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Front Wheel Scrub

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Offline johnmckinley

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Front Wheel Scrub
« on: June 24, 2011, 08:40:13 AM »
On my '93 Tracker, I have a lot of scrubbing in hard turns, left or right.  I suspect the steering just doesn't have a lot of Ackerman built in, but who knows? Is this just the nature of this beast, or is there something wrong?
John McKinley
'93 Tracker 4WD Auto

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Re: Front Wheel Scrub
« Reply #1 on: June 24, 2011, 11:34:38 AM »
What does a manufacturer stand to gain from not building the correct amount of ackerman into a vehicle?  It may sound like a strange question, but it is a serious one - why would they do it? Why would you think they would do it?

It is in the nature of this vehicles to scrub especially on tight turns - I believe it results from the fact that they are, by design, relatively short wheel base, rear wheel drive vehicles with small diameter turning circles - this means that the front wheels turn sharply and you are pushing the front tire sideways, forcing it to slip - a front wheel drive car in same scenario whould have considerably less slip, since the direction of the force is being applied in the same direction as the tire will roll.
'98 SQ420 Grand Vitara
'05 JB420 Grand Vitara
'16 APK416 Vitara
'21 A6G415 Jimny

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Offline johnmckinley

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Re: Front Wheel Scrub
« Reply #2 on: June 24, 2011, 01:23:25 PM »
Good points.  But, vehicles do get designed without Ackerman - I've long since forgotten the reason, but Ford trucks and vans with the "Twin I-Beam" front suspension were designed with essentially zero Ackerman.  Even longer wheelbase F-150s would scrub like crazy.  In my case, I was just wondering if "They all do that" or if something was wrong with mine. I think you're right, with a very short wheelbase, it may just be impossible to devise a steering linkage that can create enough Ackerman.
John McKinley
'93 Tracker 4WD Auto

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Re: Front Wheel Scrub
« Reply #3 on: June 24, 2011, 06:45:45 PM »
Now you have me wondering - what the he!! were Ford thinking!?!
'98 SQ420 Grand Vitara
'05 JB420 Grand Vitara
'16 APK416 Vitara
'21 A6G415 Jimny

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Offline wildgoody

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Re: Front Wheel Scrub
« Reply #4 on: June 24, 2011, 10:25:18 PM »
My Chevy truck does it too, the front tires chirp on every line
in the Home Depot Contractor parking spots
Real Trucks Are Built, Not Bought,
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Offline johnmckinley

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Re: Front Wheel Scrub
« Reply #5 on: June 24, 2011, 10:26:06 PM »
Memory fails me.  I was VP of Engineering for the company that made F250 and F350 brakes as well as the beams for the Twin I-Beam.  I think there was something spooky about twin I beam concept that made Ackerman impossible, but that was a long time ago and I just don't remember the details.  I do know that under certain conditions, like big cobblestones, that suspension would just go to full jounce and stay there until the cobblestones were over.  No one ever figured out why that happened. And, the beams were almost impossible to make.  Three foot long steel castings (not forgings) with incredible tolerances.  Almost bankrupt us until they re-sourced it to John Deere who had a ton of excesss steel casting capacity during one of the agriculture downturns.  They gave it back after a year.
John McKinley
'93 Tracker 4WD Auto

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Re: Front Wheel Scrub
« Reply #6 on: June 25, 2011, 12:18:34 PM »
My Chevy truck does it too, the front tires chirp on every line
in the Home Depot Contractor parking spots

Yup - I think most rear wheel drive vehicles will.

Last fall I was in a strip mall parking lot out in, I believe it was, Citrus Park - my wife & daughters were inside one of the stores and I was sitting in the car out in the parking lot reading - which is not too unusual - and I noted which vehicles "chirped" across the pedestrian crossing paint as they turned - every one a rear wheel drive truck or SUV.

The rear tires are pushing the vehicle and the front tires are at an angle so they slip sideways and scrub.
'98 SQ420 Grand Vitara
'05 JB420 Grand Vitara
'16 APK416 Vitara
'21 A6G415 Jimny