Preston's 1967 Mustang

[Ron in SoCal]

VERY impressive Preston!

How did you model the frame dimensions and suspension design? Look forward to more updates on this one...

[preston]

I have to ask since I am about to start another project, what do you think of the square set-up you are running (335 front and rear)? I have had mixed information, some say it is great and you get to rotate the tires, others say a slightly smaller front (305 or 315) help the car turn better. Thoughts?

I'm a far cry from a driving expert, my only track cred comes from 20 HPDE's over the last 8 years and 7 years of shifter kart racing. But it seemed to turn in just fine to me I didn't notice anything strange. Just from a physics point of view that doesn't make sense to me - you turn the front tires 10 degrees and which tire develops a higher thrust angle? A small tire or a large tire? besides, any large bore racing class eventually moves to the largest front wheels they can package, Trans Am, Grand Am, LeMans. Whether that means the same for a street based car is open to debate, but unless you are going for top speed or are horsepower limited, I can't imagine you would want less tire, especially on our front heavy cars. Again what do they run in LeMan spec Vipers and Vettes ? Or the CP guys in autox ?

How did you model the frame dimensions and suspension design?

In this day and age of CNC laser cut Autocad designed wiper motor brackets, I actually find it kind of refreshing that I am so old school. My first try was a glorified ladder frame, and my 2nd try was all designed on Grape which is a freeware modelling software. Once you play around in something like Grape it doesn't take long to kind of "learn" where the structure needs to go for stiffness, and you are package limited for so much of it. So on the 3rd try I already knew the critical dimensions - firewall to rearwall distance, inside rocker dimensions, dash bar height, wheelbase, etc. So from there I literally design it in my head with a few paper sketches and just make sure I have structure near my suspension pickup points (the suspension is modelled in WinGeo). Ater that the cage superstructure is just built like any car with the roof mocked up in place. Speaking of mockup, i guess thats part of it too - constant and never yielding mockup -seat in & out, engine block in & out , roof on and off. Also the front and rear bodymount/crush structures are added later, so that final "fit" can be done with normal fabrication.

I'll go more into the suspension design later.

[preston]

One other thing I wanted to add about frame design - with this kind of "square" style space frame, everything is really easy to keep "square", vs a mandrel bent curvy or OEM type replacement chassis or Ironworks round tube piece of art. I don't remember them now, but when I was in the middle of it I could tell you the required measurement from every corner. It was relatively easy to x-measure and keep stuff square, and all the structure is flat or angle cut square tubing. It doesn't look as inspiring as a tube bent Ironworks extravaganza with the laser cut brackets and gussets, but its about 100x simpler to build and probably a touch lighter too.

[Payton King]

looking forward to future posts

[Flash68]

I saw some posts of yours on CC too and it's great you posted here. Talk about a refreshing project.

I too have been struggling with the squared vs staggered setup for the changes to my car.

[preston]

This car was built with the bigger is better concept. Staggered or non-staggered, I believe in fitting the largest tires you can where you can. If that means 255 on the front and 345 on the back, then try to tune it out with suspension. Use a lot of rear roll stiffness !

Now a pure racer would disagree, might find advantages to lighter wheels and tires. But on our big motor, heavy, pro-touring inspired rides, I just don't see a point in hamstringing the only thing that keeps the car on the ground. I think the wear patterns are different for the front and rear too, so sure you can rotate them, but I find I keep the rear tires on the car a lot longer than the front if I am tracking. Now if you are basing it on economics and/or crash replacement that is something different.

Another idea I have is the car is mostly tuned for the track and the 335 front tires. On the street I mostly leave the suspension alone (because its a lot of work changing stuff around anyay) and my street tires are 285 up front. So that gives me some built in understeer (safety). Not that I ever really push cornering on the street anyway. The downside is that the 285 doesn't fill up the fenders hella flush

I realize we are discussing performance, but c'mon these cars need big tires anyway ! And for performance, a front biased front engined car needs more front tire if you can fit it.