260DET Posted September 10, 2003 Share Posted September 10, 2003 After another track test day to see how the handling has changed as a result of various mods, the results were quite encouraging. The only substantial problem now apparently remaining is some mid corner understeer. The car brakes, turns in nicely and feels well balanced. But in slower corners of 90 degrees or more there is some understeer in the middle part of the corner, before the last part of the exit line when full power can be applied. The present front alignment specs are 2.2 degrees castor, 3.1 degrees camber and plus 1mm toe. If my understanding is correct, more castor will induce more turn camber, at the point when a bit more camber is required to aid front grip. Correct? On faster corners which require less steering input the car is fine, if it starts to run wide a little when the line is not quite right, backing off the accelerator just a bit brings the front in tighter immediately with no loss of overall stability. So the setup is just about there. Any suggestions? Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted September 10, 2003 Share Posted September 10, 2003 Adjust the easy stuff first. Play with tire pressures, shock settings if adjustable and sway bar adjustment if possible. One of the neatest tools for quick assesment of how the tires are working is a pyrometer. You can check tire temps from inside to out side and get a feel for what part of the tire is doing the most work and make any adjustments based on those readings. It is not a cure all but it does give some direction as to which way pressures should be adjusted and possible clues toward changing suspension settings. Mark Quote Link to comment Share on other sites More sharing options...
katman Posted September 10, 2003 Share Posted September 10, 2003 Generally a mid corner ("coasting", or the period between the transitions of brakes off and throttle full on) understeer indicates not enough rear spring or bar, however, slow corners in Z's migrate to understeer so yours is not a unique problem. Even though your turn in is good you might consider a smidge more static front toe out, say to 2mm total, because of the Z's nasty bump steer characteristics. We've found this to fix low speed corner understeer without affecting high speed balance. If that doesn't fix you a smidge more rear spring or bar until the high speed corners scare you. Shocks probably not it because you didn't say anything bad happens in the transitions. I've always felt castor is overrated and the effects of changing it on a Z too subtle to notice for weekend warriors. Driver technique sometimes needs to be modified since we don't have the luxury of aerodynamic devices and other stuff. What tires are we talking about and how much horsepower? Quote Link to comment Share on other sites More sharing options...
johnc Posted September 10, 2003 Share Posted September 10, 2003 > The present front alignment specs are 2.2 degrees castor What katman said except I think castor is more important then he does, especially if you're using offset lower control arm bushings. You can run the offset bushings up to reduce some of the bumpsteer and then use castor to retain the camber curve. I've never measured an early Z with that low a positive castor number (2.2). 6 degrees positive is a good number to shoot for. Congratulations on being sensitive enough to spot these issues in a Z. Quote Link to comment Share on other sites More sharing options...
260DET Posted September 11, 2003 Author Share Posted September 11, 2003 Thanks all for your suggestions. A touch more toe out and more castor it is then. The more rear sway bar suggestion is pretty perceptive because on looking at the small cable ties I'd put around the shock shafts it appears that she has been on the bump stops at the rear, presumably during cornering. So the rear bar will go up a bit in size too, even though the aim has been to keep the rear as soft as possible to aid power down. Quote Link to comment Share on other sites More sharing options...
johnc Posted September 11, 2003 Share Posted September 11, 2003 If you're on the bump stops in the rear you need more compression damping and/or more rear spring. You may also want to shorten your bump stops. But, if you're REALLY on the rear bump stops you would experience sudden and dramatic oversteer. Quote Link to comment Share on other sites More sharing options...
260DET Posted September 12, 2003 Author Share Posted September 12, 2003 Yes John, it was a bit of a surprise to see that she had been on the rear bump stops because there was no dramatic oversteer. The stops are those soft foam poly ones though. The present springs are 300lb front, 275 rear with a reasonable amount of suspension travel. So we are going to take the car to an industrial estate, hopefully deserted over the weekend, and try a few things. Use a video camera to study suspension angles and body attitude in slow corners. Quote Link to comment Share on other sites More sharing options...
Afshin Posted September 12, 2003 Share Posted September 12, 2003 Hi, I found this site a little while back, they have a great variety of pyrometers, some very cheap along with a lot of nice technical info related to them. Considering the low cost you may consider one for your next outing to the industrial estate. http://www.longacreracing.com/articles/art.asp?ARTID=16 Also can you give some more info relating to your current suspension set up i.e. tire type and size, control arm bushings new, old, strut bar?... Quote Link to comment Share on other sites More sharing options...
johnc Posted September 12, 2003 Share Posted September 12, 2003 Try swapping the front and rear springs. Quote Link to comment Share on other sites More sharing options...
260DET Posted September 13, 2003 Author Share Posted September 13, 2003 Try swapping the front and rear springs. Thought you might say that John. Did some measurements on the apparent actual suspension travel used during the track test and the available suspension travel that are interesting, jacking up under one suspension unit until that particular unit just takes the load of the relevant car end, front or rear, and using that as the 'available' measurement. NB the measurements are up to contact with the soft bumpstops which compress a further 20mm approx. Front: 65mm available, 50mm used. Rear: 50mm available, 50mm used. So, both front and rear used 50mm suspension travel on the test, NB 50mm equals 2". So, given the good overall handling, power down and braking performance during the track test the springs will stay the same, for the moment anyway. Alfshin, both front and rear suspensions are fully adjustable and in excellent condition. Tyres are Dunlop RA1's soft compound 205/55R15, a great road legal tyre, better IMHO than Advan's or Toyo's which have been used in the past. And yes, a slightly wider tyre would help. Present sway bars are 22mm front, 14mm rear. Subject to the industrial estate test, I'm looking at going up a bit on the rear sway bar size as well as tweaking the front end as previously mentioned. Another item, Dave Weitzenhof's US website states that at low speed more front shock rebound reduces power on understeer. Something else to think about as I'm using adjustable rebound Koni Sports. Quote Link to comment Share on other sites More sharing options...
260DET Posted September 15, 2003 Author Share Posted September 15, 2003 Try swapping the front and rear springs. Take 2. That is what we are now going to do for the next test. Once the springs are sorted out then look at sway bars, then tweak the other adjustables. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.