I’ve finally got around to making the Push Rod Rear Suspension. (NB. I’ll use lock nuts on final assembly)
For some reason, I think it was a lack of threaded inserts, I hadn’t made the push rods. This meant that the car could never be put down on it’s wheels.
I used push rods and in-board cantilever suspension to get a rising spring rate and the hub weight down. Hopefully, with this set-up there will be no need for either front or rear anti-roll bars.
Another reason for not getting this far, was I didn’t have the alignment equipment needed to set it all up.
Since then I have purchased a couple of highly accurate digital spirit levels and an old fashioned Dunlop Camber and Caster level.
I used 5/8″ UNF rose joints everywhere (they were cheap and with the fine thread meant adjust-ability would be excellent), so I have also bought 2 off, 3 metre lengths on 5/8″ steel tube. By removing the upright and threading the 3m bar through the wishbones you can accurately set not just the required wishbone length, but also the neutral toe in / toe out. You can double check that the bars are parallel to the chassis rails. I also have a laser alignment set-up, but at this stage, I found this easier and quicker.
When aligning the bars, I set them exactly horizontal by propping them up on spacers. The rose joints were then turned so that the centre of each hole and the sides of the chassis rails, were close to identical on both sides of the car (0.11mm in my case – although very hard to measure as the rose joint hole moves as you swivel the bearing).
With 3 metre bars through the wishbones on both sides, the distance between the bars should be exactly the same at both ends of the bar (neutral toe in). Over a 3 metre length, an difference of 1mm, would mean there is 0.019° toe in / toe out. Due the thread spacing of UNF and my adjustment arrangement, I couldn’t get mine exactly neutral. I ended up with the bars being 0.3mm closer at the far end than at the hub end (0.00573° toe in).
According to ‘Chassis Engineering’ by Herb Adams the maximum Anti Squat for a road car is 25%. Other sources suggest higher but not for 4×4.
A long time ago, I’d had one of those facebook messages that gets you worried. The guy was ranting on about how my suspension design would never work and the bushes would bind up and destroy themselves in seconds. So over the last year or so, I’ve re-visited my Kangaloosh, rFactor and SuspEdit calculations and bought some more books. I’ve also spent several hours reading just about any blog site I could find. I couldn’t see anything obvious wrong with my calculations but as I’d never done this kind of thing before, I was obviously missing something.
Most of the blogs I found were for some form of racing e.g. Brisca, Touring of Offroad. My car is 100% designed for the road. It has a very low centre of gravity, it is only elbow height and quite short. It has double A-arm suspension rather than struts etc. Therefore I couldn’t ‘rob’ their calculations, so anything I’ve done is unproven and essentially ‘made up’.
Push Rod Rear Suspension Geometry
The above drawing shows how I would set-up for 100% anti squat. My IC (red dot) is below the 100% anti squat line and several feet further forward.
Whilst researching, I found this blog of someone setting up a ’67 Mustang for Pro Touring Racing.
The wheelbase is similar (8″ difference), the tyres are slightly wider but similar, the CG (centre of gravity) is roughly 50/50 front to back etc.
When looking at pages like this, you must remember that this ‘touring car’ is set up for racing, on perfectly smooth tarmac, with the centre of gravity (CG) being relatively high. If I was to use these measurements, the CG would be roughly the same height as my eyes and with the IC falling above the 100% anti-squat line, away from the track, on British tarmac, this car would hop and jump all over the place.
Therefore, although these images are very interesting, from which much can be learnt, they are no practical use to me.
I was a little nervous when I put the hubs in place, everything would bind and jam solid. I’d left the shocks disconnected so everything could move freely.
Whilst re-attaching the uprights, a huge penny dropped. The guy giving me the hassle probably hadn’t spotted that I was only using urethane bushes at the inner end of the arm. At the hub end I’m using rose joints. These allow the hub to rotate; very slightly; preventing the bushes from locking up.
In case you’re wondering; everything moves very freely. I hate to say ‘friction free’ but it certainly felt that way.
Push Rod Rear Suspension Equations
I don’t want to publish too many of my calculations as I find doing that just ends up with tons of questions for cars I have no clue about.
My worry is that someone could copy my dimensions and use them on something completely different like a monster truck.
Beside before this car hits the road, they may change and after I drive it – change again.
My suggestion is to plot everything 1:1 scale in fully up and fully down positions. Once happy with that, graph a few positions in between.
As usual; with most things I start; there were still a few nuts missing. In a couple places, as a temporary interim, I’ve used secondhand items, but once this car hits the road everything will be new high tensile items Grade 5 (8.8). All the joints are TIG welded in case you’re wondering.