JustinOlson Posted May 20, 2009 Share Posted May 20, 2009 I will be welding in my camber plates to the tubular front end I'm currently designing. I've been doing a lot of reading regarding caster, camber, ect. I'm wondering how much caster is too much? I will be running a subaru power steering rack, so I'm not really concerned with the increased steering effort. I just figure there is a point of diminishing returns here. I'm curious what you guys think. Currently I'm shooting for around 10 degrees as my target. edit: I will be using radial FA tires. 9.5" front 12" rear. I will be doing autocross and track days with the car. Justin Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted May 20, 2009 Share Posted May 20, 2009 Anything past 90 degrees is WAY too much. I normally hear around 5-7 degrees on fast cars, and I've heard some drag racers like closer to 10 degrees because it helps with the straight line stability. Quote Link to comment Share on other sites More sharing options...
johnc Posted May 20, 2009 Share Posted May 20, 2009 Make it as adjustable as possible within a +3 to +10 range. There's no one number that's the best and sometimes more is not better. More caster increases tire scrub which slows the car down in a corner. This is a bad thing for horsepower limited cars and cars racing in a spec series. Conversely, more caster increases the weight on the outside rear tire in a corner improving traction and allowing more throttle coming out of the corner. That helps high horsepower, traction limited cars. I used between +5 and +9 degrees of caster on my racing 240Z depending on the track and the speed of the important corners. A track like Willow Springs with high speed corners required less caster then a tight Buttonwillow configuration. The trend in new performance cars is towards more caster. My 350Z came from the factory with +9.1 degrees of caster. Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 I was thinking, as I increase caster, I will need to move the steering rack back to not negatively effect ackerman. This is getting more complicated then I first imagined. Fun Fun... With the camber plates I have, I won't have any caster adjustment up top. I will have to change the length of the TC rod to adjust caster. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 20, 2009 Share Posted May 20, 2009 Moving the rack back gives the tie rods a bit of a bell crank like effect which increases Ackerman. I can't see how moving the control arm forward doesn't do the same thing. Am I missing something? Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted May 20, 2009 Administrators Share Posted May 20, 2009 (edited) Am I missing something? You speak the truth. Lengthening the the TC rods increases Ackerman. Edited May 20, 2009 by RTz Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 Moving the rack back gives the tie rods a bit of a bell crank like effect which increases Ackerman. I can't see how moving the control arm forward doesn't do the same thing. Am I missing something? I'm moving the top of the struts back, reducing ackerman. If I where moving the ball joint forward, I would be increasing ackerman. I might do a bit of both now that I think about it. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 20, 2009 Share Posted May 20, 2009 Moving the top of the strut back has no effect on Ackerman at all. Ackerman is determined by the relation and angles of the tie rods to the rack and ball joint. Moving the top back has no effect. Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 ...thinking... It would have a slight effect on ackerman due to the steering arm attachment point being angled more. It would move that point up and back slightly. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 20, 2009 Share Posted May 20, 2009 The ball joint should allow for the movement of the strut without affecting the angle of the steer knuckle. Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 The steering knuckle attaches to the bottom of the strut rigidly. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 20, 2009 Share Posted May 20, 2009 Dammit. Now I think that I may be wrong on that. Regardless, I think a 5 or 6 degree change in the angle from stock is going to make a very minimal difference in the Ackerman, probably more than compensated for my by lengthening the TC rod even just a little bit. EDIT--You got there before I did. Still don't think it's worth worrying about. I'd be much more worried about installing short steer knuckles like in the picture and it's effect than I would about tilting the knuckle back. Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 Yeah, I see how changing the length of the TC rod has a much greater effect on ackerman then simply tilting the struts back about the ball joint. I just wanted to talk through it to clear things up in my mind. Quote Link to comment Share on other sites More sharing options...
johnc Posted May 20, 2009 Share Posted May 20, 2009 Medium and high speed corners need less Ackerman then low speed corners plus you use less steering angle in medium and high speed corners. I all comes back to what you're planning to do with your car. There's no one Ackerman, Caster, SAI, Camber, Toe, Ride Height, or other setting that works well everywhere. Build adjustability into the suspension and steering and then be very, very willing to use that adjustability. On my old race car there were a lot of things that were not "ideal" per the books and the Internet message boards. But it was very adjustable and I kept fiddling with things and taking notes all the time. Inside front tire temps too hot on a fast track? Add 1/4" wider front wheel spacers and reduce caster by 2 degrees. Lap times drop, tire temps even out, slightly loose. Remove front wheel spacer. Lap times increase, inside rear tires get hotter, loose. Add 1/2" front wheel spacer. Laps times increase more, front tires get hotter, rear tires cool down, push. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted May 20, 2009 Share Posted May 20, 2009 Keep going. Dave Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 Thank you for the response John. I do realize that there is no "best" I'm trying to get to a good middle ground that I can fine tune from. Anyone have a recommendation for a good camber + caster plate? It seems to me that it would be nice to have the caster adjustment up top so that I'm not moving the wheel in the wheel well. Too many other variables change IMO when you just change the length of the TC Rod. Justin Quote Link to comment Share on other sites More sharing options...
260DET Posted May 20, 2009 Share Posted May 20, 2009 Went through this Ackermann stuff years ago with the S30, from memory there is not much Ackermann possible with a front mounted rack like the S30's. Although the angles can be tweaked a bit by bending/altering the steering arms, which I did gaining a slight improvement with turn in on slow corners. But most rear mounted racks are set up so there is Ackermann, its much more practical in terms of available space to locate the relevant steering components for that purpose. Quote Link to comment Share on other sites More sharing options...
JustinOlson Posted May 20, 2009 Author Share Posted May 20, 2009 There is plenty of ackerman to be had in a front mounted rack layout. You just need to move the steering rack back. I can model it up for you in CAD later today. Justin Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 21, 2009 Share Posted May 21, 2009 Are you going to use a stock crossmember? If so, you're limited by that piece. I've got my rack attached directly to the crossmember and although I haven't measured the Ackerman I'm pretty sure it's less than 100%. I would also venture a guess to say that Richard was at less than 100% with his modified steer knuckles. With both you might get close. Some formula cars are running 150-200%, so there is an argument for going more extreme... Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted May 21, 2009 Administrators Share Posted May 21, 2009 How BMW gets 'some' Ackerman in a front steer package (E30 M3)... I can't say what percentage the E30 has. What I can say is, a while back I CAD'd up a rearward movement of an S30 rack (around 1.25"), and a surprisingly notable improvement was made. Not anything close to 100%, but enough to warrant consideration, in my opinion. Quote Link to comment Share on other sites More sharing options...
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