Front - Rear track width and transition while cornering..?

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killaz
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Front - Rear track width and transition while cornering..?

Post by killaz » Sun Oct 23, 2016 2:39 pm

Hi!
Need some advice from you track wolves about this topic since I have quite nervous rear end. Do you build your cars within factory proportions (front/rear) or you're changing track width?

Current setup on my track car:

Front: Coilovers 80kg/cm springs + factory torsion bars, Camber -3, Toe out 1mm, long nose upper ball joints, factory anti roll bar, tyres toyo r888 195/50-15'

Rear: Coilovers positioned next to rear wheels 60kg/cm springs, toyo r888 215/40-15'

Front is +20mm spaced per side / Rear is +24mm spaced per side

Any thoughts about current setup?

Cheers!
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Re: Front - Rear track width and transition while cornering.

Post by 75evo » Thu Oct 27, 2016 9:30 am

I don't know how stiff my rear springs are (Beninca), but it's not not too stiff. I've driven a very stiffly sprung rear Milano and all it wants to do is to slide the rear end. Have you tried slightly softer rear springs?


My car also has about -2 degrees of rear camber, 17 x 8 ET35 rear wheels and 6mm rear spacer. I think when braking hard it can get a bit nervous but cornering is really good, as good as S2000s with 245 rear tires.
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killaz
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Re: Front - Rear track width and transition while cornering.

Post by killaz » Thu Oct 27, 2016 12:58 pm

Softer in the rear will be too soft, Im afraid... I think I should increase rate of front springs since rsr coils come with 125kg/60kg rear.

Cambered dedion is on my wishlist!

Your front track width is stock?
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Re: Front - Rear track width and transition while cornering.

Post by 105gta. » Fri Oct 28, 2016 2:39 am

Hi Killaz, seems most of your mods are heading in the right direction.
From my experience you can't go too wide, the rear end of transaxle cras have a very high roll Centre and respond very well to lowering it. If that's an option I highly reccomend it. Otherwise remove as much flex as possible. Rose jointed watts link and sz style dedion bearing has a huge effect on feel and stability.
If you play with roll Centre you'll have the opportunity to add some camber if you're cutting the dedion, I can't say for sure which had the most positive response but before my front end mods I did the rear end. -2deg camber and lowered roll Centre,sz bearing and full rose jointed watts link. Instant confidence in rear end behavior. Not that anything was worn out to start with I just never felt confident with it. Turn in and mid corner behavior was much crisper as well. the only thing I want to add now is a little toe in on the rear for more bite.
As you've already addressed the front roll Centre, I'd suggest the rear roll centre as a focal point for your rear end mods to match the setup.
You mentioned 215 wide tyres, what rim width are you running?
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Re: Front - Rear track width and transition while cornering.

Post by killaz » Fri Oct 28, 2016 9:43 am

215/50 are on 7' wheels, they were better on 7,5' but i have some clearence issues.

On the rear, i have SZ uniball + poliurethane bushings all arround and I'm thinking to switch watts levers to uniballs...

I've read some threads about changing RC on the rear but it is not clear to me how they calculate rear RC and what they modify there, so I'm not conviced that it should be touched. I'll probably go only with camber & toe in.

Can I change camber and toe with welding only (without cutting)?
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Re: Front - Rear track width and transition while cornering.

Post by KevinR » Fri Oct 28, 2016 1:04 pm

Hi , to start off I would either raise the front ride height or lower the rear ride height . This will transfer more weight to rear but induce more under steer . Reason I would play with ride heights is you got all the kit the does make a really good handling 75 . I spent a morning watching Ron setting up his RS kit on a car in South Africa many years ago which was running a24v motor and he spent most of the time tuning the rear spring platforms after getting tyres pressure to his spec .
What is the current rake on your car . Measure under firont and rear jacking points .
Cheers
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Re: Front - Rear track width and transition while cornering.

Post by killaz » Fri Oct 28, 2016 1:46 pm

Nice suggestion Kevin, but I removed front coilovers to switch springs for harder ones, so now is not possible to check previous heights. Do you have inputs regarding that topic for future setup?
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Re: Front - Rear track width and transition while cornering.

Post by KevinR » Fri Oct 28, 2016 2:16 pm

Well theoretically you have just made car higher in front by going harder springs . Another thing you can do is introduce rear anti roll bar gap which means you slacken off the drop links on the rear anti roll bar . This small gap will let the rear roll more on corner exit as you put down power and reduce the snap over steer . So I would play between those two factors as discussed in last post ie adjust rear spring heights and then slacken rear anti roll bar a bit .
I had 65 kg rear on my old race car with 180kg front which worked perfectly for me . Even the chap who bought my old car has found it perfect for him . It had 2cm difference in rake from front rear ie front lower ( I think they call that positive rake ) . I was just following Rons set up tips .
I also had my rear tyres 0.2 of bar softer in cold than front on the semi slicks as I found this made them more equal when I came off track and took hit temps .
My big issue was turn in on hair pins as I used to set up with touch toe in while it should have been 1mm overall toe out when checking what the old touring car guys used to do in the 80s .
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Re: Front - Rear track width and transition while cornering.

Post by killaz » Sat Oct 29, 2016 12:50 am

Kevin, these are all great informations and I am really thankfull! Comparing to your data, I had difference for rear tyres which had to be 0,3 bar softer to grip better - obviously because of bad f/r setup. Front was set -3 camber and 1mm toe out, so it was pretty good in hairpins, but one tyre took more "beating" on the inside edge.

Still lot to do to prepare for next season, but now I have excellent basics!
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Re: Front - Rear track width and transition while cornering.

Post by KevinR » Sat Oct 29, 2016 2:09 pm

Also if you have gone harder on front springs and depending on your front tyre temps taken immediately as you come into the pits on the outer edge , middle and inside of tyre you could possibly bring in camber to 2degrees which will help more in braking . This is the data you need to get and not to difficult . Just need a tyre temp gauge now .
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Re: Front - Rear track width and transition while cornering.

Post by 105gta. » Sat Oct 29, 2016 3:09 pm

Killaz, to answer a couple of your questions.
The rear roll Centre is the central pivot for the watts link, if you move this you move he roll Centre. Standard it is on the top side of the dedion, most racers move it somewhere between the Centre and lower level of the dedion transverse tube. By lowering it, you will induce slight more rear roll placing more weight on the outer wheel,
You asked about welding only to achieve toe/camber change. At a minimum I think you have to cut half way through the tube and Re-weld to achieve any real change. It may be possible but I'm not sure how successful it would be. If you're prepared to cut may as well go for it. Do it at the Centre and address the roll Centre at the same time ;) can be made to be adjustable to with multiple hole heights and using a large bolt through the dedion instead of one fixed/welded pin!
You mentioned clearance issues with 7.5" wheels. Could you try different spacers with them.
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Re: Front - Rear track width and transition while cornering.

Post by Duk » Sat Oct 29, 2016 9:27 pm

Without crunching the numbers, you're spring rates will have the car with very high natural frequencies both front and rear. The rear natural frequency would be VERY high!

What rear anti roll bar are you using?

With the long nose top ball joints, have you done anything to correct the bump steer?
On my car, a similar set up caused noticable toe in with bump travel and that causes bump over-steer. Everything I read about toe in with bump travel/bump over-steer, said it should be avoided.

And what dampers/shock absorbers are you using?

Personally I think your car is way over sprung, especially at the back.
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Re: Front - Rear track width and transition while cornering.

Post by KevinR » Sat Oct 29, 2016 11:49 pm

Good point written above on back being to over sprung compared to front if you only had 80kg/cm on front plus torsion bars which is from previous discussion is about 130kg/cm total front . Ron used to start with 50kg/cm rear plus 115kg/cm front coil over plus torsion bars ( 165kg/cm total ) . This was with 30mm front antiroll bar .
I ended up on my gTV 60kg/cm rear and 200kg/cm front coil over ( no torsion bar ) plus a thinner 28mm front anti roll front .
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Re: Front - Rear track width and transition while cornering.

Post by killaz » Sun Oct 30, 2016 4:34 am

105gta: Thank you, those mods sounds interesting. But first, I'll have to adress general spring setup, ride height, camber etc...

Duk: Rear anti roll bar is factory bar, I'm thinking of removing it completely since I've changed position of shocks and springs - now they are located next to rear wheels. Shocks are revalved Bilsteins to spec for 170/60 springs. Bump steer is not corrected yet. Are there any proportions available or I'll have to measure and draw diagram to determine correction needed?

Kevin: I was thinking of measuring temperatures too and again, excellent info regarding starting setup! Yes, front springs were soft and rear was over sprung as Duk said.

Guys, you've opened my eyes, thanks a million! :D
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Re: Front - Rear track width and transition while cornering.

Post by Duk » Sun Oct 30, 2016 11:19 pm

killaz wrote:
Duk: Rear anti roll bar is factory bar, I'm thinking of removing it completely since I've changed position of shocks and springs - now they are located next to rear wheels. Shocks are revalved Bilsteins to spec for 170/60 springs. Bump steer is not corrected yet. Are there any proportions available or I'll have to measure and draw diagram to determine correction needed?
Can you share a photo of your rear spring set up? It sounds interesting and a lot like something I have been thinking about too.

As for bump steer correction with the long nose top ball joints, I slightly modified an R32 Skyline (actually any of the Silvia/180/200SX or R chassis Skylines with 14mm track rod ends) bump steer correction kit.
The taper of the stud had to be machined down to suit the Alfa's tie rod end hole and the inner track rod end threads had to be cut a bit shorter.
The I positioned the outer rod end with a 5mm spacer above it and made a 1 piece sleave to go down to the lower nut. Remaking/modifying the stud would have been nicer, but.................... :P

Any ways, when bouncing a laser mounted on the brake rotor, off a mirror and onto a vertical plate also mounted on the rotor, it showed very little bump steer. And I probably could have got it even better again if I had wound the caster angle back to the factory 4.5* rather than the (from memory) either 5 or 5.5* of caster.
I never took any before and after measurements, but the initial bump steer was so significant that you could see it with out any sort of measuring set up and working the front suspension with the torsion bar in place and no engine in the car.
The photo of the bump steer correction is during the test and measuring.
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