Rear Suspension Design Review

[Richard Timte]

Hey,

I was in the process of putting the drive train in my car and had a thought three weeks ago. What about making the engine and transaxle a stress member. Since my car is a space frame and not a mono it does not even look like an original with the tubing in the rear. So by putting in front and rear motor plates with some stabilizers mounted to the stock motor mounts will secure the drive train to the chassis. Will also make a small cage that mounts to the transaxle that has the suspension pickups on it. I was also considering doing a pushrod setup, but did not want to change the look of the exhaust so I am considering a pull rod setup (Lower CG). One other good thing is I have some rear uprights off a 1999 Reynard Indy Car that are already a 6 pin setup and the 8 piston Brembo’s I have bolt right on. Below is some renderings I put together.

And a couple pictures of the rear uprights.

Here is a picture of a plastic engine with the hewland in the back of the car. Also the tires and wheels I plan on putting on the car.

 

[Chris P.]

Race car engines/trannies are typically hard mounted even if the block and/or tranny aren't really stressed members. Formula Fords are a good example of that - some trannies being a bit stressed. Remember that hard mounting puts ALOT of high frequency vibration into the whole car - this will require much more frequent checks of components, nuts, bolts, etc. How you mount electronics becomes alot more important too.

From an engineering standpoint, I would have to ask what is your real goal? If you wanted to stress the drivetrain, one would think you are taking out structure elsewhere, but I don't see that. With the amount of structure going on in those images, you don't need to stress the drivetrain - just some strategic tube placement and there is plenty of structure otherwise. Sorry if I sound negative - thats not my intention, just pointing out things from my perspective. I always love to see other people's designs no matter what.

The race boxes might be able to take some external loading, but others like the Audi I wouldn't even think of it. Not sure of your particular Hewland unit. I have an Audi and I will need some bit of structure back there for suspension too, but I don't think I am going to rely on the tranny at all for structure.

I also don't think you need an entire plate at the front like that. A light steel tube structure that picks up 2 or 3 points on the block should suffice I would think. Remember - buildability and even more importantly - serviceability!

Also - I don't know your experience with tubular structures - for all I know you have lots. I'm not some huge expert, but I do have some experience myself and one point I would make is to stay away from bends if possible. It's difficult to get things to line up and difficult to properly have tubes meet at common nodes for good load paths. I tend to visualize the important mounting points that are the very things requiring the structure I am designing, then build inbetween them avoiding any obstacles along the way. Remember too that you have a 2000lb car that will be pushing it's weight sideways while that rear structure is trying to keep it from doing that. If you are relying on the tranny case to give you all of the triangulation you need, you need to know if it is up to the task.

Good luck - looks like you're having lots of fun!

 

Its a tricky buissnies to make the engine become a stressmember using engine plates. The plates lack stability
lenghtwise. I have been thinking a lot in this direction myself, but not come up with any good solution.

If you look at the Reynard A-arm design, notice that the centreline of the arms intersect at the uppright ball-joint.
While your blue arm has strange bend on it and a ball-joint that sticks out therby recieving bending forces.

It is important to create straight loads in all tubes of the chassis, which means NO bent tubes.

Goran Malmberg

 

[Richard Timte]

Chris,

Thanks for your comments. The front and rear plates are made from Aluminum and the front plate will mount the dry sump pump, alternator and the a/c compressor so a lot of the plate you see will have holes in it. Also the lower edge of the both plates will bolt up to a cross tube in the chassis. I was already going to solid mount the engine and trans before designing the stress version so all the comments concerning solid mounting the engine will need to be addressed anyway. As for the Hewland LG600, these are the strongest boxes that Hewland ever made and were stress mounted in the CAM/AM cars of the late 60's and early 70's (1000 HP cars). If you look at the current rear structure in following picture and you compare it to the following image you will see that a lot of rear structure was removed. Keep in mind that the tubing for this is 1" thin wall 4130 alloy and the sections that are being removed are 1.5"x1.5" and 1.5"x3" thick wall mild steel.

As for goals I was trying to address:
1. Get the 14" wide rim's in the back and not have 2" long suspension links.
2. Solid mount the engine and trans in the car.
3. Lower the engine in the car as it the engine has a dry sump pump.
4. Reduce weight.
5. Have more modern suspension design.
6. Have 6-pin drive.
7. Ease of removal of the transaxle from the car.
8. Ease of removal of the engine/transaxle from the car
9. Most of all have something different.
10. I’m sure there is a 10, 11 and 12 but can’t remember.

Goran,

Thanks for you comments as well. I have had a few versions of the arms and making them straight was one of them. As for using plates, the lower edge on both plates mount to a cross bar in the chassis. Also, there will be a link between the engine mounting pads on the block and the frame. I believe this will keep both the engine and transaxle from moving forward or backward.

 

[Chris P.]

Wow - nice tranny! Guess most of your stress worries are taken care of in that case (literally)!

14" wheels!!! Hoooooo-eeeee. One thing I'm wondering looking at the new rear structure - are you going to be bracing the rear mounts for the upper arms to the tranny at all? That main tube is going to take a hell of a beating in cornering AND in braking, especially with those big tires. I can see that those green parts are the cast mounting bosses at the top of the tranny side covers - when that main blue tube flexes from suspension loads I see lots of fatigue where the tube meets that side cover interface.

Some ideas running thru my head:

1) It's hard to see what the tub bridging across the top is doing, besides making it difficult to put a fastener in there. Why not make that a rectangular cross section and put in mounting bosses for the top of the side covers? Or instead of a cross tube and separate mounting pieces like you have, combine the two if you want those fasteners in double shear. Just remember that you're somehow going to have to install the tranny into this frame!

2) don't forget that you do not have to have the 4 suspension arm mounting points on the frame make a perfect rectangle. Place them wherever you need to in order to achieve a rigid frame without giving up strength in the suspension arms. I see lots of blue tubing that isn't really doing anything.

3) any cross tubes on the lower red portion should be between the blue/red intersections if possible. It looks like the red is getting rigidity from the tranny already where you have the 2 cross tubes now - overkill?

4) There doesn't seem to be many external mounting points on the LG600 case - how was it used as a stressed member?

5) there is going to be alot of stress at the belcrank pivot on that pullrod setup. Remember too that pullrods don't have the same geometry as pushrods. The pullrod itself does not move much per wheel travel - not as much as a pushrod can, simply because of the geometry. The mounting point for the belcrank should be very, very rigid. I don't see why you couldn't mount a shock as standard right between the bottom arm and the frame. You won't be adding very much unsprung weight and you will be saving sprung weight. Also remember with belcranks you're adding more wear points for slop, not to mention complexity.

 

[Richard Timte]

Chris,

Good info as before. Below is an older design that I did. Lay down shocks and a sheet metal box that goes across the two side bell-mounting bosses. Also the A-arms are of a straight design. The reason I changed to pullrod design is to lower and bring more to the center the CG. Also it would look cool. This is why I posted here, to get some input on my design and not some problems on the track.

Here is a picture of a Mclaren M8F with the rear body section removed. Its hard to see but the upper link connects to the side bell bosses and the lower inverted A arm connects to a substructure that is bolted to transaxle mounting points.

The blue and red are separate pieces with a threaded slug in the upper cage and a drilled through round bar with a bolt holding them together. That way the upper and lower cage can be put on the transaxle from above and below respectively.

 

[Trevor Booth]

Richard,
Why do you not follow the Mc Laren set up with reversed lower A arm and trailing links/radius rods. The accel and braking loads are fed into the main frame. Tranny then only carries the locating and cornering loads. This would reduce your "blue" frame considerably. You could still use Pull rod for shocks. Your rear subframe to carry the body could be a bolt on component similar to original 40's.

If you look closely at the McLaren photo you will see a bracing tube running forward and downwards to the tub to provide some triangulation to the engine/bulkhead system. Lola T332 ran a similar setup to McLaren but the bracing tube ran forward and up to the tub.
Trevor

 

[Chris Kouba]

This link may provide additional inspiration for review.

 

Hi Richard, like the drawings what cad program is that, will it give the lengths to cut your steel from.Do you need to do a 5 year course to understand how to drive it.heheh

Best Regards
GraemeS

 

[Richard Timte]

Trevor,

The problem with the trailing links is that the original GT40 uprights or the Reynard uprights need to be so far inbound to get around the tire/wheel combo. This makes the wheel hang so far out from the bearing that it puts a lot of stress on them. Also, since the uprights are further inbound the length of the other arms become very short. We all know that very short arms are not a good thing.

GraemeS,

The CAD Program I use is Turbo CAD Pro. And sure you can find out how big each piece of material is. As far as how hard it is to use, I picked it up quickly and is very intuitive to use. I am running version 10.5 and have a copy of version 10 with docs if you want a copy. Just PM me with you address and I will be glad to send it to you.

 

[Trevor Booth]

Richard,
But do you really need 14" wide wheels or is it an appearance thing. If you intend racing the car the wider the tyre the longer it takes to heat up. If you run typically short races your tyres might not be up to temp before the race is over.

Suitably rated bearings would cope with the overhang. (The Gulf cars ran 14" ?? wheels. )
Your trailing links need not go to the upright, they could go to the arms at a location to clear the tyre. the arms will need some local reinf at connection point.

I suggest you talk to Hewland about feeding all loads into the tranny as per your proposal.

 

[David Pozzi]

I see the hewland you have is a later MKII type with beefier sideplates and extra ring and pinion bracing. If you intend on using this box on the street it will probably be very noisy. It depends on each individual gear set, some are VERY NOISY, others are pretty quiet. A friend built one and I could hear him coming due to the whine which sounded like a blower on a drag car. Another friend has a reasonably quiet one, (they all make some noise). The gear teeth are straight which is a very noisy method.

I wouldn't want to intentionally build the box as a stressed member. If you have an accident in the car the box is more likely to be damaged and removing the box to work on it would require dissassembly of the suspension, the whole car would have to be supported and it would be difficult to move the car with the trans out.
Just my 2 cents, what are you intending to do with the car, race or street?
David

 

good cad model, should be a nice car. what is happening with the light blue structure thats picking up the suspension? Are you planning to feed the bellcrank reaction load straight into the gearbox? if you can diagonalise the sides of the box straight back into the transverse plate at the bellhousing it should be a really stiff structure (lower rear inboard+ bellcrank node to upper foward inboard next to the plate). you may want to be carefull about taking the pullrod reaction load to the that upper point on the upprights,- that point is only designed to take loads in the plane of the upper wishbone on the indy car, while the the lower foward point is made to take the vertical loads. If you put the vertical loads through the point with the clevice and shims, you put a very big bending load through the those bolts. they may be fine, but if you flip the uprights upside down, and the swap left and right, i think they will be a lot stronger and better suited to the pull rod system (that single top point on an indy upright would have been designed to take more load in the upper wishbone plane on an indy car than it will see from the lower wishbone plane on a gt40. and the toe/ steer link will be in the same spot.) just thought you may like the suggestion.

anyway, im building (or designing at the moment) a mid engine sports car around a v8 engine (looking at gen 4 at the moment, or a similar ford unit. emmision control laws in Australia basically demand that the engine be less than a few years old, and these are the big v8's we can get without fly by wire throttle and variable valve timing which make them impossible to tune).
So what im asking is if where you got the basic dimensions of that CAD engine in the drawing? Im only after a clearance model at the moment (length, width, height, output shaft height, port locations....). so if you, or anyone can help me out with a cad engine ( or line drawings even) that would be much appreciated.

good to see people making such great cars, and better yet to see them designing their own!!

cheers, bob (melbourne, australia)