[The CyberPoet]

On a separate note, is there a limit to the size of the airbox (either practically, or by regulation)? Could the airbox be built in a 50 or 100 liter size without an issue?
In theory, if you could get the airbox large enough, you could run multiple superchargers or turbochargers to induce such a vacuum level at the restrictor plate as to make it almost moot (imagine increasing the vacuum level on the inboard side of the restrictor to something in the neighborhood of 8 ATMs). The sound levels would be attrocious, but the result would probably prove to be quite effective, expecially if the compression (vacuum creation) were driven by batteries or something else not very parasitic to the engine's output.

Cheers
=-= The CyberPoet

[alriclam]

Cyberpoet, thats a very unique idea! Though the cost issue (car needs to be less than 27,000 dollars) and the weight issue from multiple superchargers or turbos would probably outweigh the added gains. Oh, and sound level has to be less than 120 dB or so, measured a certain distance from the exhaust....

The airbox isn't constrained other than by the constraints placed upon the size of the car, and the fact that all parts have to be within the envelope made by the top of the main roll hoop and the rear wheels.

Thanks for the wheel sensor info, I'll pass that along to the data acq/electronics team. Might I ask what you mean by "unsprung weight equation"? We have multiple electronics sponsors, so finding the individual sensors shouldn't be too hard, we already have some tiny hall effect sensors from Allegro Microsystems. But I'm still not sure if thats the one we want to use. I was thinking we could just encase them in epoxy right to the upright surface, and mount the magnet on either the hub or brake rotor mount. But I dont like how we have to power them.

For the front wheels, we'd only be able to place them close to the wheel, though in the rear, we'd have more options, we run two brakes right next to the diff.

[The CyberPoet]

Might I ask what you mean by "unsprung weight equation"?

The car is broken down into two basic masses -- I think I have the English terms correct here, but I might have the terms backwards (the concepts still apply):
The unsprung weight (wheels, tires, a portion of the axle, i.e. -- the masses that sit outside the springs for the suspension) and the sprung weight (the rest of the vehicle -- the engine, body, driver, etc). Handling and ground cohesion improves the higher you can make the sprung weight to unsprung weight ratio (i.e. - the less mass is unsprung, the faster the wheels return to the ground after a bump, and the faster they respond to changes in acceleration, braking, changes of direction). A long time ago, Alpha Romeo (the car firm with the huge racing heritage that first invented the aluminum block and the twin-spark-plug per cylinder and overhead cams and a host of other things) figured out that one way to radically reduce the unsprung weight was to move the rotors and calipers inboard, as close to the centerline of the vehicle as possible (such as part of the rear axle right next to the rear pumpkin for rear wheel drive cars). This permitted use of broader slotted rotors for better heat capacity, larger calipers to both keep the brake fluid cooler and to give more pad area... all while shedding that weight off at the wheels (i.e. at the unsprung masses) and improving weight centralization (which improves the CG behavior in cornering). The downside to this design is that the brakes require getting under the car to work on, but the upside is far better handling. You can find a good example of this on most of the Alpha GTX models of the late 70's, early 80's for example...

Cheers
=-= The CyberPoet

[Anonymous]

CP - you have your terms in the correct order.

The drag racer's rule of thumb (for cars, at least): for every one pound you remove from wheel & tire (the unsprung mass) you need to remove eight pounds from the 'body' weight (the sprung mass) to acheive the same performance gain. In other words, if you wanted to reduce the vehicle weight by 100# so that it goes quicker (handles better, etc), you could achieve a similar performance gain by reducing the wheel/tire mass by 12.5 pounds. This could be as simple as changing from steel rims to aluminum, or perhaps removing the tubes if you're running slicks, etc...

[tzortn]

Unsprung mass is a bad thing because it cannot be controlled. The sprung mass can be controlled by your suspension. In board brakes can help reduce unsprung mass, but they are often a packaging issue. Another issue with in board brakes is cooling. You need to make sure that you are not shrouding them too much or do as CP mentioned and increase the rotor/caliper sizes.

As for your air box size you do want it to be as large as possible. That gives the engine a little bit of a reservoir to tap into, especially if you can take advantage of ram air effects. The restrictor will limit your air flow not matter what sort of trickery you devise. It has been sized to cause choked flow. You need to make sure that you have a sound intake design, but you may be better off spending time testing the entire car instead of tweaking the intake alone.

We used to epoxy magnets to the hub and our sensor bolted to the upright when we were sensing wheel speed.

[alriclam]

Thanks guys. I understand the idea of unsprung and sprung weight, I just was wondering what you meant by "unsprung weight equations" and their relationship to placing the wheel speed sensors inboard or outboard. but I probably was misreading what you were saying.

Yeah, we've wondered alot about our braking system (in the past we've had one brake in the rear that was attached directly to the diff), and the effect of going to uncoupled rear brakes (two brakes on the CV axles). We've still decided to put them inboard, in order to reduce the unsprung mass. Alot of work has gone into understanding the thermodynamics of the braking system, and I think we'll have a very sound model, though heat build up is still only a theoretical issue. We have four thermocouples that will hook up to our data acq system which should tell us whether or not the model was right. We're running aluminum matrix rotors on Wilwood racing calipers, I'll snap some pics when the system gets put together.

[The CyberPoet]

OK, so now it's two-and-a-half years down the road since the last post and I'm curious how this all turned out ???
alriclam, can you give us an update?

Cheers,
=-= The CyberPoet