My CAD models

[RacingManiac]

Forging is alright and all, but are you planning to make a ton of these to warrant the tooling? I'd imagine at somepoint it'll be worthwhile to tool it up for forging, but if you are not making a ton you might be better off just welding tubular styule a-arm....

Not sure what the fidelity of the CAD model is, so maybe you've omitted some details. But that rear box does not look very stiff especially if you are running pull rod layout where most of your load will feed through upper ball joint and the upper a-arm, the rear suspension pickup "box" is not closed off?

[culver]

I assume this CAD model is a for the fun of it project (I haven't read the whole thread) so don't take any of my questions too seriously.

Why did you decide pull vs push rods? It looks like you connected the pull rod to the upright rather than the suspension arms. Wouldn't that risk a steering moment as the wheel is deflected?

Anyway, it looks like a good bit of work and a fun CAD project.

[RacingManiac]

I've seen that done on a pushrod layout(quite common in F1), which can be engineered to vary your rate and load transfer per steering input, but as the pullrod are more critically designed to minimize weight, I tend to be more conservative in loading them...

[culver]

I'm not sure that I've ever seen a pushrod actually connect to the upright. F1 might but thanks to the current rules which all but mandate balloon tires F1 cars have rather strange suspension setups that most race cars would never choose to run. The camber gain curves on the current F1 cars are horrific. However, F1 cars typically have very little suspension articulation (outside of the tires). It's more important to maximize down force via better aero. So ultimately the engineers decided great aero and compromised suspension was better than reversing the compromise.


I've seen lots of pushrods that connect right at the end of the A-arms. Still, it could simply be I haven't seen it on the sorts of cars I normally look at. Racecar Engineering had a recent article talking about pull rods. Their conclusion was similar to what I had already heard. It's hard to say that pull rods save weight. The pull rod it's self is lighter than a pushrod taking a similar load. However, a pull rod setup put more load on the rest of the suspension arms, especially the upper A-arm.

Incidentally, I'm acting as an informal adviser to a FSAE team. We just looked at this issue thus it was on my mind.

[Knuto]

@Racingmaniac. I intend to make an upper frame to stiffen the rear "box" once I've positioned some more components.

@Culver. I placed the pickup point quite close to the kingpin axis. At maximum steering angle the torque generated by the pullrod on the upright is about 4Nm. At low steering angles the torque is very low. The pacejka equation plus torque from castor angle tells me that the tire generates 16Nm of torque around the kingpin axis under high cornering g's. So the most torque comes from the tire grip, not the springs.

I attached the pullrod to the upright to take away the bending load on the front upper A-arms.

Thanks for the comments guys. Great to get some advice from the proffessionals

[Knuto]

The car is pretty much finished now. Just some final testing left now and finding a good default setup

Here's some screens from a race at Monaco (Interior shots are from Thermolite)
ImageShack -