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Strange transitioning understeer problem

Steve
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Steve Grand Master

Steve

Posted
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Posted

I have a 2 way, but the understeer actually reduced when I fitted it - probably because I could push harder through the corners under acceleration.

The thing is the understeer is occuring under constant speed cornering, not if trail braking (must admit, I try and avoid trail braking:))- and when I power on, it goes away as well. I could understand it if it was only under accel or decel.

Jay95R33 Grand Master

Jay95R33

Posted
Posted

Ahh cool,

In that case I'd say the tyres. Constant speed corners should (I think) have the car balanced, but of course as your cornering you'll wipe off a little bit of speed (due to the change of direction) which would push the weight bias forward, working your front tyres harder.

Anyway, enough of my ramblings..

J

ADM Contributor

ADM

Posted
Posted

Steve,

I got my kg/mm to lbs conversion all wrong. Your right 8k (450lbs) is not massive and is certainly what I would use on the front end of the skyline on the road. What I said before may still ring true in this case. Your spring rate, front to rear is significantly different. The front springs (even with the extra weight of the front end) will stop compressing, causing the front tyres to slip before the rear end. 3K (170lbs) is pretty soft and creates a large ratio difference between front to rear. Id also be surprised if that 2 way locker LSD isn’t causing some turn in understeer. Like a welded rear diff, turn in is traditionally shocking.

ADM

whatsisname Grand Master

whatsisname

Posted
Posted

Steve, I agree with the other guys on this one mate - I'd say a large part of your understeer issue is due to the differing grip levels of the Revspecs and RE540S's.

I also agree wholeheartedly with Pete (Fatz)... when the 540's do let go, they go BIG TIME!

Sydneykid Grand Master

Sydneykid

Posted
Posted

Hi Steve, I would do 2 things and 2 only, until you thoroughly test the results.

1. Same tyre compound all round, you have rear tyres that are capable of 3 seconds per lap faster times at Mallalla than your front tyres. Even them up and you can then make a sensible decision.

2. Remove the rear HICAS, as you know we have NO Skylines with the HICAS still on them. A computer is deciding the rear toe angles for you, this is never a good thing.

When you have done those 2 things, then test it. At the same time you need to check for bottoming out, mid corner understeer is typical symptom of the lack of front suspension travel. Put a cable tie tight around the shock shaft, so that when the shock compresses, it moves the cable tie to the lowest point. Then you see how much it travels, measure the distance and make sure it is not hitting the bump stops too hard. If it is, then raise the ride height and see if that makes any difference.

If you still have mid corner understeer then soften up the front bar all the way. If you still have mid corner understeer then I would recheck the front spring rate. As you know I never run more than 450 lbs in a 2wd Skyline, which is the same as you have. With the 27 mm front bar on full hard, it lifts the inside front wheel off the ground. I can feel it, so I back the front bar off a bit to keep it on the ground, then adjust the rear bar to balance the handling. You simply can't get less body roll than that, you need the tyres in contact with the track as much as you can.

Lastly I assume you are running the 24 mm adjustable rear bar, there are some 22 mm ones out there, but we only use them on GTR's.

Hope that helps

Sydneykid Grand Master

Sydneykid

Posted
Posted

Ooops I nearly forgot, I have to check in the box, but I don't think we have a rear spring rate lower than 190 lbs for the circuit race cars. I used 150 lbs on the drag GTST that I did, it worked a charm for that purpose. Maybe the 170 lbs is a little light, but 20 lbs (up to 190 lbs) is really fine tuning. If you can borrow a set of 200 lb springs and try them, it may be worthwhile. On high speed circuits, like Philip Island, we run up to 250 lbs, but it is one smooth track.

I may be able to find a set around 200 lbs for you to try, what dimensions are your springs? Top and bottom ID and free height, we use 90/75/250. They are ground flat both top and bottom. Let me know.

Sydneykid Grand Master

Sydneykid

Posted
Posted

Hi Guys, tyres sizes vary lots. A quick example, we use Yokohama A032R's and there are 2 sizes that work OK for us on 8" rims, 235/40/17 and 245/45/17. Now from the sizes you would say the 245/45/17's should be 245 mm wide, but they are 242 mm wide, (-3mm or 1.2%) so not too much difference. The 235/40/17's are 239 mm wide, (+4mm or 1.7%) again not too much difference. But what looks like a 10 mm difference in width on paper is in reality only 3mm and that is from the same tyre manufacturer and the same tyre type.

The Yokohama A032R's have quite rounded shoulders, typical for an "R" type tyre. I recently tried a set of Hankooks that were labelled 245/40/17 and they measured 254 mm because of the square shoulders (+9mm or 4%). So if I compare them to the 245/45/17 A032R's there is a 13 mm difference between 2 supposedly "245" tyres.

Bottom line, tyre sizes are really only "indicators" the tolerances seem quite large at up to 4%. If the widths are this varied, makes you wonder what the real world circumference differences are as that affects speedo accuracy.

Sydneykid Grand Master

Sydneykid

Posted
Posted
Interesting.

How did you find the hankooks?  Any chance of a review?

cheers

Sorry I can't, there is a secrecy agreement on that contract. They are not available for sale yet anyway. When they are, I think the contract releases me and I can then comment.

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