Modal Isolation

Sorry for the absence of a post yesterday, but I was flat out on the CAD preparing for my evening class. They're going to be making a pipe vice, a mill stop and a plumb bob (or at least parts of them) as assessed pieces, so they all have to be modelled in CAD and then dimensioned up as 2D drawings before being cross referenced to the specifications of the qualification units. Anyway, here's a couple of quick renders of the two main items:
But getting back on with the car, I've been thinking long and hard about modal isolation, mainly because this month's issue of Racecar Engineering has a review of the Formula SAE/Student/FISITA World Cup events - annual competitions for university undergraduates to build their own deathtraps 600cc racing cars. Back when I was just a sprog in ride and handling terms, I worked closely with a certain British sports car manufacturer on fitting an active suspension system to a tank. The system worked on the principle of modal isolation. In other words instead of each wheel station having a spring and damper rate associated with it the overal vehicle was considered to have four specific spring and damper rates: roll, pitch, heave and warp. Warp was effectively torsion of the chassis, but that's probably better known to racecar engineers as either Roll Moment Distribution (if you read Milliken & Milliken) or Magic Number (If you've been trained by Claude Rouelle). Sitting in a car with a laptop you could change any of these rates and make either a dream handling vehicle or one that would disappear off into the undergrowth at the prod of a key. In fact if you had a half-decent egotist test driver you'd prod the key mid corner while trying desperately not to laugh as you sailed off into the undergrowth.


So, where does this come in for us - I'm hardly likely to specify an active ride system (especially as they cost around £500k - Moog servovalves tend to be around 10K each and you need at least eight per car). Well, on most cars the spring and damper units have to do all the work for all four modes and as a result you tend to have a bit of a pigs breakfast when it comes to getting an optimum solution, especially in damping of modes other than wheel heave. If you could have separate springs and dampers for everything, you could make large gains in handling without compromise.


Such systems are fairly popular on racecars - F3 have been using monoshock systems for over a decade, although these don't have any roll damping whatsoever. I'd be looking for a system that replaces the two coilover units and an antiroll bar with a pair of coilover units and a set of linkages and bellcranks. I've got a couple of ideas that I need to get modelled up so I can check the kinematics with Solidworks. I would ordinarily start designing my own rotary roll damper but I reckon that would be overkill so an extra bellcrank might be the order of the day. The basic idea is to have no ride springs and a T-type anti-roll bar connected between a pair of bellcranks operated by pushrods (or pullrods, for the benefit of my other reader). A coilover unit would be connected between the chassis and the centre of the T-type bar and thus be operated in heave. A second damper would be connected between a floating frame on the anti roll bar and the bar itself to give separate roll damping. It's the latter part that's causing some grief at the moment, mainly because the whole application lends itself wonderfully to rotary damping but chucks up a load of interesting kinematics problems when it comes to linear dampers.

Oh well, time to get the back of the fag packet out and start sketching...