Tony Posted July 25, 2009 Report Share Posted July 25, 2009 Sounds like trying to evolve a 4WD set-up could get tricky To stock there's nothing to aim for.... Cambers around zero, toe's zero, castors fixed, so it's a boring calibration in my opinion. Quote Link to comment Share on other sites More sharing options...
CIH Posted July 26, 2009 Author Report Share Posted July 26, 2009 You have the Roll Center, which is the exact instant center that the sprung mass will rotate about at the given position of the individual front or rear suspensions. The Roll Center, itself, is an Instant Center meaning it moves up and down, and even side to side, depending on the geometry, as the suspension cycles. Then you have the vehicle roll axis, which is the line connecting the two roll centers. This basically depicts how the entire spring mass will roll, at a particular position of the suspension. Remember, since the Roll Centers move, so will the vehicle roll axis. Do you mind if we go through roll centre again ? I'm not totally clear on what you meant exactly. We can take it in the contect of a specific car if that makes it easier to exlain ? Quote Link to comment Share on other sites More sharing options...
Tony Posted July 26, 2009 Report Share Posted July 26, 2009 Study this Quote Link to comment Share on other sites More sharing options...
CIH Posted July 26, 2009 Author Report Share Posted July 26, 2009 Presume the Instantaneous Centre (IC), and by action RollCentre (RC), are dynamic and react to dynamic camber changes ? Quote Link to comment Share on other sites More sharing options...
Tony Posted July 27, 2009 Report Share Posted July 27, 2009 Very much so.... In truth roll centres are more for the design stage. You need some powerful software to calculate matter of fact roll centres. For us it's more a case of be aware of the centre and the couple when working with lowered cars. Quote Link to comment Share on other sites More sharing options...
CIH Posted July 28, 2009 Author Report Share Posted July 28, 2009 Yeah but it's interesting to see how SAI can have dynamic infulence. So if I were adjusting camber on an E46 3 -series then obviously I'd be manipulating static IC. Are there "ideal" positions i e; not too high or low ? I assume there's a roll centre for the rear too ? or something of a similar vein ? Quote Link to comment Share on other sites More sharing options...
Tony Posted July 28, 2009 Report Share Posted July 28, 2009 Scrub radius is also known as steering offset, and scrub geometry. It is the distance between 2 imaginary points on the road surface - the point of center contact between the road surface and the tyre, and the point where the steering-axis center-line contacts the road surface. * If these two points intersect at the center of the tyre, at the road surface, then the scrub radius is zero. * If they intersect below the road surface, scrub radius is positive. * If they intersect above the road surface, scrub radius is negative. The effect of scrub radius - positive or negative - is to provide a turning moment which attempts to turn the wheel away from the central position, when the vehicle is in motion. On a rear-wheel-drive vehicle with positive scrub radius, the vehicle’s forward motion and the friction between the tyre and the road causes a force which tends to move the front wheels back. This would cause the wheels to toe-out. If it has negative scrub radius, the front wheels again tend to move back, but this time, they toe-in. On front-wheel-drive vehicles, the opposite occurs. Positive scrub radius causes toe-in, and negative causes toe-out. During braking, on any type of drive, if braking effort is greater on one side of the vehicle than the other, positive scrub radius will cause the vehicle to veer towards the side with the greater effort. Negative scrub radius will cause the vehicle to veer away from the side of greatest effort. How much it veers depends on the size of the scrub radius. This is why, vehicles with a diagonal-split brake system have negative scrub radius built into the steering geometry. If one half of the brake system fails, then the vehicle will tend to pull up in a straight line. Since the offset of the wheel rim determines where the centerline of the tyre meets the road surface, it is important that the offset is not changed if wheels are being replaced. Changing the rim offset changes the scrub radius, and also the predictability of the vehicle handling, if brakes should fail. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 1, 2009 Author Report Share Posted August 1, 2009 how does all this tie-in with the couple between front and rear camber (and by action over/understeer) ? Quote Link to comment Share on other sites More sharing options...
Tony Posted August 1, 2009 Report Share Posted August 1, 2009 how does all this tie-in with the couple between front and rear camber (and by action over/understeer) ? One of the duties of camber is to maintain the tyres contact patch during the suspensions compression and droop, in effect the camber actually simulates the roll curve by degrees, meaning the compressed radii of the suspension linkages and the dynamic camber own the same curve. The best way to maintain the contact patch is to reduce dynamic roll but exercise a more dramatic static camber position since the actual dynamic camber gains will be less. A full "cover all" explanation of "over/ under steer" would go on for pages but an overview is this. Our model is a track car, RWD, fully braced and a low COG. The cambers are -4d front and rear. This car on the track has little or no camber gains during transition and unremarkable track times? If we changed our model to front camber -2d, rear -4d then corner-in the front transition and lat acc would compromise the grip limits (pushing) the car, the roll centres and camber gains front/ rear are unbalanced. If our model has cambers, front -4d rear -3d, then corner-in during transition, lat acc the grip limits would be compromised at the rear, the camber curve is not maintaining the radii, this is desirable because the rear is now "loose" allowing the driver to progressively manipulate the grip limit. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 8, 2009 Author Report Share Posted August 8, 2009 The BMW I spoke to you about today, I noticed the rear camber would move about -/+ 10 minutes when I was looking at rear toe tendencey. Quote Link to comment Share on other sites More sharing options...
Tony Posted August 8, 2009 Report Share Posted August 8, 2009 The BMW I spoke to you about today, I noticed the rear camber would move about -/+ 10 minutes when I was looking at rear toe tendencey. The camber will change, it's the natural curve. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 8, 2009 Author Report Share Posted August 8, 2009 "camber tendencey" ? We've discussed camber curve already, but can this be accurately observed with the car on the geo rig ? Quote Link to comment Share on other sites More sharing options...
Tony Posted August 8, 2009 Report Share Posted August 8, 2009 "camber tendencey" ? We've discussed camber curve already, but can this be accurately observed with the car on the geo rig ? No you was saying about the camber change during the toe tendency test, I'm just saying that's natural. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 20, 2009 Author Report Share Posted August 20, 2009 That's right. I'm wondering if observing the camber change can be of use ? As it is with toe ? Quote Link to comment Share on other sites More sharing options...
Tony Posted August 20, 2009 Report Share Posted August 20, 2009 That's right. I'm wondering if observing the camber change can be of use ? As it is with toe ? No not really.... Toe tendency only needs mild compression of the suspension whereas camber gain needs a heavy pull down to map the transition. Domestically this tells you little and for the track car the gains due to coil rates would be little to nothing. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 21, 2009 Author Report Share Posted August 21, 2009 Pretty much what I figured. I only saw about -/+ 10 minutes and I presume the actual range is much larger ? Got another Q too. We've discussed how camber puts more weight on to the contact patch. I presume this increases satuaration limits ? If so, how about toe ? Quote Link to comment Share on other sites More sharing options...
Tony Posted August 21, 2009 Report Share Posted August 21, 2009 Domestically the objective of static toe is a calculation of dynamic bushing compliance, ultimately the objective is a dynamic zero. Modified or race cars can have a static zero toe.... Rule of thumb for all cars it to minimize how busy the contact patch is. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 21, 2009 Author Report Share Posted August 21, 2009 but toe has no direct impact upon saturation limits ? Quote Link to comment Share on other sites More sharing options...
Tony Posted August 21, 2009 Report Share Posted August 21, 2009 but toe has no direct impact upon saturation limits ? Toe the rear out 30' or so and see..... Toe is an interference angle. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 29, 2009 Author Report Share Posted August 29, 2009 I made some interesting toe observations doing FF trackings today. Two yr2000-ish Astras. One, a cooking model, had mimimal toe tendencey, just a smidge toe-out on pull down. The other, an SRi, saw almost 2mm (about 18" I think ?) toe-out on pull down. I forgot to do the first but the SRi saw about +1mm when I lifted the front. I would have guessed the SRi would have seen less migration due to "sportier" suspension. Quote Link to comment Share on other sites More sharing options...
Tony Posted August 29, 2009 Report Share Posted August 29, 2009 That's a huge variation.... That's why it's important to ascertain the tendency when you set the toe..... each and every car will have a variable. Quote Link to comment Share on other sites More sharing options...
CIH Posted August 30, 2009 Author Report Share Posted August 30, 2009 I followed the tendencey on pull down (braking) but I was a little concenrned as it meant +2mm static, which is a fair bit off factory parallel ("0mm") ? Last time I did tendencey I pulled the lower grille off of a facelift mk1 focus! Luckily it just clipped on/off. Quote Link to comment Share on other sites More sharing options...
CIH Posted September 7, 2009 Author Report Share Posted September 7, 2009 AARRGHHH!! I hate 3series! I like Astras though! Quote Link to comment Share on other sites More sharing options...
Tony Posted September 7, 2009 Report Share Posted September 7, 2009 AARRGHHH!! I hate 3series! I like Astras though! What's gone wrong now Quote Link to comment Share on other sites More sharing options...
CIH Posted September 8, 2009 Author Report Share Posted September 8, 2009 Just takes bloody ages drilling out those damn strut top screw-tab things. And this one fought me all the way so it took like 2hrs total. Astras are good. The rear beam has like 20" toe adjustment if you shove it hard enough. It didn't actually need that much, but it was interesting to see. Quote Link to comment Share on other sites More sharing options...
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