Tarmac Rallying Perpetual Thread

[MBS206]

The thing that has me mostly beat is the whole "Fast on new tyres" thing. This is why we thought it was suspension. Because the car now on old tyres under-steers into corners and then moves to oversteer as soon as its neutral even before you even apply power. So we thought the problem was it was too soft - so we changed back to the old well known Teins that I still have with heavier springs and the car was no different and no faster (or slower).

I'm trying a different set of upper arms before TWP and if that doesn't work I will try swapping over the entire suspension and drive train out of a known fast car and see if that changes anything. LOL

Just from reading how it's handling, it's the rear diff. Sounds too tight, acting mostly like a locker, forcing understeer, and then breaks loose.

Have you tried putting the old diff in?

If the old diff was "shagged" it would allow the car to turn better under brakes and neutrally, and then should want to wheel spin a bit more then a good working LSD.

If you have an over tight LSD in the rear, it will make the whole car understeer until the rear shakes loose, and then any power put down will result in wheel spin, until attessa kicks in.

[Marlin]

Silly question, have you measured the difference in rear track between the new and old car?

Its my hunch from my own observations that V-Specs run less rear track, making them more stable everywhere.

[NickR33]

sorry maybe a dumb question Marlin but how does less track make the car more stable?

[Beer Baron]

sorry maybe a dumb question Marlin but how does less track make the car more stable?

it's not so much the less track. it's that comparatively (front to rear compared to a non V) it has more front track than rear which makes the car much easier to control.

think of it like swaybars. stiffening the rear gives more front grip. but how it does that is by reducing rear grip. the track thing is similar. it takes away some stability from the rear and thus makes the car. so it's desirable to add front track if possible. if not reducing rear track a little may have a similar effect.

The diff could be a factor. you running a front LSD snowman? and if so where you running it before? wbat else can you think of that's changed from the old car to the new one?

[Marlin]

Rear track narrower than the front naturally makes the rear end want to "follow" the front.... in basic terms.

The 34 was the first GTR to have rear track wider than the front.

[Snowman]

Richard - I have a Nismo front and rear 2 way LSD.

But that was the same as the old car. Only difference is the rear diff is a brand new one.

The only 3 major changes are the gearbox, the weight difference (150kgs lighter) and the fact its now a Non-V base. Otherwise its the same damn car.

Well sorry - there's 4 major changes. It's now white instead of blue but I'm pretty sure that ain't it.

[NickR33]

ah yes narrower track in relation to the front makes perfect sense, thanks guys

should've got a red one Snowy, everyone knows they're faster... also maybe needs more stickers?

[blackrex]

Have you checked that the plates in the diff are setup correctly? They may have to much preload on them, making the diff snatchy in it's operation.

[Snowman]

the question is though - if its a diff problem why is it fast on new tyres? Surely if the diff was too tight this would be more noticeable when there is more grip?

I'm starting to wonder if the whole problem is just the tyres themselves. I've done so little testing work with these A050's as they are definitely the bee's knee's when they are new and if its wet but I just wonder if the whole problem is simply a case of them being no good on a hard driven GT-R over long runs (aka - if you give them a baking they are shot forever). Because it was only after I cooked them at THC on the stage before lunch that the car lost speed. And at TT10 I was definitely "faster" relatively speaking at the start of the event compared to the end - but I had put that down to other factors (such as crashing into a tree). And all of the track "testing" I have done has been on these older A050's.

Food for thought - maybe I will try and track down a set of trusty RE55's for TWP to knock this theory on the head.

[JAGR33]

Richard - I have a Nismo front and rear 2 way LSD.

But that was the same as the old car. Only difference is the rear diff is a brand new one.

The only 3 major changes are the gearbox, the weight difference (150kgs lighter) and the fact its now a Non-V base. Otherwise its the same damn car.

Well sorry - there's 4 major changes. It's now white instead of blue but I'm pretty sure that ain't it.

Hasn't the car dropped quite a bit of horsepower compared to the old one since you're running a smaller exhaust and different turbos???

[Duncan]

I can't come up with any other ideas, but I can answer re the A050R. They are consistent from new to medium wear, and drop off slowly. Plenty of testing of this on heavy cars 1800kg commodores up here, and the guys are very happy with them, including racing up to an hour straight (a lot longer than any targa stage). I'm not happy with their 600 a tyre price tho.

Very different to the previous A048R which did indeed drop off significantly between brand new and medium wear - in the order of seconds a lap.

Not to say your problem could not be tyres though...depends how old they are, how they were stored, how they were heat cycled when new etc etc.

Also, if you are testing the impact of colour on performance, maybe paint it yellow/purple/silver and find out if it becomes cursed. Either rolls down hills or blows up motor depending on the order of the colours.

[blackrex]

If the new tyres are capable of applying (figures used as an example) a constant moment of 100Nm to the differential and the plates can only hold 95Nm, the extra moment will be enough to overpower the friction internally within the plates and thus making the diff work more as an open unit, but worn tyres can only provide that figure at maximum Fy and Fx, thereby not providing enough load consistently into the differential to overpower the clamping force of the ramps, causing them to lock and unlock dependent on the transmitted load. When the diff is locked then it will cause under steer on turn in and over steer on exit. It may be the due to the old diff being worn out, that it was not generating full lock under loaded conditions and masking the issue.

[Snowman]

Hasn't the car dropped quite a bit of horsepower compared to the old one since you're running a smaller exhaust and different turbos???

No the regs changed Jack and we can run the old Benno 3.5" exhausts now. So for all intensive purposes my car is the same as Scotsmans car - at least engine / turbo / power wise.

And when we went to Winton just before Xmas for the test he ran a 1.35.1 (and said it was a bad lap) to my 1.37.5 and I reckon I got everything possible out of it and we were on the track at the same time. We were also both running Teins with the same springs. The old car used to run 1.34's around there easy with that set-up. So in theory with my car being so much lighter than his now I should be quicker than him all other things being equal. I reckon the car should be good for 33's around there now but its miles off that right now.

[JAGR33]

No the regs changed Jack and we can run the old Benno 3.5" exhausts now. So for all intensive purposes my car is the same as Scotsmans car - at least engine / turbo / power wise.

And when we went to Winton just before Xmas for the test he ran a 1.35.1 (and said it was a bad lap) to my 1.37.5 and I reckon I got everything possible out of it and we were on the track at the same time. We were also both running Teins with the same springs. The old car used to run 1.34's around there easy with that set-up. So in theory with my car being so much lighter than his now I should be quicker than him all other things being equal. I reckon the car should be good for 33's around there now but its miles off that right now.

Yeah there's definitely something not right there. You mentioned something about camber and castor not being as good as previous car, although it should account for a significant difference I'd doubt it would be 4 seconds worth. I say get back out there as from what you were saying you only got a couple of dry sessions that day at Winton and after all it's been quite a few years since you were last out there so you might have been a little rusty. Don't give up, I say get back out there and give it another go before pulling the car apart......

[Ant97GTR]

I concur with Dunc about the A050's....Last year PI when Russ did his 48 (yes I know its not a GTR) the tyres were well worn and obviously still had plenty of grip. Sure it would depend on their life and how they have been stored but in my experience they seem to be pretty consistant during their life.

[Snowman]

The thing is with my last set of A050's - when they over heated about 3cms of rubber from each of the loop part of the inside set of "hooks" completely tore out so the tyre there was as flat as the tread depth. The outside set of hooks weren't so badly effected - but the whole tyre looks..... ugly. And not used R comp ugly. This is different. And back from each of those sections where the tyre was completely gone the rubber looks blistered and bubbly rather than the usual grainy effect. This especially happened on the rear tyres and as I said it happened suddenly after only limited use. Hot pressures were 32psi.

Now I am not sure if they were maybe not cycled properly or something (they did the HWY drive to Mansfield and one semi-spirited drive up Mt Buller 2 days before they were used which is about all I normally do) or whether it was just a funny batch or whether its what ever is wrong with the car is doing to the tyres as the first set didn't do this - but then that was at Targa where it was basically wet the whole time anyways - and they were on the old car.

[Beer Baron]

hmm sounds like it could be a problem with the tyre. which compound are they?

is the front diff 1 way? 1.5way? surely not 2 way? is the front diff out the old car too? so it's just the rear that's new?

it's possible that with the new different alignment (lack of camber and castor) that the tyres were not getting used evenly and that they were damaged and from thereafter where just gone?

[Snowman]

Front is Nismo 1.5 way. Rear is Nismo 2 way. This is the same set-up as the old car.

Anyways - doesn't matter for now. Tomorrow we are putting new arms in the car to get the alignment right (Thumbs up for that rule change). Will take it for a drive and see if that makes any noticeable difference in feel. If not I will try swapping back to the original non-V 4wd computer. And if that fails will pull the rear diff out and play with that.

After that it will be time to look for the matches!

[No Crust Racing]

Keep us posted

Despite being flat out with building a house, work, preparing to go on our cruise, moving house etc.. If you need me too I can look after the car for a while and conduct some scientific testing and report back on what I find.

I'm just that generous

[paulR32gtr]

Guys whats the current ruling on rolling guards. Reg's seem to suggest it's not legal.

I have some 18x9.5 +22 te37's going on mine so should fit ok, but before it gets painted I wanted to make as much space in there as possible.

I'm not looking to increase track at all for Targa events, only maybe in future for track days.