The challenge: New rear-suspension for a Tiger R6 kitcar

[drakkie]

Me and a classmate got an assignment about a month ago. We would have to redesign the rear suspension geometry to fit to two needs:

- Adjustable ride height for varying conditions (for example track and road)
- All the other things should be easily adjustable

We went to work with analyzing the current layout. We found that it uses a so called De-Dion suspension now. The unsprung weight of this is quite high and can affect handling negatively. Also it is quite hard to adjust anything,except for the shock absobers.

The simple sketches we made to work from and use as reference:



Then we set our own demands to the new suspension. It had to be relatively easy to design and build and as light weight as possible.

We looked at various design options:
- Double Wishbone
- Multi-link
- MacPherson
- Any of the above combined with Citroën's Hydropneumatic system.

We cut these options down to two options that we thought were best. The Double Wishbone and the MacPherson with Hydropneumatic. The Multilink suspension would be to difficult to create. It would involve an enormous amount of wrok on the frame/chassis, that wouldn't be justified by the advantages. The MacPherson with normal shocks would be too heavy and not adjustable easy enough.

After that we made our final choice in a debate with the school / teachers. We chose for the Double Wishbone layout. It was not economical to use Hydropneumatic's. The weight would be too big and the system's pumps and lines would likely not fit in the engine bay without major changes.

This phase of the design process was done for now, now we started work on a Planning and a document with what we would do, the risks, costs etcetera. The school found it ok and we started out with the work.

Because there was no 3D-CAD model available of the rear section of the frame, we had to make one. We took every measurement we could think of and set to work. My mate sketched it and I measured it. Within an hour we were done. We also looked up the kind of steel used in the chassis and the inside thickness of the pipes.

The following 3D-CAD drawing was then made by me in a weekend's time.


The drawing features the chassis from the passenger compartiment to the back.

After this was done we started sketching more detailed idea's about the chassis modifications that would be needed to suspend the wishbones. We made many sketches and after a lot of coffee and calculations we found that it would look like this according to our preffered adaptations:


The steel used for our adaptations is the same as is used for the chassis,namely steel-360.

Because of the extremely busy times with other subjects, we had to slow this project down for a week. After this we got back to calculating and drawing again. We are now not completely finished but have done most of the work. We achieved this result:




We now need to calculate what shocks to put under it and how to connect the axles onto this. This will be our task for the weekend. Next tuesday we have a presentation for various teachers and high people of school again. In this meeting we present our solution. After this the school will make a decision if they will use our design, but I allready got hints that it is likely to be funded. The parts would be CNC'ed and mounted and we will probably have to do the welding ourselves (TIG).


Hope you enjoyed it as much as I did,
Sjoerd

[Matra et Alpine]

Do the forces math.

The de-dion layout with adjustbale trailing arms means you can easily adjust the rear suspensions AND the rear chassis is not subject to large lateral deflection forces.

You'll need to "beef up" the rear to prevent it distorting under power/braking/bumps as the wheels try to push/pull the arms backward/forward. Original Lotus 7 chose trainling arms to enablt the rear to be very light as it didnt' need a lot of strength.

WHere are you going to mount the shocks ?
Watch out for angles. At worst you want the shock to be operating about 70%, for optimum control ity woudl be better up at 80-85. You might need to provide additional towers to mount the top in or consider using lever arms.

[H12]

Have you considered track width?
The arms look like they are parallel and have the same length. This will not work well as the tires will lean outwards in roll. You have to consider camber gain which means that you have to figure out things like roll centers, track width, spring rates, tyre characteristics, CG height etc.
Are you going to manufacture your own uprights?

[drakkie]

The shocks are not yet drawn completely. We will use custom-made (by my uncle probably) Koni FSD dampers. The top mounting points are the same as in the current situation, which is shown in the attached picture. It will be connected to the upper wishbone.

In the past 30 minutes I have designed a wishbone that will be adjustable. not done the force calculations though yet... That should answer most of your questions

The uprights will be manufactured too by us. Everything will be. That is IF we get the funding.


My apologies for the worthless quality pictures. It was made with a P&S because it is quite unhandy to carry my bigger camera plus a laptop and books and an overall through rush hour subways.

PS:
I notice we might need to change some brackets for the shocks too.

Edit:

The easy adjusting of the De Dion is not the case here. The trailing arms are fixed and have no option to adjust. Everything else is solid cast iron/steel. With two people we couldn't lift it up (offcourse with everything attached).

[McReis]

Now you can put up the comps!

[drakkie]

Now you can put up the comps!

If only I wouldn't have to make extensive Matlab graphs And a 20 page report And a presentation And offcourse I need sleep sometimes.. Been working full days on this for the past week.. 6am to 11 pm

[H12]

drakkie:
Why make a bend in the arms? How will you be able to adjust camber?
Have you considered the length of the arms? Maybe you should think of designing a subframe around the differential and put the wishbones there?

That being said, why not go for an adjustable de-dion?
http://www.alfagtv6.com/gallery2000/gallery5j.htm

[drakkie]

drakkie:
Why make a bend in the arms? How will you be able to adjust camber?
Have you considered the length of the arms? Maybe you should think of designing a subframe around the differential and put the wishbones there?

That being said, why not go for an adjustable de-dion?
http://www.alfagtv6.com/gallery2000/gallery5j.htm

At the place of the bend in the arms is an adjustable element. This way the toe-in / toe-out can be adjusted. In my latest version I have a kind of A-shape. It is this way to fit in the brackets for the shock absorbers.

The length of the arms is calculated to be ths length by my classmate. I hope he was correct ! They are relatively short to fit in the car (around 280mm iirc).

The subframe seems like an enormous amount of work. We got to consider costs too.. We have a 20.000 euro fictive budget now, but we allready have trouble fitting everything in.

Why not ? Because we basically didn't even think of it Luckily I am still learning

[H12]

drakkie:
Well, take the width of the chassis 1.05 (right?) and then add 0.28 on each side. That means that your car is 1.61 m wide before adding the uprights, hubs and wheels. Suddenly you have a car that is 2 m wide.
This should give you a few ideas, don't use the bad ones

The wishbone that you have done has a bend in it which means that when it is put under load it will flex. Also, you can't adjust camber with that design (unless you intend to do it some other way). Both frame ends of the wishbone needs to be adjustable.

Designing a good IRS is not trivial so you have your work cut out for you.

[drakkie]

drakkie:
Well, take the width of the chassis 1.05 (right?) and then add 0.28 on each side. That means that your car is 1.61 m wide before adding the uprights, hubs and wheels. Suddenly you have a car that is 2 m wide.
This should give you a few ideas, don't use the bad ones

The wishbone that you have done has a bend in it which means that when it is put under load it will flex. Also, you can't adjust camber with that design (unless you intend to do it some other way). Both frame ends of the wishbone needs to be adjustable.

Designing a good IRS is not trivial so you have your work cut out for you.

Thanks. The current width is about 1.55 m excluding tires and rims. Our design wouldn't make it too much wider then. Apart from that we have just two days left to finish it off ! For three days of work on this, I can't say we did a poor effort.

What I was thinking of as a way to adjust camber was two adjustable elements in each wishbone. If you adjust them toghether at once, you get a change in camber. If you adjust one solely you get a change in toe-in/toe-out...

The planning is to redesign the wishbone as a whole,instead of just parts, tonight after my job. We will try to finish the assembly on saturday night and get cracking on the necessary calculations / reports / presentations from then on.

For now back to work, my boss is getting annoyed.

[BjD]

Good effort for 3 days work. Some observations/suggestions.

Im sure you've increased the track. Most Seven IRS have the inboard joints much closer to the centre line of the car. If you've increased the track you will need new driveshafts, unless you have enough plunge on the originals. Also, the longer the wishbones the less track change you have on bump/rebound. Compared to over Sevens the uprights "look" too far out.

The inboard mountings are single shear, ideally have a second bracket on the opposite side of each mount. This eliminates bending on each bracket.

Need a lot more triangulation on the frame at the back.

Need a fifth link Sure its just not present on the CAD pictures.

Personally, I would have the inboard ends of the wishbones fixed, and use a screw in ball joint/rose joint at the outboard end to give camber adjustment. Exactly the same as the Locost front upper 'bones. If you then make the inboard mounting on the chassis wider than the wishbones use shims to put the wishbone in the right place longitudinally, giving you your toe adjustment. With your method of adjustable inner wishbone mountings you can't change the toe without affecting the camber. We had a similar issue with our FS car which I helped to solve with new uprights this year.

As mentioned equal length parallel upper and lower 'bones are not the most ideal solution.

EDIT: This pdf is very useful

[Matra et Alpine]

If you rose joint all and mount one of the inboard on a sliding joint then you can have adjustment in all 3 dimensions to cover all needs.

I'm still worried that you've no cross bracing at the rear inbord connection for the arms. Road forces will push that chassis leg in. It needs additional strengthening.

[drakkie]

I'm still worried that you've no cross bracing at the rear inbord connection for the arms. Road forces will push that chassis leg in. It needs additional strengthening.

Believe it or not, it can face the forces

However to stay within or safety factor of 2 we need to make some minor adjustments. We opted for a triangular shaped leg there, which however needs to fit around the axle. Next challenge !

[revetec]

The lower wishbone needs to be very strong. This wishbone transfers most of the force to the chassis. Then mount your axle as low down to that wishbone. The top wishbone needs to be shorter to create the desired negative camber when pushed down to give the tires maximum road contact. The heavier the car, the more negative camber you will need when the wheel is forced up.

Caster and Camber adjustment can be done with shims between the top "A" frame to chassis mounts. Take one out of the rear and put into the front should not effect camber but will increase caster. Taking both front and rear out will give more Negative Camber, and visa versa.

On your design you also need to add an arm off the hub and connect it with a rod for toe control. If these are the drive wheels then a bit more toe in will probably work better as the drive wheels try to pull outwards at the front a bit more than coasting wheels.

Look at a radio controlled car suspension for an idea on the type of layout as most use a double "A" frame at the rear.



Here is a rough layout that I quickly drew which doesn't have toe adjustment although toe could be adjusted if the lower wishbone had shims like the top, or an offset washer at one of the lower mounting bolts. This setup would be neoprene or rubber bushed to allow proper adjustments to be made. This type of setup is reasonably ridged and stable under power due to the wide lower wishbone and hub mount. Coil over can be mounted either side (Front or Rear) of drive shaft. Hub has extended arm to top "A" frame creating better leverage of top "A" frame for control. Uneven "A" frames provide negative camber as wheel rises towards top of travel and reasonable consistent camber in the central position. The wide lower hub mount provides toe stability without the need of an extra toe rod. Hub is mounted forward from the centre line of the lower "A" frame providing better stability and strength under power also. Top wishbone can be central to the hub as it controls movement rather than transfers loadings.