Way wider tires, spacers, Ackerman who?

Working on my Challenge car, and currently the rear track is some 9" wider than the front track. Yeah, it looks pretty dumb right now. So to compensate I was planning on taking the 145 wide tires off the front, replace them with 275 wide tires, machine some 4" spacer assemblies out of billet aluminum and take a sawzall to any bits of body work that disagree with my plans.

Then I start to think to myself, "self, I bet you're going to totally eff up the ackerman geometry by just widening the front track like that with no other changes." but then I think, "no, no I'm not, because the angle change of the left and right wheel will remain the same, though they will translate fore and aft some now with the kingpin pointed clearly to the inner side of a stupidly wider tire. The wheelbase of the car remains the same. No compensation needed to increase the front track some 9 inches.

So learn me about Ackerman geometry and the practical implications of making the front track way wider by simply putting a big offset rim and fat tires on it.

Your going to change "Scrub radius" hugely Thats what you need to look up.

Unless awd involved I would take sawzall to A-arm, steering ARM, and all other such things and widen your actual flange to flange by the 4" per side it takes. Or whatever keeps scrub near stock.

yep you could do that. but it will become the squirlyest car uve ever driven. with horrendous bumpsteer

hey KATYB... we haven't heard from you in a while ... glad you're back

AWD is involved, sort of. there's a Modular Ford V8 and front wheel drive power train in the back seat. Two engines, more like Front wheel drive X 2.

Go slightly wider and live with the apearance then. Large scrub plus fwd is bad.

Ackerman is affected the amount the steering rack is behind the outer steering rod end. Widening the contact patch point with spacers isn't going to effect Ackerman. Other things, perhaps, maybe even noticeable, maybe not, but not Ackerman.

Ackerman may or may not have an effect on suspension performance. In Smith's early books he says "who cares about Ackerman." In the later books he says "Anything that makes the car easier to push around the pits can't be a bad thing." Paraphrased.

Here's the solution ;-)

Ackerman won't change with wider tires. Ackerman is (as you correctly described) the way the angles of the steering components cause the inside tire to turn more than the outside tire. The opposite would be reverse ackerman which is used with plenty of success on street cars and race cars.

The main thing (as has also been mentioned) is the changes to scrub radius. Draw an imaginary line from the upper strut mount through the lower ball joint. That is the kingpin axis or steering angle of inclination (SAI). That line (on the stock setup) was probably intersecting the ground just outside the centerline of the tire's contact patch. The distance between the CL of the tire and where the SAI intersects the ground is the scrub radius. Highly positive and negative scrubs were sometimes used by the factory to reduce steering effort or as a trade-off to larger under-hood space.

Some race cars use a largely positive scrub radius (where the tire CL is further out than the SAI). This has benefits in steering feedback. It tends to be easier to feel the transition from grip to slip with higher scrubs. The downsides are increased steering effort, increased "squirm" under unequal braking forces, and increased torque steer in FWD applications. It also means that any bumps you encounter will have a much greater effect on the steering wheel since the tire has more "leverage" on the steering components. That is not to be confused with bump steer which describes a change in toe as the suspension swings through its arc.

What I may suggest is doing some math and guessing. You can add a tiny bit of scrub radius by adding camber, but since the spindle (and tire) move along with the strut, the difference in scrub radius is minimal. What you may be able to do is elongate one of the holes where the strut mounts to the spindle. Then you can relocate the upper strut mount without changing the camber. Bingo... adjustable scrub radius.

You may also find that its not a big deal. Put the tires on and test it out.

In reply to wbjones: thankyou. im around a bit now. not super active. prob wont be again. one well im working again 2 im doing alot of activism work and 3 my 2 lil boys keep me awfully busy.