Spool + Weight Jacking

[swong46]

Hey everyone, we are currently designing the suspension around a spool setup and want to ask a question on how to effectively use a spool.

So first I said put the rear RC as close to the CG height as possible, this way the weight transfer will purely be geometric WT; thus causing the rear inner tire to lift at corner entry/mid. As soon as the RC migrates, we will generate a small roll moment and the springs will then deflect to prevent erratic wheel lift.
Now with all the weight of the rear on one tire, we use our dynamic toe to maintain peak lat. grip. I am afraid this would have an oversteering effect for the drivers because it should happen rather instantaneously.

Another thing I thought about was diagonal weight jacking, but this deliberately changes loading in the front to effect the rear. This I think is not the way to go as front grip changes (but I guess you can toe in the inner front tire to make up for this load change) but more importantly, I would think this method would be too slow to react.

Whats your opinion guys? Anyone from Monash care to step in? I know their spool setup was done wonderfully. The car was pulling hard throughout the skidpad with just one rear tire through almost the whole thing!

[swong46]

Hey everyone, we are currently designing the suspension around a spool setup and want to ask a question on how to effectively use a spool.

So first I said put the rear RC as close to the CG height as possible, this way the weight transfer will purely be geometric WT; thus causing the rear inner tire to lift at corner entry/mid. As soon as the RC migrates, we will generate a small roll moment and the springs will then deflect to prevent erratic wheel lift.
Now with all the weight of the rear on one tire, we use our dynamic toe to maintain peak lat. grip. I am afraid this would have an oversteering effect for the drivers because it should happen rather instantaneously.

Another thing I thought about was diagonal weight jacking, but this deliberately changes loading in the front to effect the rear. This I think is not the way to go as front grip changes (but I guess you can toe in the inner front tire to make up for this load change) but more importantly, I would think this method would be too slow to react.

Whats your opinion guys? Anyone from Monash care to step in? I know their spool setup was done wonderfully. The car was pulling hard throughout the skidpad with just one rear tire through almost the whole thing!

[Chris B]

diagonal WT is the way to go in my personal opinion. theres alot of reasons for it which i wont go into but to answer your question, everything in my experience with locked rear ends is diagonal WT. and dont worry if the drivers adapt to the locked rear properly the effect will be quick enough and not sluggish.

[Tom Wettenhall]

Here's the other side of it:

Check out go karts for your caster-based DWT. Bear in mind that with a go-kart, you have a short chassis with removeable/replaceable members which can be used to tune torsional rigidity and thus the net amount of DWT. Some SAE cars have adjustable caster, which might be worth a look for a similar effect. All the same, easy cornering, no understeer. Unless you've got the torsional rigidity to work it, don't even bother with DWT.

MUR '10 has little caster and a pretty soft chassis, so its cornering comes from an epic rear roll rate and much right boot. (MUCH right boot)
The advantage is that it's way easier to tune out that weight transfer if you need more grip, and you can make the car a bit more predictable than it otherwise might be 'cos the weight will even up across the rear when you oversteer, whereas with caster you have to centre the steering to plant the rear again.
Also, once you do, it's gonna want to go straight even if you've still got some lateral g, so (Speculation starts here) I suppose you might be able to get cleaner corner exits without DWT. (Speculation ends here) Certainly the promo videos look pretty cool when even your green drivers can do full-bore 10,000rpm four wheel drifts.

My personal opinion would be, for a single with a bit less power you'd probably go for DWT as you don't have to try so hard to minimise the inherent understeer, (It's possible, even with our 2010 car which has been a bit... ah... in the past) whereas with a four like ours, (many herbs) the roll rate method might be the go. (Speculation begins again) From what I understand of the M10's (Monash) dynamics, the rear wing keeps it in line pretty nicely if you overcook it. (End of speculation)

Oh, and DWT is probably the way to go if you're chasing skidpad, because our method sort of plays your four tyres off against each other, rather than the kart method which just takes the inside rear out of the equation and lets the other three do their thing.

General spool advice: Wide front track. Big tyres. Don't even think about running without adjustable sway bars.

[Tom Wettenhall]

Hey, what do you mean when you say 'dynamic toe'?

[wagemd]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tom Wettenhall:
Hey, what do you mean when you say 'dynamic toe'? </div></BLOCKQUOTE>

Thinking bump steer... I don't have much experience with spools, but I know they do things like that to make them work a little better.

[Tom Wettenhall]

Ah...

Do things like that: Yeah, sure. Two-position toe link pickups. ~0 bump steer, or moderate roll oversteer. Simple.

Work a little better: It's a nice idea, but it severely limits the amount of power you can put down if your rear wheels are facing outside the corner. It's essentially wet weather in a can in my (limited) experience.
Seems like spool setups nearly always have to be driven lairy and hairy to really work, so your driver needs to have a great deal of confidence in the rear end because they're always going to be pushing it, unlike say an open diff where your best speed comes from clean driving with no wheelspin.

On the other hand, maybe the Aussie teams who run locked diffs (Or damn close, like Swinny) have just seen too much of the V8 Supercars...

[wagemd]

Oh, not advocating, just defining what I think Swong was talking about.

I'm somewhat partial to my diff...

[swong46]

Hey guys, thanks for the inputs. Wagemd is correct, bump steer is what I meant.
We will be running a single cylinder so perhaps DWT is the way to go. But I like the idea of tuning with roll stiffness rather then caster/scrub and all that.

Tom, I totally agree with the driver style; a go-kart wont "work" unless pushed hard and it definitely takes a lot of confidence.

But as for my ideas on design, am I on the right track? Keep the feedback coming hah

[Tom Wettenhall]

Wagemd, I wasn't intending to have a go at you mate, I was just trying to elaborate for Swong's benefit. Sorry about the confusion.

Swong: I can tell you what I know, heaven knows it's damned hard to find information on designing for a locked diff, but you need to evaluate how you want to build your car based on your own understanding and the priorities of your team.
For instance, you may want to use caster to create DWG, but can the team afford the extra weight of tubing that has to be put in to your chassis to create the necessary rigidity?

A couple of things from your OP which may be errors: First, unless you have physical movement, such as excessive roll, which makes your suspension run out of droop travel, your inside rear wheel will not leave the ground. Weight transfer alone will completely unweight it but will not remove it from contact with the ground. This is a good thing.
Second, apart from the tiny movements caused by body roll and other stuff like fuel burn, your CG is basically static. Did you mean RC?
Finally, remember that changing load conditions at the front as a function of steer angle can be corrected dynamically and in kind with Ackermann geometry. That's mostly what it's for.