Mdoifying the suspension brackets to meet SFRO (Sweden) regulation

Tuesday October 25th 2011
Last Minute !

Please read this before going further :

I finally decided to buid the suspension as per the book. Not that it will not work with the set up I studied, but I don't want to change the geometry of the car which has been designed with a 45° angle. I hope my SFRO inspector will pass my car with the original set up as they passed a fair amount of other cars who does not respect this 30° angle recommendations from SFRO.

But if you want to make the modification to meed SFRO recommendations, and for that getting inspiration of my design or even the complete construction of the front upper bracket extension, please post a comment, and I'll come back to you with all the details. You will not find the final design details here, just a few pictures, as I unfortunately have no time for that, for now. But the design exists, it has been finalized thanks to the help of some members of the haynes Forum, so do not hesitate to contact me.

On my side, if in the worse case my SFRO inspector ask me to change the suspension set up, at least, the design of this extenstion is ready to be fabricated.!

Thanks.

Sylvain


Intro


On the Haynes Roadster, the original design gives a shock absorber angle of 45° if you respect the books shock absorber brackets' position :

(for the need of the drawing, FF3 is "wire framed" (see through) in order to be able to see the upper shock absorber bracket behind it)

In Sweden, to meet SFRO recommendations, this angle has to be 20°-30° (see page 32 of SFRO manual)

This implies to change the suspension brackets' position.

Fist, where to measure this 30° (max) angle. Is it correct on the picture below ? (this question is asked to the swedes who understood SFRO's manual better than I did !)

Front suspension

I first wanted to move the lower bracket inward on the lower wishbone, as well as moving the upper bracket outward by welding it on the side of TR1/TR2. After exchanging and collecting opinions on the Haynes Forum, it appears that moving the lower bracket inward is a bad idea, as there is a risk of bending the lower wishbone (the overhang between the lower shock absorber attach point, and the front upright lower pivot point is lengthened).

So, the only solution to get the 30° shock absorber angle is to move the upper bracket further out. For getting there, an assembly has to be built. As the new shock absorber upper attach point will be quite far our, a bracing needs to be done as well.

This implies using shorter shock absorbers than with the original design (45° angle). With a 30° angle and no modification of the lower wishbone, the right length will be 11'' (280mm). This is considering this shock absorber size (11'') that this bracket extension has been designed.
Note that changing the position of the upper rear shock absorber bracket to get this 30° angle (see "Rear suspension" chapter), will also make us use 11'' shock absorbers.

This is the final design of this extension. As said in the "Last Minute" at the beginning of this post, if you want more details about the construction, just post a comment and I'll get back to you with all the details.

The bracket is welded to a support, and the whole assembly is "dressed up" with shaped plates, just to make it look good (or, at least, not too ugly !)

The bracket's support, as it is shown on the drawings, is made out of 4 tubes pieces (box section 20x20x2mm) :
- a first assembly of 2 tubes welded on the side of TR1/2,
- a longer assembly of 2 tubes welded on the lower face of TR1/2
This longer assembly give the main bracing to the whole bracket extension.

It could of course be made out of 3mm steel plates, cut to the right profile. But I think cutting tubes is easier than cutting a profile on a steel plate, and we all have a lot of small sections of square tube left at this stage of the building. So why not using them, as they will be hidden by the "casing" plates ?

 (I've added transparency to the side plates to see the bracket support assembly through them)

 
As TR6/7 already braces TR1/2, there is no extra tube to add (as it was my first idea).

I would be glad to know if someone has or will realize this assembly, so please fell free to tell me by posting comments here on this blog !
Thanks.



Rear Suspension

1st option : using a shock absorber shorter than 13''

Regarding the rear suspension, if we want to avoid modifying the rear uprights (lowering the attach point of the rear shock absorbers) or cutting a hole in CP3/CP4 plates, we will have to accept using a shorter shock absorber than the 13''. The only solution is to move the upper shock absorber bracket outward 93,3mm away from its original position (= the position in the the Haynes Roadster book). We obtain a distance from bracket centre to bracket centre of 261mm (10,27''), which allow to use 11'' shock absorber but no longer ones.

Very important: this implies that tube RS15 has to be moved 93,3mm outward too, as its use is to brace the upper bracket.

 

2nd option : using 13'' shock absorbers

Modifying the rear upright is useless.

I explored the possibility of modifying the rear upright. The only "small" doable modification is lowering the lower shock absorber mounting point on the rear upright, as well as the upper wishbone mounting point on the rear upright (= rear upper camber adjuster mounting point). The conclusion is, if you want to keep the same clearance between these two points, which is about 42mm, you can not lower them more than 13mm (and this is the extreme maximum, as the tube of the rear upper camber adjuster will touch plater RU3 of the rear upright, so, practically, it will have to be even less).

In this conditions, if you wan a shock absorber angle of 30°, you can get a maximum length of 273mm (10,74''). The upper shock absorber bracket will have to be moved outward 90mm away from its original position (its position in the the Haynes Roadster book). But the gain is very small in regard to the mess of the modification, and it will never allow us to use 13'' shock absorbers. So, unless you are ready to make big modification on the uprights, in my opinion, this possibility has to be given up.

Positioning the upper bracket higher is the only option.

To keep the 13'' shock absorber, as it was ThomaZ idea, it is needed to make a big hole in CP3/CP4, and weld the upper bracket on top.

(The line between the two bracket axis shows the shock absorber axis)

To get a 30° shock absorber angle, the pivot point of the upper bracket has to be 111,6mm from the inside edge of CP3/CP4. In that case, the distance from bracket centre to bracket centre is 317mm (12,46 inches). This allows to use 13'' shock absorbers.

Important: if you chose this option, you will have to brace the bracket in a way or another.

Wheel clearance.

According to the dimensions and measurements I've got, with a 7x15 ET38 or ET40 wheel and 45mm (1-3/4'') diameter shock absorber, the wheel does not touch the shock absorber with the 30° angle, so no issue regarding this point.

(sorry for the poor modelling : the shock absorber is just a 45mm diameter cylinder,

and the wheel a 45mm wide ribon to make the shape...)

Roll bar issue

This new position of the rear upper shock absorber bracket will imply to change some things on the roll bar fittings.

1 - Moving the rear inside M8 nuts backward of 20mm (the original position is in the hole to be cut on CP3/CP4)

2 - Because of that, extending the length of the roll bar plates RB1 backward of 15mm

3 - Making a cut in this roll bar plates RB1 matching with the cut in CP3/CP4

As I don't have a shock absorber with me, it is difficult to know the size if the cut in CP3/CP4/RB1. If we want to keep the roll bar dimensions and position, the cut can go up to the inside edge of the roll bar. I think this way, the hole should be big enough for the shock absorber clearance, but it has to be checked.

Making a box on top of CP3/CP4

The other idea of ThomaZ was, to weld another box on top of the bracket, and, I suppose, shortening the roll-bar and fitting it on top of this box (M8 nuts will have to be moved in wart to fit the new dimensions of the roll bar.)

If it is possible to manage the cuts in CP3/CP4/RB1 as described above, moving one of the M8 nuts, and keep the original dimension and position of the roll bar, hopefully this will not be necessary.

But if the cut in these condition is not big enough for the shock absorber clearance, yes, the roll bar will have to be repositioned, and this option of welding another box on top of the other can be good, even though, aesthetically, it will not look very nice.

Conclusion.

With a few modifications on the shock absorber bracket positions, it is possible to get the 30° shock absorber angle required by SFRO, and even, with bigger modifications, to keep the 13'' shock absorber for those who already bought them.

The unknown part remains this cut in CP3/CP4/RB1 (rear suspension) that I can't calculate as I don't have the dimensions of a shock absorber