"Correcting" geometry on a narrowed front track

Now this is very non-GRM of me, as it's basically form over function, BUT I know this crowd would have a better knowledge base than the typical slammed/stance brotards.

Theoreticaly...

If I were to narrow the front track of a FWD mcpherson strut vehicle by physically moving all suspension pickup points inwards by say 2" overall, what effect would this have on Ackermann and/or other geometry that could/should be modified for a street driven vehicle?

This is for a low and slow cruiser, not a race car, and heavy on the sawzall and welder in building, but I'm trying to retain original structures/control arms/struts/etc. but just narrow the track width overall. Obviously I'd have to narrow the tie rods.

Just narrow it and run it? Need to change something else? etc.

Ackerman would be messed up, can't tell you which way, but it wouldn't be the same. You'd need to change the tie rod length and steering rack position to correct it.

If you moved everything inboard equally, including the strut tops, the only other thing that would change would be the roll center (and therefore the RAI).

GameboyRMH wrote: If you moved everything inboard equally, including the strut tops, the only other thing that would change would be the roll center (and therefore the RAI).

Right. Everything in including the strut tops.

Keep in mind this would be for a lowrider type cruiser, not a track car, but I'd like to set it up to at least be as "correct" as possible despite the heavy modification, rather than just creating a terrible driving contraption.

Basically, doing a hairbrained idea, in a minimal hairbrained way

Width of the steering rack (inner pivot to inner pivot) is going to be REALLY messed up.

you will have god awful bumpsteer unless you put in a narrowed rack. Not to mention that this throws ackermann all over the place.

Also, custom driveshafts?

I would suggest this book as a real source. http://www.amazon.com/Race-Vehicle-Dynamics-R146-Publisher/dp/B004XEA3E2/ref=sr_1_2?s=books&ie=UTF8&qid=1392144209&sr=1-2&keywords=milliken+dynamics

Note, this is a textbook and is priced as such. (I have a copy of it)

You might want to go over to the LocostUSA forums as they have good suspension design support, but they primarially focus on SLA types rather than struts.

Apexcarver wrote: Also, custom driveshafts?

Yes, with others who have done this they cut down and narrow the driveshaft.

^ Good point about bump steer. You can fix the ackerman without changing the rack length, but not bump steer. Shouldn't be a huge problem for a slammed cruiser though since it won't have much suspension travel.

You will need to find a similarly narrower steering rack to not mess up bump steer, (the rack it's self, not the tie rods). If I recall correctly Ackerman geometry depends on the fore/aft placement of the rack and the angle of the steering arms, so I don't think it will be affected.

^It depends on the track width too though. Change that and your ackerman doesn't change, but becomes wrong for the new track width.

True, but I doubt you would notice the difference when driving.

hadn't thought about bump steer due to shortened tie rods.

hrmmm. Hadn't thought about running a different rack, that throws a bit of a monkey wrench at it.

Obviously narrowing 2" and finding a 2" narrower rack would put it back to "stock like", but that may not be feasible.

EDIT: Found some decent reading material regarding rack length/tie rod orientation: http://www.woodwardsteering.com/images/cat05%20pdf%2064-79.pdf

Not sure how to differentiate instant center on a macstrut car. Sounds like there's some ways to cheat the rack length by moving the rack up/down to get the end points into the correct orientation.

EDIT AGAIN: (learning here...)

I see it's not due entirely to the shortened tie rods, but also potentially due to the the racks changed relative end pivot point position, lending the tie rod angle to be incorrect relative to the instant center (fixable by moving rack up/down)

I swear I've seen racks effectively "shortened" by making up a C shaped bracket to bolt on either end of the rack, and attaching the tie rods to that farther inward.

I know they do something like that in drifting to move the tie road attachment points forward...once tie rod never hits the rack I don't see why it wouldn't work.

In reply to Kenny_McCormic:

Yes, I forgot about that option. Heavy and more complicated, but it would work.

Posts: 1320 (05/05/07 10:27 AM) • Reply • Quote • Edit • More o My Recent Posts This is going to take awhile to write so print it out and take it to the car and look at stuff.

1) measure the distance between the two inner ball joints (hiems) Lets say for example its 22 inches

2) look in catalogs that have rack dimensions for a rack with inner tie rods at 22 inches. You'd like to get as close as possible.

3) make sure the rack you pick you can get the steering shaft to with out going through the motor. Tried to help several racers fit racks that just didn't work.

4) Rack height. This depends on a few measurements so take you time and get it right! 4a) Measure from the outer ball joints center of swivel to ground. Becarfull if you use real ball joint like me it can be deceiving as to where the center of rotation is. Cut and old one to make sure. lets say its 8.0 4b) Measure from outer tie rod center of rotation to ground. Lets say its 8.5 4c) so you have a outer tie rod that 1/2 inch higher then the ball joint this means you rack need to be 1/2 inch hirer then the inner ball joints. This will set the angles of the tie rods the same as the lower control arms thus minimize bump steer.

5) I race pavement and have found some Ackerman to be a good thing. Most big car set the racks 1.5 to 2 inches off the center line of the outer tie rods In DWARFS 1/2 to 3/4 works good. 5a) Point both wheels straight now tie a string around the outer tie rode bolts measure to a cross member in-line with the inner ball joints let say its 12 inches. So if you have front steer spindles (most dwarfs do) you should set the racks center line at 11.5 to 11.25 from the cross member.

Keeping a rack in the right place is hard!! An easy way to help is this. Measure the rack diameter and find some tubbing that will snuggly fit over the racks ends now weld on some thin sheet metal to the tubbing this can be clamped to the 1x2 frame that comes out to hold the bumper.

So to sum it up you want the tie rod angles to match the lower control arm angles when viewed from the front and you want the rack set back toward the motor .5 to .75 from the outer tie rod ends.

Best of luck 44dwarf "Smok'in the competition NOT Tobacco"

"Transplant organs, Don't bury them!"

Not to be confused with dwarf44, 44 or Buck44 I wrote this for someone a few years back. (good thing i look to see if it was still on the old pentium1 box) It needs pictures but I'm just to busy to take them. Hope this helps. CHECK THE BUMP STEER before you weld it all up.

This is going to take awhile to write so print it out and take it to the car and look at stuff.

Kenny_McCormic wrote: I swear I've seen racks effectively "shortened" by making up a C shaped bracket to bolt on either end of the rack, and attaching the tie rods to that farther inward.

Fancy. Good idea as well.

There are also some cars with the tie rods attached next to each other in the center (Saab maybe) that can be installed just forward of the original location and attached to a dummy rack narrowed properly. The one car I saw done up that way had a dummy rack that was adjustable in length. Sure it took a while to get set up, but from a fabrication standpoint it was a breeze.

I still don't get the why part??

Timely post I am planning on doing this to a RWD front end.

kevlarcorolla wrote: I still don't get the why part??

Simply put, in my specific scenario, to get wheels further inside the fenders so they can tuck more and go lower.

Basically I'm looking at the equivalent of a body drop, or chanelling like an old lead sled or minitruck, but applied to a unibody vehicle. Cut out the entire structure from the firewall forwards and move everything "up" relative to the body by about 2". At the same time, wanting to move things "in" to get the wheels/tires to tuck inside the fenders farther.

I'm not asking anybody to understand the logic...

Great ideas on the dummy rack.

To make the concept more applicable to racing though, I imagine in a lemons contraption or otherwise I could see somebody hacking up a complete FWD drivetrain and suspension to fit into a smaller vehicle, or converting something to RWD (with a narrower track), etc.

Aircooled VW guys, and even old truck guys, narrow the heck out of the front beams/tracks to get low, but most of them take no account for ackermann correction. I'm just trying to do it a little more right, and almost nobody cuts up unibody fwd vehicles so I'm pretty much playing in a playground by myself.

Here's a VW Rabbit truck that has had the strut caps raised and moved inwards to get the wheels inside the fenders like I am talking about: