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Wider rear track effect on oversteer

239 Views 3 Replies 2 Participants Last post by  cerial
I am building a mid engine pile out of a collection of easily accessible junk.

Here is how it sits. For several reasons I have decided to run swap the ifs for a solid axle.

I am running the numbers on a triangulated 4 link. That will have an extension off the upper links to airbags making this a cantilevered setup.

I am at the point I am ready to build a crossmember right in front of the engine mounts that will go under the engine and cut the frame in front of that engine out.

Ran my numbers and they look good then my dumb butt asked myself one question.

Is having a wider track going to mess things up?

So my pile has a 81" wheelbase with a center of gravity at 26" with a weight bias is 60-65"% rear.

I am running a 78/79 full width 9" and a 91 2wd jeep cherokee dana 30 for axles.

My current front axle and tire width won't change much. I am running a 24 3/8" tall tire that is 8 3/4" wide with the outside of the tires being 63" wide overall for a 54.25 track width.

For the rear I am currently running a 26 3/4" tall tire that is 9.5" wide with the outside of the tires being 73" wide overall for a track width of 63.5.

Now this is the minimal rear tire width I plan on running. Where things get fun is what I plan on bolting up after changing the 3rd out for the proper gearing for the application as I am only using 2 of the sm465's 4 forward ratios(3rd/4th straight throw). I plan on running UP TO a 18.5" wide 31" ish tall pro street or UP TO 13" wide 33" tall paddle. I have 16" between the frame and the 78/ 79 F150 stock 9" wms. I should be able to keep most of that tire close to the frame with rim offset moving that rear track inward with wider tires.

Now I "think" having the wider track in the rear and having the weight biased towards the rear is going to promote understeer. Especially under acceleration. What I am trying to eliminate is oversteer and excessive pro roll under breaking in a turn. You know things that could break some glass.

With the even wider tires in the rear I should maintain the track being wider then the front at all times.

But I need to be really careful I get these 4 link numbers close so I dont need to build this front suspension a 2nd or 3rd time.

Right now I am planning on making gusseted tabs on the frame and axle that the crossmember and front axle link mounting brackets can bolt to. Just building and then testing it kind of deal.

I would like to get things close on paper first though. If I need to redo the numbers to account for the wider rear tracks effect on the front I will.

I just have no idea how to actually figure out how to actually get that diffrence in track width number to then change things based off that number.
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Some great tech I found below as I try to grasp corner weight distribution due to track width diffrences.

Lots of info in that article.

Kevin Wilson has some really good videos on front and rear suspension setup on yootoob. But his stuff is for going straight, and leaving hard.
Maybe you could glean some from his videos too?
He does get a little bit into caster, and camber, and how they affect turning, but mostly for going straight.
What all will you do with this "thing?"
I am guessing you live near some dunes, for the paddle tires to work with. I like watching cars and trucks flying up those giant sand dunes, then soaring over the top!!

Chassis building is definitely a science, and I admire anybody who goes beyond what the factory puts out the door.
Its just a fun driver. With the intent of driving/parking it anywhere. I hate not being to be able to see around a car relying entirely on cameras or having the door open.

The cab actually is tilted down for easier visablity to have minimal blocked areas letting me parallel park this thing in a city like Chicago.

The goal here is to eventually after alot of tuning at 300 NA. Run a turbocharged setup on basically a junkyard ring and gasket longblock.

Drive around nice around 400hp. But if I want to have some fun I can bolt in diffrent gearing, tires, turn up things closer to 650hp and push them hard.

When things break. I have spares upon spares with cheap spares available to bolt in.

The thing has a 2" reciever as its rear bumper. I will eventually build a trailer to pull around a spare longblock, transmission, 3rd, hoist, tires, etc with the intent of doing a powertour drive someday.

Hopefully out there to help others with my little moble shop more then fixing my own breakdowns.

It won't ever be track approved. Floor, firewall, cab modification, just a few of the reasons. But I may sneak in on a open night just to see what it could do before being kicked out and told to get safety stuff. 2" reciever could also facilitate a 3 point wheelie bar setup just for fun.

Of course the main reason for using 2 speeds is to keep that front end down. I am basically going to run 454 clutches and 5500 shifts. With a short life for what will be a clutch that cost around $200 and can be replaced in about an hour.

Oh yes. I have been watching wilson for a while, as well as many others. This will squat down nice under acceleration with most of that weight going on that rear passenger tire due to the engine offset. One of the benefits of the cantilevered setup is I should be able to have a good amount of front travel before those tires leave the ground without camber changes and I believe I can even eliminate bumpsteer with misalignment heims to a high degree (60) at those raised heights.

Shock and shock valving are going to be a pain to figure out. I am making it drivable without shocks. The rear is dang near perfect right now. Set up soft but still has plenty of travel.

The front not so much. With the to stiff, to firm, I need to get into custom to make it just right suspension being one of about 6 reasons.

I think I need to think diagonally here. In a right turn I am not just applying weight to the outside drivers front tire I am removing weight from the passenger rear tire.

The rear is fixed so scrub should be constant in the rear leaving just the front to be a factor. I am using factory offset rims so that should eliminate that isdue there.

Now breaking in a turn things get fun.

If I set it up with high antidive it will try to keep the frontend up under breaking. The tradeoff is that it will also try to keep the frontend up under acceleration.

Do I trade better cornering for slower acceleration? Or rely on the weight bias alone being a factor in keeping that rear from lifting? Or play with the airbag valving to make it fast evough that can stiffen up the front suspension to prevent dive in that way? Having such a small wheelbase makes for fun trying to find universal suspension tricks I can apply. But I still want the park it anywhere use and the wheelbase it requires.

The more I look into this the less the track offset will affect things. I dont think it will be a contributing factor worth changing things over.
I can just be a good boy and drive it "like a truck" in the turns. Most of my trucks end up going sideways in the turns. But I am slowly getting a bit better about self control. Still want this to handle as good as I can make it.

Fact is I just need to build and test at this point. I have went as far as I can on paper. I have decided tomake the crossmember and rest of the link brackets bolt in. This will add weight and not be as strong as a fully welded setup. But will allow me to remake several setups changing things after testing.

Onto testing I have a trailer with no suspension I can put this on. Strap down the tires and drive down the road with the thing running. Take turns, hard breaking, hard acceleration, etc all with the engine running and thing strapped. Then review and make changes to avoid rolling this out the gate. Then once I am happy get some real seat time to further adjust things.

Not ideal at all. But thats what I am doing. Back to ordering parts and building.
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