced with lower temperatures. I still maintain 0.60G is on the low side, though. Hence the point of this thread.

I'll put it down to MPS4S being a slightly track-orientated tyre, as less prestigious tyres offer improved performance in these circumstances.