When race cars get "really light" what starts going wrong.

I have projectcaritus as in all my cars are finished and finished cars are boring. I was completely on board with building a 550 spyder replica next but got sidetrack by Manx's. Got the bug and after looking at them now have a really stupid idea. Why not build a VW engined tube framed street car at right around 1K lb's total. IE a 550 spyder minus 400lbs but not the 35K it was going to cost me. All in I bet I could get this done around 9K to a seriously high spec.

So lets just say you had a "Manx" body a tube frame with a Formula Vee style suspension. Single seat on the left side, engine in the middle, transmission flipped around like a 550 spyder. Its got the weight and the drivetrain of a Formula Vee car but with say another 40hp. Formula Vee cars are scary fast for the power.

It would be around 1000-1050lb's give or take if you really went nuts with stripping a Manx out. Slam some say R888's for the street on some really lightweight 16's and put solid 100 bulletproof engine in the middle.

Would this have the same general driving dynamics of say a 3000lb tube frame racer with 300hp. Or is the weight not linear in terms of speed, same with brakes. 13 inch rotors on willwoods on a 1K car vs say 15 willwoods on a 3K lb car. I bet I could get 10 inch wide rubber on all 4 corners if I really went nuts.

So what going to go wrong, what does not behave like on a heavier car with the same power to weight ratio.

So you want a vw ghettocet? Lol. It's a really neat idea. Make sure it's STRONG.

Trackmouse wrote: So you want a vw ghettocet?

That is exactly what I want. I have been looking at building a Vette cart as well.

But having a "VW beetle" chassis or at least the bit with the vin on it under a Manx makes it a fully legal road car in California. Manx body gets me past the fender requirements, I can run a cut-down windshield. Noise and safety is almost completely thrown out the window, I can get a fix it ticket maybe.

Low weight means tires last forever, same with brakes, engine is only pushing 1K lb's instead of 2.2K or so in weight around. So engine i significantly less stressed.

What I keep wondering is , all things being equal. is say 10lb's / HP the same across all rational weights or when going this low in weight am I somehow much faster then a heavier car with the same ratio.

Having driven an 800lb F500,a sub 1000lb homebuilt,a 1600lb awd geo metro ice racer,currently a 1680lb tercel ice racer on top of all the usual weight miatas,E30,civics etc etc I will say that every aspect of a well sorted lightweight is better than a well sorted....well anything heavier.

Light cars do need light wheels and tires though,R888's aren't great on a light car(or a heavy car for that matter)but are ok.I see 0 reason for going to 16" wheels though.

I'd stay with 8" wheels and 205's,with R-comps(stiff side walls) intended for cars nearly 3 times the weight going wide won't work out as well as reasonably sized rolling stock.

For track use go with 13x10's and hoosier slicks(prefer the avons actually),their actually designed for the light car and will be a joy to drive.

Well, non-traction limited acceleration is theoretically the same. Traction limited acceleration is likely better in a lighter car because there is less to accelerate (same traction pushes light car more than heavier car).

Turning is way faster in a lighter car.

I'll be the jerk here (some things come naturally). The whole "lighter is better" while certainly not obsolete, just doesn't cary the weight that it did when there was a very limited variety of engines - all unimpressive by modern standards. To me, if you're going to put that much time into a vehicle, why not allow a couple hundred more pounds and put a nice subbie motor in a mid-engine configuration? Even the venerable 2.2 will be much better than the aircooled VW.

Robbie wrote: Well, non-traction limited acceleration is theoretically the same. Traction limited acceleration is likely better in a lighter car because there is less to accelerate (same traction pushes light car more than heavier car). Turning is way faster in a lighter car.

...until you get to a fast track, where it becomes less about power to weight than power to drag. Small, light cars usually don't have correspondingly less drag than heavier car with the same power:weight, so the heavy car will be going a lot faster at the end of the straight.

C&D does the "Lightning Lap" at the big, fast VIR track, and this is really obvious if you look the overall results.

In reply to Kreb:

At that point I would almost be better just building a Project 818 but I agree with the sentiment.

I am bascially trying to talk myself out of building a exocet proper as its going to be 17-20K USD when I am done. I just know it. I want cheap and chearful and easy to fix.

As for the tires, that was a big concern, at what point does the low weight just make them ineffective at getting the heat in.

Any reason a motorcycle engine couldn't work?

Snrub wrote: Any reason a motorcycle engine couldn't work?

Cost and lifecycle. A VW or Crvair engine to a VW tranaxle is pennies and a phone call to get sorted out. Motorcycle is much more expensive to figure out. Plus reverse, also bike engine are for pushing 500lb bikes aorund not 1000lb race cars so it would be a lot more stressed.

The overstressed bike engine stuff is overblown for the most part imho,I beat on mine with slicks and hundreds of drag starts autox'ing with 0 issues in 3 yrs.

2 seasons of standing starts(60 races)in the awd ice racer with studded tires(way heavier than most consider safe for a bike engine)again with 0 issues.

Appropriate gearing is the right answer,litre bikes are geared for at least 170mph for the most part.Shorten that up and the clutch bits a much happier.

Your street driving it though so a bike engine isn't well suited even though I've never tried it.

For tires your probably best to run the latest UHP tire like the Rival or Re71R's than the R888's(no idea what the new R888 r is like yet)due to the softer sidewall.From what I gather they are likely a softer compound than a R888.

I think you're more or less describing a lowered short wheelbase mid engine sandrail, the mid enigne ones tend to be longer though.

I'm with kreb on this one. The idea sounds great but why in the world would you use a air powered vw when you can have twice the power for the same price

If you ever do a track day the drag limitation is really going to suck

When race cars get really light they become very loose but very easy to recover, and yes there can be trouble getting heat into the tires...look at some videos of Sevenesque cars on the 'ring to get an idea of how they handle. Downforce can make them handle a little more...traditionally.

GameboyRMH wrote: When race cars get really light they become very loose but very easy to recover, and yes there can be trouble getting heat into the tires...look at some videos of Sevenesque cars on the 'ring to get an idea of how they handle. Downforce can make them handle a little more...traditionally.

A sevenesque car isn't loose because its light,they tend to be loose due to design constraints.

Take a wishbone front suspension with decent geometry and combine with a heavy stick axle(especially relative to overall weight) with no camber and bingo instant drift machine.

I didn't mean "loose" as in "sidewaystastic," although you're right that sevenesques also have a handling bias well toward oversteer. I mean "loose" sort of like the "knife edge" handling a car has when its handling bias is very close to neutral. Lighter cars seem to be naturally more "knife-edgey," but also easier to recover...so not exactly the same effect, but it is easier to break traction on whichever end. Downforce gives you more margin for error.

In reply to GameboyRMH:

Dunno,lighter cars respond faster than marshmellows but well setup just means going where you ask it too WHEN you ask it too.

I consider light cars responsive to drive,heavy cars reactive to drive.

Reading what you are looking to do makes me think you are wanting something similar to The Lot Lizard but with mid-engine.

if you were on a budget i would say find a 4 seater buggy on CL so that your cage work is already done (or atleast a solid base to start from)and has room to flip the engine to the middle and go from there.

Eddie that is EXACTLY WHAT I WANT TO DO>

How you sold that for 5K and did not have people beating down your door to pay more is haunting me right now.

Back when the NorCAL UFO (United 510 Owner) was active. A number of locals used to show up with myers manx dune buggies all kitted out for autoX. They were really cool to watch. They tended to fight understeer as the weight bias was strongly to the rear. One guy was rumored to have concrete in his front bumper to help balance the car.

If you take a manx and cut out the parcel shelf behind the drivers seat, there might be room for a drive train from a FWD car.

In reply to icaneat50eggs:

I'm not sure what the OP is thinking, but for me the strikes against the Subaru are weight, complexity, and plumbing.

If the goal is really light and cheap, air-cooled has an appeal. It seems like the Subaru might be creeping the scope a bit. If he keeps it largely Bug/buggy-based, it simplifies figuring out where the Subaru bits end and the Buggy bits begin. Fewer worries about axle shafts, no need to mess with a radiator and water lines, plus no need to build a fuel system for EFI.

Although many of the above concerns aren't really that hard for many of us on this board, I feel like they are going to be hard to do for air-cooled VW money with an ambitious weight target.

My dad and I did what you are talking about. 8 years ago we built a sand rail. 2 years ago we got into autocrossing and slowly started modifying it for that purpose. We lowered it, moved weight forward, and got better tires. It has been an awesome buggy and we went from 90/120 to 30-40/120 at our local autocross

Nobody ever asked for a heavier airplane. The lighter the car, the less wing you need for the same cornering speed increase. If you can pull 5g in all of the corners, the only thing the engine needs to do is overcome the drag.

A lighter car will have less centrifugal force in a corner.

Ergo !