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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / March 2007Not your typical spark plugs & wrench type ??
My question concerns SAI(steering axis inclination): How much SAI is "too much"? I know this depends on other design and adjustment variables, especially camber & caster, but just curious, since I know that SAI helps the most to center a car(straight line) and doesn't have the same negative effect on backing up as does high degrees of caster. -ChrisCoaster > My question concerns SAI(steering axis inclination): How much SAI is > "too much"? I know this depends on other design and adjustment [quoted text clipped - 4 lines] > > -ChrisCoaster __________________ Hmmm. I was right. Too scientific for this NG. I'll ask elsewhere. > > My question concerns SAI(steering axis inclination): How much SAI is > > "too much"? I know this depends on other design and adjustment [quoted text clipped - 7 lines] > > Hmmm. I was right. Too scientific for this NG. I'll ask elsewhere. Bye On Feb 28, 6:59 am, <H...@nospam.nix> wrote: > > > My question concerns SAI(steering axis inclination): How much SAI is > > > "too much"? I know this depends on other design and adjustment [quoted text clipped - 13 lines] > > - Show quoted text - Steering axis inclination is set so that the axis passes through the lateral center of the tire's road contact area. If the tire hits something, it won't yank the steering wheel out of the driver's hands. People who put wide wheels with their attendant outward offset (or just offset wheels of regular width) on their cars often find them unpleasant to drive, as every little irregularity on the road wants to make the vehicle pull one way or another. Fashion over function again. Just like politics. Why would anyone want to increase SAI unless they wanted to recenter the axis on offset tires? And why mess with SAI if it's going to upset the camber? Is that scientific enough? Dan On Feb 28, 10:27 pm, Dan_Thomas_nos...@yahoo.com wrote: > On Feb 28, 6:59 am, <H...@nospam.nix> wrote: > Why would anyone want to increase SAI unless they wanted to > recenter the axis on offset tires? And why mess with SAI if it's going [quoted text clipped - 5 lines] > > - Show quoted text - ___________________ Sorry Dan(and others). It's just that I post to a lot of forums out of curiosity and a willingness to learn or enhance what I already know. It's frustrating when I ask a question and it takes days for a reply to show up, when most other posters' questions are answered within hours of being posted. Nothing personal. So basically SAI is designed to aim an imaginary steering axis through the midline of a tire specified for a specific vehicle - correct? Makes sense, and it's not something usually considered by customers who want to "plus-size" their factory tires, unless the rim offset allows for the tire's width to be distributed equally on both sides of that center axis. > On Feb 28, 10:27 pm, Dan_Thomas_nos...@yahoo.com wrote: >> On Feb 28, 6:59 am, <H...@nospam.nix> wrote: [quoted text clipped - 14 lines] > reply to show up, when most other posters' questions are answered > within hours of being posted. Nothing personal. You are posting through 'Google groups' on the World Wide Web or WWW by the looks of your header. This means it can take days for your post to show up on some 'Usenet' newsreaders. If you were posting through a Usenet server instead of a web page, your post usually shows up instantly and can be replied to instantly. A lot of Google folks and WebTV'ers get their noses out of joint because they think they are being ignored due to the propagation delay. Then again, maybe you 'were' just ignored.... ;-) Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > So basically SAI is designed to aim an imaginary steering axis through > the midline of a tire specified for a specific vehicle - correct? > Makes sense, and it's not something usually considered by customers > who want to "plus-size" their factory tires, unless the rim offset > allows for the tire's width to be distributed equally on both sides of > that center axis. See http://www.familycar.com/Alignment.htm#Steering%20Axis%20Inclination%20(SAI) Pay attention to "scrub radius" being negative or positive. This is the offset of the SAI from the tire's centre, and different vehicles have different specs. As before, any offset of the wheels messes it up. A better picture: http://www.rqriley.com/images/fig-14.gif Dan On Mar 1, 11:22 am, Dan_Thomas_nos...@yahoo.com wrote: > > So basically SAI is designed to aim an imaginary steering axis through > > the midline of a tire specified for a specific vehicle - correct? [quoted text clipped - 13 lines] > > Dan ____________ Ahh, now I get it. So the axis of the SAI might be designed to fall slightly inside or outside of the centerline of the tire to give a particular vehicle a certain feel and handling. > On Mar 1, 11:22 am, Dan_Thomas_nos...@yahoo.com wrote: > [quoted text clipped - 20 lines] > slightly inside or outside of the centerline of the tire to give a > particular vehicle a certain feel and handling. Handling, yes to a degree. But the single most obvious effect of a non-zero scrub radius is to increase the sense of steering input ("pull") during braking (and acceleration on front-drive cars) when the front wheels have unequal traction or unequally performing brakes. Take, for example, a right front brake that is not working. IF the car has a positive scrub radius, the car will pull toward the working brake on application of the brakes. A zero scrub radius- no pull (and incidentally no direct indication that the brake is not working!) And a negative scrub radius will cause the car to pull toward the DEAD brake. Byeeee,,,,, tell her I love her. cuhulin > ChrisCoasterwrote: > > On Mar 1, 11:22 am, Dan_Thomas_nos...@yahoo.com wrote: [quoted text clipped - 37 lines] > > - Show quoted text - ________________________ Now if one of you could explain to this dyslexic ole' fool the difference between positive and negative scrub radius. Is it negative when the steering axis hits the road outside the centerline of the tire and positive when the steering axis hits the road inside the center line of the tire, or vice versa? In either case, theoretically I could design enough SAI into a car depending on how far in(toward the car's center line) the upper and lower links can be positioned. That of course in turn depends on how big the engine is and how much offset the wheel rims possess. Ahhhh science.... : ) -CC "Who needs SAI to stay in his lane on the purntike, nyuk nyuk..." Doggy,she needs to take me out in the front yard now,for ''whatever''.Byeeeeeee,,,,,,,, cuhulin > ________________________ > Now if one of you could explain to this dyslexic ole' fool the > difference between positive and negative scrub radius. Is it negative > when the steering axis hits the road outside the centerline of the > tire and positive when the steering axis hits the road inside the > center line of the tire, or vice versa? Its the way you said it- negative when the axis is OUTSIDE the center line of the car. I apologize.I guess I had one beer too many.I only drink two six packs every week and a half. cuhulin >>So basically SAI is designed to aim an imaginary steering axis through >>the midline of a tire specified for a specific vehicle - correct? [quoted text clipped - 13 lines] > > Dan FWIW, front-drive vehicles are a lot more persnickety when it comes to scrub radius than RWD. I think most people interpret a scrub-radius effect- the wheel jerking to one side or the other if a front wheel passes over a slick spot or loses traction during hard acceleration- as torque steer. That effect does not, of course, exist with a rear-drive car. I can imagine that the trend of tiny donut tires on huge offset (outward) rims on front-drive econoboxes a few years ago made those things virtually un-driveable on slick roads. I certainly always gave them a wide berth.... ChrisCoaster Wrote: > I know this depends on other design and adjustment > variables, especially camber & caster You appear to have answered your own question...or maybe you should learn how to ask an intelligent, comprehensible question instead of just posting to show off your limited knowledge.  SignatureGohan Ryu http://www.automotiveforums.com |
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