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11/27/2015 4:52 pm  #1


Introduction of Myself and My Project

Hello, I am new to this forum.  After hours of research into different types of suspensions, and an even longer search into possible vendors, I just couldn't find what I was looking for.  So I decided to go on an adventure to build my own.  I've got nothing to lose.

My project is a 1979 VW Rabbit, that I am converting to a mid-engine, RWD hill climber.  Obviously, the torsion beam suspension that the car came with originally, is going to just be in the way of a mid-engine design.  So, it's got to go.  What does that leave me with?  Well, I'm not interested in complex multi-link - too hard to set-up, align, etc.  The mac strut is simple, but it does somewhat complicate wheel availability and spacing. (I want wider, staggered wheels on the rear)  So for me, the only choice is the SLA double wishbone.

Now, the fab part isn't hard for me.  The design is new to me.  I understand most of the concepts.  Maybe what I'd like, is a realistic baseline.  Has anyone else done a similar design or application?  I'd also be interested to see clever ideas that people have come up with for tools or simplified designs.

Anyway, I look forward to a prosperous and rewarding experience here.  Thank you, in advance.

PS - webmaster - is it possible that you could either change the font, or load some additional BB skins for those who would like to change it?  This is a very unfriendly web font. (sorry, but thank you!)

 

11/29/2015 2:37 pm  #2


Re: Introduction of Myself and My Project

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?

 

11/29/2015 3:35 pm  #3


Re: Introduction of Myself and My Project

I'm going to use a European model Audi TDI transaxle. (01E FWD)

With the mid-engine, I've still got a fair amount of weight in the front of the vehicle.  I've got the radiator, battery, gas tank, water tank for the intercooler, and ? up there. 

Even when you go to the VW/Audi AWD, you're still getting FWD with on-demand rear wheel engagement. (when the wheels slip)  Unless you get one of the few models with longitudinal engine placement that had full time AWD.  But then, you've got to put the engine in the bay, the other way, and you've put more engine ahead of the driving wheels.  More understeer.

The transaxle that we're using puts the engine in a longitudinal configuration, for a TRUE mid-engine design.  I purposely avoided using a transverse engine, because I didn't want a rear-engine car, for the reasons that you just mentioned.

To be honest, my son is in high school, and they have no automotive program at his school.  He really likes cars, and wanted to do something completely different than the standard LS engine swap in a small car/truck.  I couldn't help but jump at the chance to do a silly and impractical garage project with my kid.

To answer your question about the suspension swap...  Couple of reasons we don't do that.  Number one - I want to use OEM half shafts.  That sets my width perfect for the widebody kit we are going to use.  Number two - the mac strut setup really limits what wheels you can use. (because they end up rubbing)  I dont' want to use spacers to buy me real estate, because now I'm risking floating outside my flares.  Nope.  Number three - there is a good chance that once we build this car, we'll put it through it's paces.  Hill climb cars are meant to go fast around curves and bends.  I plan on doing a cross country in this car with my son, when it's safe to do so.  

Last edited by solid7 (11/29/2015 3:38 pm)

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11/30/2015 10:18 pm  #4


Re: Introduction of Myself and My Project

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)

 

12/01/2015 5:02 am  #5


Re: Introduction of Myself and My Project

I can't agree that the  strut has nothing to do with wheel selection.  Either you're stuck with what you've got, or you've got a clean slate.  If you're dealing with a built car, the strut is absolutely a constraint for wheel selection.  If you're starting from scratch - like we are - you choose the wheel first, but only after you understand why you're choosing it, and what constraints it places upon you, thereafter.

VW/Audi cars are a tight package, and require someone knowledgeable in their quirks to do suspension tuning, lest rub issues present themselves. (and it's quite common for people who try to tune their suspension to destroy ties through rub)  I've chosen some OEM components, which basically give me a stock track width.  So here is wherein the problem lies.  The coilover is in very close proximity to the OEM wheels.  So close in fact, that when I brought this particular car home on a dolly, I got the wheel strap stuck between the rim and the coilover.  Now, using your suggestion of using a VW/Audi subframe, rather than building my own suspension is an OK thought, but you can only do so much with that, because of the close coilover placement.  As I mentioned earlier, I know how wide I want to be overall. (vechile overall width)  So, any width that I want to add to the wheels, has to come inboard.  I'll be using wider than stock wheels, but I can't come any farther inboard, on account of the OEM placement of the coilover, because I now have a wheel/tire rubbing. (strike that - it's not a rub, it's a collision)  That's half the reason I'll be going to a double wishbone.  (The other half is the dynamics of the vehicle)  Of course, you are right that the vehicle designers selected a strut placement that had safe HP of the vehicle in mind.  But we're throwing that out the window.  We're effectively changing every parameter that was initially designed into this car.

To add a little more depth to my rationale, do a Google search for "Berg Cup".  Or better yet, check out the YouTube videos. 

It sounds like you are giving me advice to save me some work and expense.  While this would normally be highly appreciated, I have to say again...  this is a project that I've undertaken with my son, and I want to expose him to as many different facets of automotive design, fabrication, and tuning, as possible.  Because quite frankly, he's just not going to get this kind of education anywhere else, anytime soon.  No book or classroom will ever  beat putting your hands on something, afterall.

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12/01/2015 10:05 pm  #6


Re: Introduction of Myself and My Project

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)

 

12/02/2015 4:46 am  #7


Re: Introduction of Myself and My Project

tyrellracing wrote:

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.

 
Yes, I agree, and that is exactly what I have in mind.  There was never anything else in my mind, but an LSA. (OK, I’m saying double wishbone, but I consider this a variant of it)  Regardless, there will be no strut suspension in this car.  There are some cars that we’ve looked at that have LSA subframes that would work for us.  But I’m not so sure that the geometry of those subframes will be ideal for our car.  Plus, I just want to say that we’ve designed and built our own suspension.
 

tyrellracing wrote:

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.

 
That is more the direction that we want to go.  (again, getting away from OEM subframes)  I’ve never been a pure machinist for a living, but I’ve owned my own shop equipment in the past, and spent a lot of time in the machine shop as an R&D engineer.  I’m definitely looking to do something similar to this.
 

tyrellracing wrote:

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.

 
Again, totally agree.  In building this project, I want to bias my son from the outset, to the world of performance driving.  No more conversations about flashy cars.  Let’s talk about how it handles. J
 

tyrellracing wrote:

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?

 
Not following…  Did I say something about hill climbs without turns?  The bends and curves are EXACTLY why we’re building LSA suspension.  In fact, some time ago, I pulled the rule book for Pike’s Peak Challenge, and am trying to build the car as close to spec, as possible.  My reasoning is, if it’s in the rule book, it probably works well.  Suspension has nothing to with that, talking more about roll cage, weight, etc.
 

tyrellracing wrote:

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.

 
Rabbits are definitely monsters on road courses, because they’re so light.  They are one of the first vehicles that make many people fall in love with small, nimble cars.  And there isn’t any part of the body that you can’t buy for cheap.  So it’s an excellent vehicle for experimentation.  Plus there are hordes of others who have gone before, so info abounds.  That being said…  They’re also an economy car.  The torsion beam/trailing arm that causes them to lift in corners, is not only a starting point for suspension tuning, it’s also a significant contributor to excess weight.  And it just doesn’t work for a mid-engine conversion.  If I’ve got to adapt a lower control arm, anyway, Why not just get everything I want out of the experiment?

I’m in the process this week of trying to find the wheels that we’ll be using.  I’ve all but decided on a 2.7L biturbo engine.  So, wheels…

 As for Vette owners – funny or not, I have never regarded a Corvette as a vehicle built for great handling. (any car can handle, with some work, obviously) It’s a staple of the American “go fast in a straight line” genre.  LOL
 
 
 
 

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12/06/2015 2:16 pm  #8


Re: Introduction of Myself and My Project

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. 

 

12/06/2015 2:25 pm  #9


Re: Introduction of Myself and My Project

PS As to corners... My bad. must have miss read it. At 52 yo my eyes are not what they use to be. Especially when staring at my computer monitor for hours on end. Well I need to turn this dam thing off, go outside and do something productive with my time. Or atleast make my wife think so...

 

12/07/2015 4:30 am  #10


Re: Introduction of Myself and My Project

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.  http://cdn.boardhost.com/emoticons/lol.png
 

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.

 

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12/07/2015 8:25 pm  #11


Re: Introduction of Myself and My Project

Great software to get your lay out pretty dam close before you cut into anything you had to spend money on. Isnt technology great these days? We use auto cad at work for all of our Haas machines and it is very similar to your computer aided drafting software. Beats the hell out of the trial by error or poke and hope with a welder. The weight bias you are shooting for and the power train you chose should make one hell of a ballistic rabbit. Please post pictures when you have something assembled far enough to be proud of.

 

12/08/2015 4:09 am  #12


Re: Introduction of Myself and My Project

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.

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