Why not just move the front suspension to the rear or go to one of vw's all wheel drive set ups and leave the engine where it is? I was thinking about doing something very similar to a Honda Del sol I have had too long. I put a B18 V tec with a turbo in it 5 years ago and was never happy with the FWD traction. I have always liked the engine over the drive wheels concept. However in a FWD config. it falls short in the first corner you encounter severe under steer. In the mid engine config. you are still dividing the weight between the front and rear axles. Thats a desired trait.  The vw and honda trans axles place the engine almost directly over the axle center line so both set ups moved to the rear would make them rear engine rear drive and the polar moment inertia would kill the design. This would be a bad design due to Uncontrollable under steer/ over steer. Too little weight on the front and too much at the rear would make the vehicle spooky to say the least. This was where I was stuck in my design decision.  Were you going to use the rabbit trans axle or did you have something else in mind?

I had no idea that the VW trans axle did their torque split favoring the front so heavily. Typically a differential is used between the front and rear drives with a viscious coupling controlling the F to R bias. These set ups always have all wheels driven. The bias doesnt take effect until one tire begins to loose traction. At least thats the way the AWD trans axles I tore into worked. Who knows I learn new things every day. I agree in that there will always be fuel, battery and so on to off set the rear engine weight on rear engine rear drive vehicles. I just like to see all heavy items placed between the axles. All heavy parts placed out board of the axle center lines behave like a pendulum. This is a bad trait in any vehicle.. I like your planned vehicle. It sounds like you are on the right track as far as drive train parts and lay out. Since the rear wheels do not turn I didn't think there would be a rub issue. I would be inclined to cut more sheet metal out of the inner fender wells and use wider wheel tubs to put enough rubber at the rear to keep the mid engine set up safe. Personally, I don't care for wheel flares. They seldom look right bulging a classic body line. Some astute engineers have pulled them off successfully like turbo Porsche. That's the exception.. Letting the tire width grow inward with tubs would solve any rub issues caused by converting to rear drive no matter which lay out you use. Rear struts are Chapman struts. Chapmans do not allow the wheel to turn. IE 240 Z.  Macphereson struts are used on the front and allow the wheel to turn on the center line of the shock stem or ram. The strut doesn't have anything to do with wheel selection. The hub and spindle's carrier bearing offset controls the wheels back spacing and distance from the strut. The strut will typically be bolted to or clamped to the spindle. The spindle design is entirely responsible for wheel clearance. Chapmans do not turn so they can be positioned further from the wheel than Macpheresons. For this reason using Mac struts at the rear is a poor choice because they are placed by the spindle at the center of rotation for a wheel that turns. If you draw a line through the center line of the front Mac strut to the ground it will pass through the center of the tires tread patch. Any distance off center is called scrub radius and no stock FWD vehicle has scrub radius. That would rip the steering wheel out of your hands if you hit a pot hole. If you draw the same line on a rear Chap strut it will be inboard of the tires tread patch by at least 2 inches . This added distance makes a massive difference in wheel choice and is why all manufacturers that use struts on the rear of a rear drive vehicle with any intention of agressive driving will use Chapman struts offset inward by the spindle or hub carrier enough to fit the width of wheel they need to make horsepower delivery safe...

Last edited by tyrellracing (11/30/2015 10:25 pm)

If you make your own spindle, you can locate your strut as far inboard as necessary to clear the wheel.. However, at that point a LSA suspension would be a superior route to take. I am a machinist, I made a copy of a Griggs racing race type GR350 front suspension for my old shelby. With that I made my own 3" drop spindles out of 4140 steel. This allowed me to use 19x10 wheels in front with 14" rotors and 6 piston calipers. I made the brakes too. I built the suspension around the wheel size and back spacing so even lowered they tuck into the lightly modified front fenders. Fender lip was raised 2" to allow the wheels to turn lock to lock. I have a Jag rear in it with custom built bearing carriers.  The aluminum jag carriers devoloped cracks and I needed bigger rear inboard rotors so I made new inner lower control arm mounts that moved the pivot 1.5" down from the stock jag location. By doing this I was able to go from custom made 11"x.75" rotors to 13"x1.125"with stock 67 mustang Kelsey Hayes 4 piston calipers and.250 spacers to accept the wider rotors. This required the bearing carriers to grow in height the same 1.5" to maintain geometry.  Now the carriers are thin wall steel.. I built a T5 suspension for it to get rid of the inherant Jag wheel hop {see the ford irs section} ran it for a while then decided I needed more rear traction and went back to the Jag set up because the T5 limits wheel back space with the leading and trailing links.The rear wheels are 19"x11"now, the T5 only allowed me to use 9" wide rims. I will not modify the rear fenders or the exterior appearance on my 67 gt350 for wider rear wheels. Front fenders are easy to replace and are readily available on e-bay. I am a firm believer in negative camber gain suspensions so I dont like struts in any performance applications. You will get near identical performance by welding the carrier to the frame. With struts when the body leans the wheel leans with it. All cars lean in corners, unless you weld the carriers to the frame.  That is not a desired trait. With SLA suspensions set up properly the outside wheel leans into all corners like a motorcycle would.The inside wheel leans into the corner as well but weight transfer makes its contributuin to traction less than the outside.  Negative camber gain is a desireable trait that all modern performance vehicles have designed into them. You will never find a strut under a Ferarri. This is not intended as any kind of insult. Just explaining my pedagree and the facts surrounding the negative side of struts vs LSA suspensions.. You said you were building the rabbit for hill climbing. I have never heard of hill climbs that were not on knarly windy mountain roads. IE Pikes peak. Just curious. Where are the hill climbs with out turns?  Rabbits are cars that can be monsters on road corses in modified form. Both my little brother and I went through our novice year of road racing silly rabbits and did very well. They look funny when they lift a leg in hard corners but on 3 wheels, trail braking will out corner vettes doing it. The vette oners dont find that too funny. The way you get a strut suspension rabbit around a corner fast is to make it so stiff it beats your kidneys out. We did not encounter any tire rub we could not fix with our GTI's. We ran racing slicks that were aprox. 3" wider than stock. This problem is not specific to strut vehicles. It can occur with any vehicle you change wheel and tires on.  Back to welding the hub carriers to the frame.

Last edited by tyrellracing (12/02/2015 12:10 am)

I agree with you. Especially in regard to Vettes. The only reason I used them as an example is because in SCCA at the tracks around here they will run multiple classes simultaneously. You will not find formula fords and open wheel cars racing with sedans but you will see vettes, rabbits and mustangs running together. Rabbits were a natural choice for me because I was young with limited income, unlimited enthusiasm and they were CHEAP. I think I got my first lo mile GTI for $500. It had been hit hard from behind. Then I found an older one with a million miles and a perfect body for scrap price. The solution was obvious. That car was a terror and I loved driving it. Rabbits are light to start with but gutted  they become almost too light. The 8 pt roll cage solves that nicely. I feel almost jealous hearing about your project. I had as much fun as a twenty something guy can have with out a woman, with those little cars.  LOL  I have no doubt you will have fun too. 

I'm sure that it's not just your eyes.  I had a hard time at first, on account of this insidious font that is used here. If you want to change it, a simple solution is to download a plugin for whatever browser you use. (I use both Firefox and Chrome)  There are a couple of different ones that will allow you to selectively change the font for either specific websites, or all websites, and it can be toggled on and off.  Great for the older guys, too, because you can even set it to Readers Digest Large Print Edition.   

At any rate, I'm paying close attention to both weight, and weight distribution.  What I haven't said thusfar, is that since we are basically doing a full tube chassis, we are modeling it in CAD.  I can mock-up (in block type geometry) the engine and transaxle close enough to determine a good CG, and the mass is already known.  Obviously, designing the entire chassis in the program easily lets me arrive at a combined CG of my subframes/cradles and the rest of the chassis.  And, it gives me a good cut list.  So, yes, if we start to look a little light in any areas, we may have to move some components, or possibly even ballast, if the weight just isn't there.

I'm shooting for a weight distribution in the neighborhood of 55/45, with a rear bias.

 

Thanks for the comments.  I have my build log posted in my sig, but I'll certainly throw something out, when I get a chance - especially the suspension.