Tyre Pressures

[Django91]

Hello All,

I am on Goodyear F1 SuperSport tyre's in the 235/40r18 size. Pressures are 30-32 psi all around cold.
The car is on mild lowering springs and thick roll bars front and rear. Together with fast-road geometry I find that the front end is very sharp on turn-in and it's the rears that unhooks first when you get frisky, probably because front grip is very high.

1) Is there a way to change the way behaves on the limit with tyre pressures. I would prefer an understeer-neutral type balance. Can the steering response be slowed down a bit with tyre pressures as well, as I find it a bit too sharp/quick at the moment.

2)Ride quality and bump absorption on these tyres seems a lot less compliant than on 225/45r18(vredestein sessanta) the car was on, possibly due to stiff sidewalls, can this be helped with lower tyre pressures possibly?

3) Can tyre pressures be used to change the steering and road feel of the car? Whilst the current tyres are very grippy they don't give any audible indication of when they're about to let go with no squeal, I am sort of having to guess where the grip is at based on the amount of lean I have in the car.

Quick google search shows that lower tyre pressures could soften the ride, and altering them front/rear axle can change balance of the car. Any tyre gurus care to comment?

Thanks
Toby

[warpc0il]

Oversteer is the natural on-the-limit attitude for a rear wheel drive car.

If you were to adjust front/rear tyre pressures sufficiently to impact this behaviour then the rolling diameters would also be sufficiently different that the TCS, DSC, ABS and EBD systems would all stop working correctly, which would create chaos.

[Django91]

Oversteer is the natural on-the-limit attitude for a rear wheel drive car.

If you were to adjust front/rear tyre pressures sufficiently to impact this behaviour then the rolling diameters would also be sufficiently different that the TCS, DSC, ABS and EBD systems would all stop working correctly, which would create chaos.

Ok, I find the oversteer behaviour off-power when transferring weight onto the front. Again this probably what the FR layout is meant for but is there a way to alter this behaviour whilst 'learning' the car?

Since, I can't alter spring/damping rates wondering if something can be done with pressures. I was wondering if my pressures were high if I'm at 30-32 psi when cold. I say cold based on a quick drive to my local pump to change pressures, don't think its enough heat to raise the pressure sufficiently.

I'm guessing there's no solution for steering/road feel then.

Have kept away from things like spacers, as not sure how this would affect the car.

[Django91]

That's also typical of a "fast-road geometry" which is designed for experienced drivers that want to be able to control the stance of the car with the throttle.

If you're "learning" the car then you'd be better with a more conservative geometry, or just take your time and get a feel for it.

There are two other things that can also cause (what appears to be) excessive oversteer;
- a stiff universal joint on the steering column - see viewtopic.php?f=25&t=47002
- too tight a nut holding the steering wheel...

Seriously, your grip on the wheel should be really light and, other than parking, you should hardly be "gripping" at all.

[Django91]

That's also typical of a "fast-road geometry" which is designed for experienced drivers that want to be able to control the stance of the car with the throttle.

If you're "learning" the car then you'd be better with a more conservative geometry, or just take your time and get a feel for it.

There are two other things that can also cause (what appears to be) excessive oversteer;
- a stiff universal joint on the steering column - see viewtopic.php?f=25&t=47002
- too tight a nut holding the steering wheel...

Seriously, your grip on the wheel should be really light and, other than parking, you should hardly be "gripping" at all.

Lol, fair enough. Its been a bit of a struggle finding a decent seating position without reach adjustment. Steering always feels in a weird place. I'm more used to it now, but the front end feels mucjlh quicker than standard and like I'm rushing the car.There is not even a hint of understeer in any kind of bend except in very tight turns.

With the oversteer I get the countersteering right, but natural tendency is to panic and lift off than feed in throttle when rear starts to come around.

[Dr. FrankenRex]

Things to adjust if you want to change the balance of the car on turn in and on the limit:

- ARBs. This is the single biggest change in feel and dynamics IMO without spending ridiculous amounts of money. Totally transformative.
- Wheel Alignment. Getting a professional to adjust your wheel alignment and add a little bit of negative camber will help
- Wheel Spacers. Sounds daft, but if you have a slightly wider track at the front relative to the rear then you'll create a slightly more 'urgent' turn in and a bit more 'control on the throttle' experience at the limit.
- Forced Induction. This is a bit of a daft one for a laugh, but ultimately the RX8 lacks torque and, as a result, will be harder to rotate on the throttle than something punchier. Forced induction solves this

[ChrisHolmes]

As your car has been “set up” then all that needs to be changed now I think is your driving technique and a track day at Castle Combe, your nearest track with instruction would be my recommendation. A cheaper alternative is the skid pan school there which may be more related to road driving.
Learning on public roads where the limits of the car and your skills lie is dangerous as there is no run off to save you, usually just a curb or a ditch and the odd tree not to mention other vehicles!

[Django91]

Things to adjust if you want to change the balance of the car on turn in and on the limit:

- ARBs. This is the single biggest change in feel and dynamics IMO without spending ridiculous amounts of money. Totally transformative.
- Wheel Alignment. Getting a professional to adjust your wheel alignment and add a little bit of negative camber will help
- Wheel Spacers. Sounds daft, but if you have a slightly wider track at the front relative to the rear then you'll create a slightly more 'urgent' turn in and a bit more 'control on the throttle' experience at the limit.
- Forced Induction. This is a bit of a daft one for a laugh, but ultimately the RX8 lacks torque and, as a result, will be harder to rotate on the throttle than something punchier. Forced induction solves this

Yes-I have the racing beat roll bars which are non-adjustable, but they did make a massive difference to the handling of the car. To be honest turn-in is super urgent at the moment, have never experienced anything like it before.

I've entertained the thought of spacers before,IMO it would enhance the look of the car. But in performance terms it is a can of worms of sorts, you do get a wider track, but I wonder if its worth the added rotating mass penalty(don't know how much they weigh) plus possibly detracting from the way the suspension geometry has been designed. From what I understand, in general if offset isn't managed properly it messes with the effective spring rate at the wheel due to the extra leverage the offset puts on the suspension. But maybe it doesn't have a big effect on the rx8?

[New Duke]

Leave the tyres at the standard pressures. We only really raise them by upto 10% for soft sidewall tyres, or lower them for track conditions. But you won't get them hot enough on the road to make any real difference, even on a very spirited summer drive. More likely to end up wrapped around a tree messing with them to alter the handling. All tyres feel different, maybe the F1s just aren't for you.

I agree with Frankenrex's suggestions in general but it sounds like driving style is first base here as per Warpc0il and Chris' advice. The behaviour you've explained is as expected. In normal circumstances oversteer is preferable to the alternative and sharp steering is what most us aim for . Maybe before the geometry was done you had some odd toe at the front that generated understeer at the expense of speed/cornering that you got used to. 'Fast road' is pretty broad to be honest.

Sorry if I'm reading your posts wrong, I don't know your driving experience, but a couple of comments rang alarm bells:

Ok, I find the oversteer behaviour off-power when transferring weight onto the front.

Weight transfers to the front under braking. Although there are exceptions, as a general rule you don't want to be braking while turning. Brake phase first - then turn phase. Applies on the track, roundabouts, country lanes, everywhere.

and it's the rears that unhooks first when you get frisky, probably because front grip is very high.

(Assuming this is under throttle and not the same braking cause as above) Grip at the front won't make the rears unstick. If the rear unhooks it's usually because you've over-rotated the car, by turning in too sharply, or too much throttle, changing gear, or wet/bumpy surface etc. Especially if you're driving like that with the DSC off...

As suggested by the others time on track or at a skid pan with the setup you have will clear it all up. At least that's cheaper than many mods.

[Django91]

Leave the tyres at the standard pressures. We only really raise them by upto 10% for soft sidewall tyres, or lower them for track conditions. But you won't get them hot enough on the road to make any real difference, even on a very spirited summer drive. More likely to end up wrapped around a tree messing with them to alter the handling. All tyres feel different, maybe the F1s just aren't for you.

I agree with Frankenrex's suggestions in general but it sounds like driving style is first base here as per Warpc0il and Chris' advice. The behaviour you've explained is as expected. In normal circumstances oversteer is preferable to the alternative and sharp steering is what most us aim for . Maybe before the geometry was done you had some odd toe at the front that generated understeer at the expense of speed/cornering that you got used to. 'Fast road' is pretty broad to be honest.

Sorry if I'm reading your posts wrong, I don't know your driving experience, but a couple of comments rang alarm bells:

Ok, I find the oversteer behaviour off-power when transferring weight onto the front.

Weight transfers to the front under braking. Although there are exceptions, as a general rule you don't want to be braking while turning. Brake phase first - then turn phase. Applies on the track, roundabouts, country lanes, everywhere.

and it's the rears that unhooks first when you get frisky, probably because front grip is very high.

(Assuming this is under throttle and not the same braking cause as above) Grip at the front won't make the rears unstick. If the rear unhooks it's usually because you've over-rotated the car, by turning in too sharply, or too much throttle, changing gear, or wet/bumpy surface etc. Especially if you're driving like that with the DSC off...

As suggested by the others time on track or at a skid pan with the setup you have will clear it all up. At least that's cheaper than many mods.

New Duke,

The over-steer I talked about is from turning in sharply I suspect, so if I enter a roundabout and let the car take the left most lane and then turn right in order to go straight( like taking an exit that goes straight) quite sharply, you're really transferring weight that's probably what caused it.
It's just the front end is soo damn sharp now, it exaggerates it even more.

I've been wary of turning the DSC off as it does this even with all aids on, you can get a few degrees of slip before the systems intervene and brake the concerned wheels. Yes, I agree with your driving tips. Perhaps it maybe best to go to the Eagle F1 Asymmetric 5's which are more road biased next time. I guess on the road you want the car setup for feel good factor and comfort rather than outright speed, so whichever tyre is great value that meets that brief. I have no doubts the Super-Sports will be great on track, just no tyre squeal at all.

Might be best to try a skid pan day first then, can you do them in your own car?

[Kamys]

Try Yokohama Advan Sport. I find these tires very hard to slide on (definetly possible tho) and very good grip in the wet even at ridiculous speeds. As for the ''learning the car'' bit, why would you make a change, learn that, then reverse the setting just to be back where you started, when you can just get used to the way it is.

Something that i didn't see mentioned here and might help with the car's behaviour is changing break bias. However considering you're in the ''learning'' stage, I'd leave that for later.

Personally I find the RX8 an extraordinary machine handling wise, and once I got used to it, it stoped taking me by surprise.

[350matt]

do you know what the sidewall rating number you have on your F1 's?

as I personally prefer a soft enough sidewall to allow the car to 'lean' a little into the turn

from your comment it sounds like you may have 94-95 ish rating which is quite stiff for the mass of the car and makes the turn in very sudden whereas a softer sidewall like a 90 -92 gives a little bit more in a turn and gives you more a feeling for when things let go

its a small thing but the devil is in the detail

Personally I'm on a 235/40 R18 91 Uniroyal rainsport which I find excellent as a all weather tyre

[Django91]

do you know what the sidewall rating number you have on your F1 's?

as I personally prefer a soft enough sidewall to allow the car to 'lean' a little into the turn

from your comment it sounds like you may have 94-95 ish rating which is quite stiff for the mass of the car and makes the turn in very sudden whereas a softer sidewall like a 90 -92 gives a little bit more in a turn and gives you more a feeling for when things let go

its a small thing but the devil is in the detail

Personally I'm on a 235/40 R18 91 Uniroyal rainsport which I find excellent as a all weather tyre

I believe the load rating is 95.

Well, as it turns out, I have been driving a completely stock Rx8 over the past week or so.
Its on Goodyear F1 Asymmetric 3 tyres in 225/45R18 size and the difference in ride quality is night and day. It just seems to absorb all of the road bumps and imperfections easily, even while cornering, while my car always feels nervous and on the verge of skipping over bumps. On a track where roads aren't bumpy, mine might be faster, but I preferred this stock car on the road,.

While you can actually feel the 'mass' of the car with the stock roll bars, the 'tempo' of this car felt much more gradual and measured without the super-sharp front end and generally I actually liked driving this car more than mine, which left me feeling a bit deflated. I think I may have gone wrong with my tyre selection, and maybe even the alignment.

[Django91]

Try Yokohama Advan Sport. I find these tires very hard to slide on (definetly possible tho) and very good grip in the wet even at ridiculous speeds. As for the ''learning the car'' bit, why would you make a change, learn that, then reverse the setting just to be back where you started, when you can just get used to the way it is.

Something that i didn't see mentioned here and might help with the car's behaviour is changing break bias. However considering you're in the ''learning'' stage, I'd leave that for later.

Personally I find the RX8 an extraordinary machine handling wise, and once I got used to it, it stoped taking me by surprise.

I'm trying to get used to the way my car drives, but its a tad too 'nervous' feeling for my liking. I'm thinking its my tyres, I put mods on my car starting with springs( on standard size tyres),new tyres and alignment(done together), then roll bars in that order. It's felt a bit off since I changed tyres and the alignment tbh. Don't get me wrong they are ultra grippy in the dry, but it seems to have taken away the pliancy in the suspension which is one of the best features of the Rx8 in my opinion. Just doesn't seem settled on the road anymore. I've been driving another car lately and it seems to back up my doubts. Might just stick to standard tyre size and more road-biased tyres when I change next.

[350matt]

try backing the tyre pressure off 3 psi or so
as the normal rating is 32 psi so dropping down to 29% is a 10% shift which you should feel

[v-rex]

When I first got my car it was on Bridgestone RE040A's and whilst fantastic turn in in the dry, they made me uber nervous in colder, greasy conditions and the wet. The car just stepped out so easily.

When I switched to RE050A's it felt more understeery to me in the dry but so much more stable in the wet. So definitely I think don't rule tyres out. All were 225/45/18.

At the mo I have RE050A's at the front and S001's at the back and it feels understeery especially as they are about 5 years old now, and I'm debating what tyre to get next. I'll probably go for 235/40/18 if and when I do so will report back here on my experience if I notice anything. Trying to get through winter on my old tyres and then change fresh for next spring but a few scary moments in the wet may make me do it sooner....

[Django91]

When I first got my car it was on Bridgestone RE040A's and whilst fantastic turn in in the dry, they made me uber nervous in colder, greasy conditions and the wet. The car just stepped out so easily.

When I switched to RE050A's it felt more understeery to me in the dry but so much more stable in the wet. So definitely I think don't rule tyres out. All were 225/45/18.

At the mo I have RE050A's at the front and S001's at the back and it feels understeery especially as they are about 5 years old now, and I'm debating what tyre to get next. I'll probably go for 235/40/18 if and when I do so will report back here on my experience if I notice anything. Trying to get through winter on my old tyres and then change fresh for next spring but a few scary moments in the wet may make me do it sooner....

My issue is probably down to the tyre model choice itself as 235/40r18 has been reviewed favorably by many on the forum.

[v-rex]

My issue is probably down to the tyre model choice itself as 235/40r18 has been reviewed favorably by many on the forum.

Yes sorry if that wasn't clear, I was talking more about the tyre model rather than the size. Just letting you know what size I had used in the past, to give you all the facts.

I will be moving to 235/40/18 on the next tyre change as well, given all the good things I've heard, just need to figure out what tyre model I will go for.

Edit: Yes re-reading my post it does look like I might be questioning the 235/40/18 size, but that wasn't the intent !!

[Django91]

My issue is probably down to the tyre model choice itself as 235/40r18 has been reviewed favorably by many on the forum.

Yes sorry if that wasn't clear, I was talking more about the tyre model rather than the size. Just letting you know what size I had used in the past, to give you all the facts.

I will be moving to 235/40/18 on the next tyre change as well, given all the good things I've heard, just need to figure out what tyre model I will go for.

Edit: Yes re-reading my post it does look like I might be questioning the 235/40/18 size, but that wasn't the intent !!

I've had the chance to drive another RX8 in the recent past on Michelin PS4, in the standard size. This is a very good tyre in terms of road noise and comfort levels, certainly enhances the balance between bump compliance and handling that the RX8 naturally seems to have. Unsure about overall grip levels, but on the basis of the above factors I would rate PS4>Eagle F1 Asymmetric 3>Eagle F1 Super-sport.

Dry grip wise I would rate F1 Super-sport>PS4=Asymmetric 3. F1 Super-sport is pretty decent in the wet, but not very good in damp/greasy situations in my experience.

[v-rex]

Yes I'm thinking Michelin PS4. I have them on my CX-5 and they are great in greasy conditions, and much more softer than the toyo proxes I had on before.

[350matt]

the other thing to consider is do you have the stock anti-roll bars still?

as it sounds like you may prefer the way the car feels on those rather than the stiffer racing beat items

perhaps fit the originals and see how you go? as a car that rolls more tends to telegraph its movements more and will feel less snappy
ride quality will also improve

[Django91]

the other thing to consider is do you have the stock anti-roll bars still?

as it sounds like you may prefer the way the car feels on those rather than the stiffer racing beat items

perhaps fit the originals and see how you go? as a car that rolls more tends to telegraph its movements more and will feel less snappy
ride quality will also improve

This my be a weird observation, but from the drivers seat, the standard rx8 I drove actually felt like it 'rolled' less. This doesn't make sense to me, but is just the way the car felt to me compared to mine. Theoretically roll bars shouldn't really affect ride quality as they 'work' only when the car is in 'roll', my ride seems unsettled even when travelling dead straight. Gut tells me different tyres might solve my problem.

[warpc0il]

I'm assuming that you've checked that none of your arb drop links have become disconnected.

Sent from my SM-G800F using Tapatalk

[350matt]

ARB's do effect ride quality as 90% of the time its 1 wheel that is coping with a bump and not both wheels on the axle at the same time

so the wheel has to push up against the stiffness of the bar thats effectively held by the other wheel
if you can design suspension to work without ARB's then you typically get excellent ride quality
however they are a very useful tool in the suspensioner designer's toolbox

if you have access to other rex's via friends cars perhaps you can ask to briefly borrow their wheels and tyres to try on your car?

[Django91]

ARB's do effect ride quality as 90% of the time its 1 wheel that is coping with a bump and not both wheels on the axle at the same time

so the wheel has to push up against the stiffness of the bar thats effectively held by the other wheel
if you can design suspension to work without ARB's then you typically get excellent ride quality
however they are a very useful tool in the suspensioner designer's toolbox

if you have access to other rex's via friends cars perhaps you can ask to briefly borrow their wheels and tyres to try on your car?

Yes, I might swap wheels/tyres to see how it does.
Might need to check roll bar linkages too.