I'm quite lucky to have a few abandoned runways near where I live, and a few hugely long private roads (not a joke here, I could even googlemap them!) so like to indulge in the odd bit of high-speed tomfoolery. I've noticed a problem recently though, the car used to feel stable up to 170+mph, now at 145-150mph the cars gets quite unstable, if feels like the front becomes very light, what could be the reason for this? The only changes I've made are switching from FK452 245 front, 275 rear to Nankang (sorry, but money was tight) 235/255 rear. I also changed the front bumper from the pre-facelift to the post-facelift, and removed the rear spoiler.

I'm going to assume it's the tyres, as the 'kangs are only rated for 155 and aren't renound for their grip, but could the bumper or lack of spoiler cause any downforce issues?

Why did you drop the width of your tyres?

budget at the time, I saw a few GTR owners were happily running 255's front and rear on wider wheels than mine with no complaints so assumed it wouldn't have a major issue. Indeed at normal daily speeds and driving there isn't any problems.

I'm strongly considering upgrading from 17 to 18 and getting 255 front and 275 back on the rear, but its one hell of a lot of cash

I have exactly the same problem. I kept my old rear subframe, which I'm fitting this afternoon. I suspect the bushes or a Hicas problem with the existing one. If this doesn't sort it, I'm going to check out the front of the car. Hope you find it too!

Strangely enough, I've just replaced the 18" wheels my Zed came with for a set of 17" so I've gone from 225/255 F/R to 235/255 and I noticed the car wandering a bit at motorway speeds the first time I had it out.

 

I put it down to a bit of tramlining, but I'd never noticed before. Now you've got me worried about a busted HICAS.

Yeah ,the rear hicas bushes etc. could be a good point to check

Err can you just rewind a bit and tell us more about these runways you speak of lol.

 

Where are these runways?

North highlands, littered with unused aerodromes. Also quite a lot of industrial roads which are v. v. long. Used to use them as a drag strip when I was a younger lad.

I think the tyres are definitely your problem mate!

 

Dropping the width won't make it any worse - the car should perform fine at high speed on stock tyre sizes. And whilst the spoiler was supposedly added to improve downforce (and consequently traction) on the TT, I'm not sure it would make much difference........

 

........Although at 170mph there's a chance it might - Nissan knew what they were doing I guess.

 

But unfortunately cheap tyres will make all the difference to the car's handling. When I bought my first ZX it had almost new Avons all round and high speed stability was suspect. Switching to Goodyears (and later Dunlop SP9000s) made is handle like a different car - it was way better!

 

Also make sure the wheels are properly balanced. Even a little bit out of balance will be magnified at 150mph+ but may go unnoticed at more "normal" speeds. Finally have the full 4 wheel allignment carried out as well.

 

Richard:chef:

Thanks Richard, I'll have all that looked at and get back with results.

I had all four wheels aligned, but the problem was still present. Today I noticed two bleed screws on the Hicas actuator - might be worth opening them with the engine running just in case there is air in there. Be careful if you do because I sheared one of them off today and had to weld a bolt to what was left of it to get it out!

Surely aerodynamics are going to be the biggest factor at that speed. What was the spoiler that you had on before? Also I dunno how confident I'd feel on Nankangs at that speed...

Sounds far more likely to be the spoiler and front bumper change are to blame for the high speed problems to me.

I can't see how the facelift bumper could be less aerodynamic than the prefacelift though... the spoiler was the standard one, it's now running spoilerless.

spoiler to spoilerless would make a big difference at 170mph - id imagine that with no spoiler there would be lift at the back end at those kinds of speeds!

Yeah I would say the spoiler would have an impact up at those speeds as well. Interesting! I assume you have repaited your tail gate? You dont have a spare do you.... pop a normal tailgate back on with the spoiler and see if the problem is cured... assuming you can of course. Would make an interesting experiment!

Sounds far more likely to be the spoiler and front bumper change are to blame for the high speed problems to me.

 

Regarding the front bumper - you mention the "facelift" one by which I assume you mean you now have a '99 spec?

 

If so then I'd doubt that would make a difference as it was still developed by Nissan specifically for the car. But everyone seems to agree that the rear one will make a difference.....!

 

Richard:biggrin:

Hehe, who knows - I don't have a spare to try.. I haven't painted up the tailgate yet - waiting until the money is easily available rather than living tight on cash. The only other spoiler I'd really want to put on is a veilside EC-1 and that's impossible with the cash I can spare.

Well, irrelevant of how big a pinch or salt you take it with, JC on TG did say that the new Jaaaaaag', (OK, it was new a while ago!), suffered from having a lip spoiler fitted to it's bootlid so as to mount a brake light. Apparently it made the nose light at speed due to the extra push on the tail.

 

My only referrence is to a Supra I had a number of years ago which was very twitchy with a hard tailwind but one of the most planted cars I've driven at high speed in still conditions.

Well, irrelevant of how big a pinch or salt you take it with, JC on TG did say that the new Jaaaaaag', (OK, it was new a while ago!), suffered from having a lip spoiler fitted to it's bootlid so as to mount a brake light. Apparently it made the nose light at speed due to the extra push on the tail.

 

My only referrence is to a Supra I had a number of years ago which was very twitchy with a hard tailwind but one of the most planted cars I've driven at high speed in still conditions.

 

True, but that was on a car designed to be spoiler-less. From factory the turbo 300zx had a spoiler. (although it could be optionally removed.)

 

Have you checked tyre pressures?

I think I might see if I can get an R34 GTR spoiler then customise it to make it wider. I'd sooner not have a spoiler, or have a ducktail lip but again, feesably they work out to be very expensive and I've no idea about aerodynamic requirements! My cars not 'nuts' enough to pull off a carbon wing either.

Erg.

 

Wish I hadn't bothered. By mentioning the front bumper I was pointing these are the two arodynamics that have been altered on the car and these at 170+mph are far more likely to be effecting things.

 

The new style bumper without a spoiler could cause a problem as it is designed for a car with a 99 spec spoiler not one without any spoiler at all, they are designed to work together as a whole so yes the 99spec being fitted "could" be causing a problem when it has no rear spoiler fitted.

 

Make sense?

Do companies really spend that much time on R&D on such items? I always figured it was more an asthetic alteration rather than an acctual aero enhancement

You'll most likely find it was redesigned primarily to assist airflow/cooling with the bonus of it being a way to spot the newer model.

 

All cars spend many many hours in wind tunnels and performance cars even more so. You cant change the front of a 170mph car without making sure it wont send your customers into a hedge at those speeds lol