swapping the front hubs to cure negative ackerman
swapping the front hubs to cure negative ackerman
(OP)
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.





RE: swapping the front hubs to cure negative ackerman
Anyway a dimensioned sketch would probably be a good idea.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: swapping the front hubs to cure negative ackerman
RE: swapping the front hubs to cure negative ackerman
je suis charlie
RE: swapping the front hubs to cure negative ackerman
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.
RE: swapping the front hubs to cure negative ackerman
RE: swapping the front hubs to cure negative ackerman
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: swapping the front hubs to cure negative ackerman
RE: swapping the front hubs to cure negative ackerman
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.
RE: swapping the front hubs to cure negative ackerman
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.
RE: swapping the front hubs to cure negative ackerman
RE: swapping the front hubs to cure negative ackerman
There's the root of the problem.. the above statement is not true.
RE: swapping the front hubs to cure negative ackerman
je suis charlie
RE: swapping the front hubs to cure negative ackerman
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: swapping the front hubs to cure negative ackerman
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.
RE: swapping the front hubs to cure negative ackerman
RE: swapping the front hubs to cure negative ackerman
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.
RE: swapping the front hubs to cure negative ackerman
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: swapping the front hubs to cure negative ackerman
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 ..
RE: swapping the front hubs to cure negative ackerman
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: swapping the front hubs to cure negative ackerman
I dont want to make comment on the parts you have without seeing them, so Ill keep it pretty general.
Running the negative ackerman the car drove fine on the street. I aligned it with with reasonable front toe-out (~0.25-0.5") and the car has a notably long wheelbase 100+". I began drifting the car at local events and it was difficult to control at high counter-steer MA. Redeveloping the steering knuckles to allow for zero-to-positive levels of ackerman made the car way easier to drive at high slip angles, but the car is slower due to the front tires dragging I presume.
Even if your plan is not drifting, toe out will help you recover from spins (think high countersteer). Static toe (alightment) and dynamic toe (ackerman) are useful for controlling initial toe for turn in, and toe at high steering angles. Drag must be balanced through the both of them.
FWIW, my car was last setup with low positive ackerman and high static toe (~0.75" out) to make it tossable and stable at high slip angle. Its a toy that I built in college though, not a seriously developed race or production car.
RE: swapping the front hubs to cure negative ackerman
RE: swapping the front hubs to cure negative ackerman
My username is the same on locostusa forums, send me a PM there if you'd like.
RE: swapping the front hubs to cure negative ackerman