Tuesday, November 23, 2010

Revised front rocker & shock arrangement

Using the two bulkhead setup allows for some extra flexibility here, including being able to package a longer damper (75mm stroke in place of 50mm). Intuitively I'm thinking that being able to use more stroke should give me some more flexibility in terms of rates and installation ratios. Not knowing entirely what kind of wheel travel to expect, I'd rather have a little much travel than too little.

Clearly I still have to flesh this out quite a bit, but you get the idea. Additional bonus is that these rockers will be stupid easy to machine, even after I add another mounting point for a F-ARB. Would be nice if I could get the force vectors of the pushrod and damper a little more in line so there isn't as much of a net force (and moment!) on the single shear rocker mount. I'll have to play around with it. At this stage of the design it's more roughing things out in terms of what space they'll have to occupy.

As an aside, you may note that when I put in blind (as opposed to 'through') weight relief pockets, I like to fillet the bottom to mitigate any stress risers. In this case I have R0.060" fillets at the bottom of the pockets on the rockers. At first, this may seem dumb from a DFM perspective - how are you going to get a DIA 0.125" ball endmill down in there without it chattering and running slow as hell?

Luckily, God gave us 'bullnose' endmills - they are clutch for this sort of application. Think of it as a flat endmill with a small radius on the end. A hybrid between a flat and ball endmill. Allows you to pocket things out with a healthy diameter cutter for efficient metal removal while leaving a nice rounded edge at the bottom. Additionally they tend to be slightly more robust. Since the cutting edges don't come to a small sharp point they are less likely to chip off. Believe they also leave a slightly better RMS surface finish. What's not to like?!

They are also known as 'corner radius' end mills. Can find them over at MSC or most places with a good selection.

Friday, November 19, 2010

Kinematic curveball - tire rates

As you probably know, the balance of a car's wheel rates (front-to-rear and on diagonals in particular) play a big part in cornering feel and over/under-steer. Tire spring rate is a component of wheel rate... so any change in how tire rate is distributed has the potential to change the feel of the car.

Before I go any further, I'll share one of the tunes I'm rockin tonight as I get my vehicle dynamics on. Don't worry about the Friday night in... tomorrow night will likely have shenanigans afoot down in Columbus.


Anyway. Obviously tire rate is going to change with air inflation. Good way to make small tweaks. Tire rate also can change with camber - significantly. Typically it goes down the more you lean a tire. This in itself isn't anything earth shattering. It's public domain knowledge, and I remembering even hearing about it at a talk on tire testing & modeling at Colorado State right as I was finishing up undergrad. Never really stopped to think about the implications.

Let's assume we have some tire rate versus camber curve like below. Note: the numbers are totally arbitrary, just to serve as an example.


We'll consider some simple car with equal corner weights and identical installation rates (Ks). Kt denotes the tire rate, Kw the wheel rate.
Now we can give the car some arbitrary ride-camber rate and an arbitrary pitch input.
Fronts are going to gain negative camber, rears are going to move toward positive. Aka, fronts will soften and rears will get stiffer. How significant is a 1.4% change? That's up to you and your driver to decide. You'd be surprised what subtle changes you can notice - even playing with 0.5 - 1.0 psi increments on different corners of the car to play with tire rate.

Can also make up some arbitrary roll-camber rates and input. Let's say in this case, a car with more roll-camber change in the front than the rear (Mustang?).
In this case there's a very slight front/rear change, but a noticeable change in cross. Yeah, and you thought cross weight was only for oval racers. In this case, going up in cross during a left hand turn I believe tightens the car up mid corner - putting extra load on the RF and LR hurts front cornering capacity but helps drive-off.

Of course this is a really simple example with identical, linear tires on all corners. In reality it's a bit more involved. Opens a whole new can of worms in kinematic design - which wasn't exactly fuckin' easy to begin with! One option is to make use of rising rate motion ratios. Presumably if you knew enough about the tires you could use that to offset the change in tire rate and keep a relatively constant wheel rate.

Alternatively it may be beneficial to take advantage of some dynamic change in cross or lateral load transfer distribution - depending on how you want your balance and braking or power-on capacity to change at different parts of the corner. Trick stuff.

Of course none of this probably makes a hell of a lot of difference on FSAE teams that decide to throw heaps and heaps of spring rate in their car, in the misguided notion that it will increasingly make the thing more responsive. Brick the suspension and you don't have to worry much about kinematics, or low speed damper tuning for that matter.

Time to figure out what the hell I was working on back in March

...and we're back - hopefully with some momentum. You may be wondering what I've been up to for the past 8 months. To put it mildly, 2010 hasn't been a great year. For a while I was busy with work - which isn't a bad thing, but didn't lend itself to much spare time. Late summer I had some woman issues... things inexplicably went to shit. The combination of depression and anxiety really killed any productivity at work or home. This went on for a couple months. Finally had to get a grip on things, make some changes, get back to the things I enjoy in life: Sarcasm, working out, racecar vehicle dynamics, martinis, and music. In any event, here I am. Enough of my personal life though...

Tough picking this up after so long! I barely even remember what I was working on. Apparently I got started on some exhaust routing (yes yes I know about cylinder matching, this is more just roughing out where things might go).
Some things to tackle, in no particular order:
  1. Packaging and 'parametrization' of the front suspension with the 2 bulkhead design
  2. ...same for the rear suspension
  3. Some legit high-level suspension engineering. I've worked hard to get things truly parametric such that I can move key points around and have the mechanical bits rebuild successfully. If you've done any amount of solid modeling, you can appreciate that it isn't a trivial undertaking! I'd like to start firming up some ideas of rates and kinematic curves though. Ultimately a big part of that is going to be picking an initial tire selection and building in enough adjustment to work with radial or bias slicks, etc. More on that in the next post.