MayorAdamWest

Hi,
Does anyone know if there is a rear sway bar available for the 991? Also, has anyone had any success reducing understeer? I've got PDCC, and there is a fair amount of understeer at the limit. I am getting even tire wear, but burning up my fronts pretty quick. Any other bolt on improvements for the suspension? I'd like to stick to things that are easy to remove for resale/trade-in. Right now, I'm running 19"s with R888, Girodisc rotors, and Pagid rs29 pads.

Thanks!

chuckbdc

Non PDCC S models have a rear sway bar (also called roll bar or stabilizer bar). PDCC is in effect an active dynamic sway bar system. It adjusts the roll rate as though there were and infinity of sway bars each optimized to a bunch of sensed inputs. If I understand it, it should not be causing understeer, it should be reducing what you would have with a static sway bar. I think you would have to remove the PDCC to add a static sway bar but am not certain of that.

You might try reducing the differential between front and rear tire air pressures. Up to a point, more more air for closer pressures should provide more "stick" to the front relative to what you have now. Beyond that getting more camber up front may help.

Porsche builds in front understeer with stock settings to reduce the chance that the rear heavy rear end will swing around and spin the car. Adjusting cornering entry and exit attitude may help.

MayorAdamWest

Thanks for the feedback. I'm running generally even pressures for F and R. Usually around 38 hot on R888s. I'm not sure how much negative camber we can get on the stock setup. Anyone know?

chuck911

You don't need to be messing with the car. First, fix your tire pressures. 38 is on the high side. The risk in running high is that traction falls off very rapidly as you exceed ideal pressure. Running a bit on the low side you lose a bit of responsiveness but overall traction remains high. I would think 32/36 will be closer.

Next, sign yourself up for some DE time. Everything chuckbdc says is right on- especially the last bit about your driving. As right as he is about Porsche building in a little understeer, it should only take a few minutes with any Chris Harris Porsche video to understand it cannot be all that great a tendency- not with the tail hanging out there like that! So its really much more about how the car is being driven than the car itself. Don't believe me? Next time you notice "a fair amount of understeer at the limit" ease off the throttle a bit, and just watch how that understeer goes away and the car turns in tighter. Problem solved.

chuckbdc

Also check out SM ATL's thread on 6speedonline or ask him there for first hand advice. He is totally on top of tracking the 991 and has a setup similar to yours.

MayorAdamWest

Perhaps I should have directed the question to the Racing and DE forum. I wasn't looking for advice on how to drive, but thanks. 38 is a generally accepted pressure for R888s, which prefer to be hot. I have tried everything from 34-42. MPSC+ preferred the lower side.

008

I can't help with the anti-sway bar question but you may check with Tarret engineering, TPC Racing and BGB Motorsport. I haven't run R888s on anything but an Elise but I was not a big fan considering how much I loved the RA1.

That aside almost every review I've read have blamed the PDCC for excessive understeer. What are your alignment settings? I ran -1.6 all around and zero toe at TWS last week as that was my max in balance I could get up front. I'm going to CoTA next month and will work to get more in the front and definitely add more to the rear with slight toe out up front which should help on turn in and slight toe in on the rear. I am on stock 20s with SPASM but no PDCC. Pyrometer was telling me -2.5 all around would be close to ideal on street tires but Rs may need more. Mid corner understeer was noticeable but not terrible. I have a feeling my new settings will trade some front to rear traction but that's ok. I may enlarge the upper mount holes to get at least -2 in the front.

I plan to leave it relatively stock but may go to adjustable control arms and solid bushings, then the slope gets slippery as even at its firmest the suspension is too soft for Rs.

lunarx

Understeer?
My car oversteers heavily.
255/F, 305/R PSS tires.
PDCC & SPASAM.
I need to check my alignment.
I suspect I might need more rear toe-in.
If alignment wont fix it, I will be looking for a front sway bar upgrade.
If one ever becomes available.



Do you know what your settings are?
Do you have the 245 or 265 in front?

Slantnose!

Actually....does any place carry anything performance related for our cars?

MJBird993

I don't think adding negative camber is going to reduce understeer. It's the nature of the beast, and having wider tires on the rear just make it worse. So unless you want to start removing that expensive PDCC system, maybe you can try some skinny tires in the rear?

MayorAdamWest

I'm actually on 235 in the front because tirerack has been out of the 245 for quite some time. I'll get 245 next time (no r888 in 255). I didn't think 265 would fit my 8.5" rim?

SM_ATL

I did not find so far that the 991 suffers from excessive understeer so far, and PTV certainly helps from that perspective, as well as the wider track.

As far as sway bars/PDCC is concerned, my first 991S has the standard suspension (and I had installed Speedart springs) and my second has PDCC (with SPASM this time). I can not say that I felt any significant difference in understeer between the two. With PDCC, anti-roll bar drop links are simply replaced by a small 2-way hydraulic actuators:



In a 'dynamic' situation, PDCC noticeably less body roll. The car feels more stable and controllable at high speed, but it is true that you have less feeling of how close you are to the limit and that there is probably more risk for the car to snap if you go beyond it.

Back to the question on understeer, I would personally not modify the stock suspension if you have PDCC (how is it going to work if you went for a stiffer rear sway bar for instance???). As mentioned by others, a lot can already be controlled by driving technique. I do not know how far you can push the camber, but I stay close to the OEM setup as my car is also a daily driver.
I am trying a new set of tires on the track this week end (Yoko AD08R instead of the usual R888 as I wanted something more street oriented when I travel long distances to the track and that there are chances of rain). Front tires are 255 and it could positively impact understeer (I used to run R888 in 235). I'll see if i can feel any difference.

just my 0.02

lunarx

Yup. 235 will give you understeer.
If you can get a 9" and 265 your front will be planted.

008

Properly inflated as you reach max sidewall deflection you are reducing contact patch to a degree as you corner. You counter this by setting your camber at a point that maximizes your contact patch under load as you corner without giving up too much straightline stability. Tire construction has a lot to do with this as well.

What you need to look at is when you are feeling the car push. Is it on turn in (you're likely rushing it or too stiff up front or soft in the rear), mid corner hunting for the apex (overcooked it or balance is off that increases with lateral load), corner exit ( on power too early, etc). Obviously there are many other reasons for all of these but it's a place to start and without changing any hardware you can narrow it down significantly with alignment and air pressure testing.

A quick google on R888 netted the below and further down from the Toyo website:

SET UP ADVICE FOR THE TOYO PROXES R888

What I would like to do is to offer some advice on tyre temperatures and pressures and how to achieve the best from our tyres when taking part in either trackdays or racing.

The R888 has a semi race construction (very stiff) and a race tread compound. The optimum tread temperature range is between 85C and 95C measured using a probe type pyrometer, and ideally a maximum difference across the tread of 9C. The maximum hot pressure we recommend is 40psi. Camber angles up to 5 degrees are permissible but the final setting will depend on tread temperatures. It is advisable to have as much positive castor as practical as castor induces a beneficial camber change during cornering. I recommend that the tyres be put through a heat cycle before hard use, this will scrub them in and remove any mold release agent.

The pressures you use will initially depend on the weight of the car, too little pressure on a heavy car can lead to over deflection of the tyre and subsequent failure.

Below are some basic settings:

VEHICLE WEIGHT COLD PRESSURE HOT PRESSURE
Very Light < 800kg 17 - 22 psi 22 - 29 psi
Light 800kg - 1000kg 20 - 26 psi 24 - 32 psi
Heavy 1000kg - 1400kg 23 - 27 psi 28 - 40 psi
Very Heavy > 1400kg 27 - 35 psi 37 - 40 psi


As a tyre gets hotter the pressure increases, this is due to the moisture in the air. The cold pressure you set to achieve a desired hot pressure will depend on the conditions on the day i.e. ambient and track temperature, wet or dry. If the day/track is cold you will need to start with a higher cold pressure as the tyre will not get as hot therefore the pressure increase will not be so great.
Hot pressures must be balanced side to side. Once the tyres have cooled you will find that you will have a difference in pressure side to side, if you have been racing on a right hand track you will find the offside pressures will usually be higher than the nearside.

Changing hot inflation pressures by small amounts can be used to fine tune handling.

Reduce Oversteer Reduce rear pressures or increase front pressures
Increase Oversteer Increase rear pressures or reduce front pressures
Reduce Understeer Reduce front pressures or increase rear pressures
Increase Understeer Increase front pressures or reduce rear pressures


Achieving the required tread temperatures will depend again on the conditions on the day i.e. ambient and track temperature, wet or dry.

You often here competitors saying “My tyres started to go of towards the end of the race”, this is usually due to the tread getting to hot.

The tread temperatures are constantly changing through out a race, hotter when cornering and cooler when on the straights and cooling even more when you are slowing to come into the pits. Therefore the temps you record in the pits will be lower than those during the race. So if you record temperatures within the range given above the probability is the temps will be too high during the race.

Increasing your tyre pressures will cause your tread temperatures to increase, more pressure stiffens the tyre’s casing which results in the tread having to do more work resulting in the tread getting hotter. Lowering your pressures will cause them to decrease.


Inevitably changing one thing will affect other things, the whole set up of your car is a compromise between anything that is adjustable.



Alan Meaker
Technical and Motorsport Manager Toyo Tyres (UK) Ltd



Operating Temperature: 71°c to 104°c (160°F to 220°F)
Tire Pressure:
VEHICLE
WEIGHT COLD INFLATION
PRESSURES HOT INFLATION
PRESSURES
Less than 800 kg. 17 - 22 psi 22 – 29 psi
800 kg. – 1000 kg. 20 – 26 psi 24 – 32 psi
1000 kg. – 1400 kg. 23 – 27 psi 28 – 40 psi
More than 1400 kg. 27 – 35 psi 37 – 42 psi

Camber: -1.0° to -3.0° (the Proxes R888 generally requires less negative camber than the Proxes RA1)
Caster: As much positive as possible
Shaving: 4/32” is recommended for competition (grooves remain until 2/32”)
Rim width: a tire size that falls within the range of midpoint to maximum rim widths is recommended
Wet conditions: increase tire inflation pressure by 6-10 PSI to reduce the possibility of hydroplaning
Driving style with less slip angle is recommended (when compared with the RA1)
Break-in procedure for new R888 at full tread depth: scrub the tire clean of any mold release chemical. Bring new tires (full tread or shaved) up to operating temperature gradually over the course of a few laps to minimize graining

MayorAdamWest

that's really interesting stuff, thanks. I'll try running with lower pressure in the front than rear. Right now I was generally at the same hot temp all around.