swapping the front hubs to cure negative ackerman 1

[mike kay]

I am building a locost 7 using a totaled 2001 blazer. The front steer arms create negative Ackerman. I want to switch the driver's side hub to the passenger side and also swap the passenger side to the drivers side and use a rear steer rack and pinion. I got this idea from looking at a Chevrolet Trailblazer as I believe this is what they di to correct their Ackerman. Appreciate any input from members.

 

[GregLocock]

Negative Ackerman, really? How do you define that? The only way i can see that is that the outside wheel turns more than the inside wheel. If you mean that the donor vehicle had negative Ackermann then I'd be a bit surprised.

Anyway a dimensioned sketch would probably be a good idea.



Cheers

Greg Locock


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[mike kay]

Thanks for replying. On s10's and Blazers the steering arm positions the tie rod to the inside of the ball joint instead of the outside of the ball joint. I believe this causes Ackerman problems since the tie rod position is in front of the hub. Please humor me and tell me if you think I can't swap the hubs and turn my vehicle into a rear steer vehicles with the tie rods placed inside of the ball joints.

 

[gruntguru]

What you are proposing is fine but like Greg, I find it very unlikely the donor vehicle had negative Ackerman. You need to look at both ball joints (or MacPherson strut mount) to locate the steering axis when determining Ackerman.

je suis charlie

 

[SwinnyGG]

You can flip the knuckles if you want, but Ackerman percentage is based on the relationship between the rack and tie rods and ball joints- it's not as simple as just moving it to the other side. If you want the car to handle right and be fun to drive, getting the steering right is probably the most important job you have.

You need to sketch the geometry out, or use ADAMS or similar software to design your mounting locations, if you have access to something like that.

Negative Ackerman or anti-Ackerman is not a common geometry for street cars. There are some odd applications that use anti-Ackerman geometry, like F1 cars and high speed trophy trucks.. but maneuverability at 2 mph in a parking lot is not a concern for either of those applications.

 

[mike kay]

Thank you for all of the input; I will take it to heart and examine it closely. I'm a fiberglasser and have already built the body parts;my front end is inspired by the Brunton Stalker and my rear end is inspired by the World Class Ultralite. We are beginning are chassis development using principles from Litton's master thesis from Cranefield university that does a thorough analysis of the Locost chassis in finite element analysis. We will use the 4.3 V6 and various LS engines. Appreciate all the help. Feel free to contact me if you wish to know more;I am located in Omaha Nebraska.

 

[GregLocock]

I can think of a couple of reasons why a truly desperate handling engineer might put negative Ackerman in, the first being to saturate the outer tire in extreme cornering and prevent untripped rollover. But there are other ways of achieving that. The other possibility is that they weren't meeting a turning circle requirement and just had to whack as much wheel lock in as they could get. I think that's unlikely, there is an optimum Ackerman percentage (not 100%), and once you go too far above that the wheels start to actually fight each so you don't get much benefit in turning circle.

Cheers

Greg Locock


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[mike kay]

Greg I agree with what you've said and it coincides with my understandings based on analysis about Ackerman by Mark Ortiz in his Chassis Newsletters. The S 10 frame and suspension was GM reusing its G body platform of the 1960's and if you look at its front suspension and steering it is as I described. I am trying to add a new dimension to the excellent work done by Dennis Brunton to make the S10 a donor platform for the Locost 7 community. Again thanks.

 

[SwinnyGG]

How do you know that the stock S10 has anti-ackerman geometry? Have you measured?

It still seems like you don't understand that just flipping the rack to the other side of the axle centerline doesn't take anti-ackerman geometry and automatically make it pro-ackerman.

 

[cibachrome]

I can assure you that S-10 and T-10s do NOT have negative Ackermann. Hundreds of steering ratio tests GM runs on its' own vehicles and competitors also produce Ackermann MEASUREMENTS. There are quite a few with parallel steering (up to high steer angles) but no anti. Some of this is fuel economy (coastdown) and some is tire wear (scrub in turns).

The reason for having anti-Ackerman (toe-out in turns) is quite clear: to get more from grip ("side-bite") from a pair of tires for which, because of load, camber and slip conditions, causes the inside tire to fight the force and moment signs from the outside tire. This depends on the tires, though, so a tire test is required, usually for a situation where VERY large load capacity tires are used on a lightly loaded vehicle.

Couple of drawbacks, though: More front grip reduces the front cornering compliance and lowers the vehicle's understeer recipe. Confused drivers, magazine airheads, and book writers will blame this on the change, not the true cause being that an equivalent change should/ought to be made in the rear to maintain the same steering gain (g's per 100 deg/steer at at a prescribed speed). Given two brands of tires of the same size pressure and rim width, its possible to get 2 opposite recommendations. It depends on the tire's load sensitivity (a math based property) and is dependent on construction features and parts (shoulder stuff).

Then there is a the reality of whether you want to push on a tierod with the highest internal force, or pull on it? Pushing on a limp you-know-what is not as keen as pulling on it, so extra beef is mandatory. Otherwise the working Ackermann is not the same as the AutoCad version because of buckling and gear mount issues.

Run some tests, do some simulated cornering maneuvers and build from there.

 

[mike kay]

Again thanks for all the input. I respect the opinions of all you highly qualified individuals. I have not measured the actual Ackerman,but am making the observation that the tie rod/steering arm location is to the inside of the lower ball joint on an s10 and blazer which is a front steer vehicle; if you will look at one you will see this to be true. I still believe that only anti Ackerman measurements can come from this situation and I believe that some of the handling problems of s10 are caused by the the front wheels fighting with the intended steering direction. I believe my suggestion will solve this problem in an easier way than was used on the series 1 Brunton Stalker. Again thanks for the input.

 

[SwinnyGG]

the tie rod/steering arm location is to the inside of the lower ball joint......I still believe that only anti Ackerman measurements can come from this situation

There's the root of the problem.. the above statement is not true.

 

[gruntguru]

Rather than "looking" at ball joints and tie rods, why not measure the Ackerman? You can get a reasonable answer with a tape measure between tyre sidewalls. Turn the steering wheel - does the measurement increase or decrease?

je suis charlie

 

[GregLocock]

You old empiricist you.

So to slightly enlarge on that, use a white paint pen to mark the centre of the tread on each front wheel. Put the wheels straight ahead. measure between the foremost point on each tire, using some highe technolgy bits of wood and a tape measure.

Apply full lock. Measure it again.

If the dimension increases you have positive ackerman. If it stays the same you have parallel steer. If it is smaller then you've probably made a mistake.

Cheers

Greg Locock


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[cibachrome]

OK, so here are just the Facts, Ma'am:

A not-too-recent shot of statistics on the GMT330 Blazers (S-10s and T-10s) shows that the average Ackermann error (Thank you Rudolf) is about 0.80 degrees at a 10 m turn radius (standard reference condition). Error being measured toe - theoretical required, with a max error of about 4.25 degrees at max steer. No difference between domestic trucks and the RHDs shipped overseas. GMT360 (TrailBlazer) stats are not much different, maybe a squeak higher at the 10m radius. Not much different from ALL the vehicles I have on my abacus.

I ought to be able to post some graphical testimony later to squash the snowflakes. BTW: it's "Reverse Ackermann" for the technocrats.

 

[mike kay]

I can't thank the contributors enough for clearing up my thinking about the Blazer geometry as regards Ackerman. We are serious about using the 1998 on Blazer to make an improved version of the series 1 Brunton Stalker and you have reassured me that GM knew what it was doing as regards Ackerman. We will measure the Ackerman as we go forward. I have invested over $20,000 in making molds based on the Lotus 7 style hood and nose so we will push forward. I hope I have not wore out my welcome as the knowledge of the esteemed members of this forum is well appreciated. Thanks again.

 

[cibachrome]

So, just when you thought it was safe to go outside, some real world STATISTICS show up and a few more facts, too.

Turns out there are a few 'reverse ackermann' vehicles out there, at least for part of their turning careers, just about all of THEM are very long wheelbase BIG (Fed-Ex or UPS , or rental moving) trucks. They start out as negative ACK_Err for a while and cross over to positive error somewhere along the turn radius. Oshkosh by-gosh type motor homes (pushers) are, too.

Then there is the Jeep Wrangler at -.18 deg going to +4.8 degrees error at max turn.

Did I mention a Cadillac Corvette wanna-be is, too, just a bit ? That's on a Michelin tire while a 'Vette is on Goodyears, so the science could be valid.

Some other gripping 'factoids' in the data, too.

 

[GregLocock]

Ackerman is strongly affected by static toe setting, so steering engineers throw hissy fits when they see plots of ackerman vs SWA. It is a good reminder though, static toe has a significant effect on low g steering response, to the extent that the vehicle I am currently working on gets a static alignment after every change I make. I only have Ackerman targets at 20 deg lock. Below that is an outcome that we adjust with static toe, above that we are usually chasing minimum turning circle for minimum inner wheel lock angle.

Cheers

Greg Locock


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[BrianPetersen]

Just to be clear, you are talking about "ackerman error", i.e. the deviation from theoretical required ackerman, right?

It's possible for the inner wheel to steer more than the outer wheel while still not quite steering enough to match what is theoretically required ..

 

[GregLocock]

(Inner steer angle -outer steer sngle)/Ackerman error % is the way I usually report it, but in practice i only really think about the top line, at full lock. Max isa is usually package limited.

Cheers

Greg Locock


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