Choosing suspension hardware

[Dennis de Rooy]

Hi Guys,

I've been lurking here for a while and found tons of interesting info. I'm planning to build my own front suspension for my Alfa Romeo 75 2.0 TS. This car has a torsion bar setup on the front from the factory, but this has long gone and I'm running a coilover setup. To lose the heavy a-arms and to correct the geometry (which is terrible on a racing car running 7cm of ground clearance) I want to start building my own. I've read some very interesting pointers here and I hope you guys can help me out with this.

My first problem is the tubing. I would like to use 25crmo4 tubing (for strength/weight), but what diameter is safe on a 910 kg car? 20x1,5 mm? Or is that overkill?

The next one are the bearings. I would like to adjust camber on the lower ball joint location (and use the rod end under the upright). What kind of rod end do I need to withstand the forces? Or should I use a uniball and adjust camber on the top of the upright? The front static weight is about 250 kg per wheel. The bearings on the lower arm (where it attaches to the chassis) would be regular ball bearings.

Here is a pic of somebodies home brewed suspension to give you guys an idea of the setup.



Any pointers are appreciated and if you know a online shop for this stuff, great.
Cheers, Dennis

[MikeHart]

Personally I would strongly advise against using threaded rod ends on the lower wishbone. It's a generally accepted 'no no' on lower A arms and not really that brilliant even on the lower stressed upper. A good article that explains this can be found on Pat Clarks column on the Formula Student Germany website here: http://www.formulastudent.de/academy...ds-in-bending/ One way of adding adjustability is to add clevises to either the inboard or outboard end of the wishbone and then shimming them to get the changes you want. Since a picture says a thousand words, here's a clevis and shim setup on the '07 TUGraz Formula Student car:



I hope Goran doesn't mind me linking to his website too....you may already have had a look since he posts on here, but if not, his very detailed diary of his 'HemiPanter' build is a great read and explains some basic suspension theory very well IMO. Take a look here if you haven't already: http://www.hemipanter.se/

When it comes to speccing wishbone diameters and rod end sizes, I'm afraid there isn't really any other answer than to use a little Maths and work it out. I'd perhaps suggest looking up Carroll Smiths 'Tune To Win' which is a good introduction to race car design, and if you fancy getting really serious, get Milliken and Milliken 'Race Car Vehicle Dynamics', although I wouldn't use it for anything more than reference when you already know the basics. A more general and mathematical book to get would be Shigley's 'Mechanical Engineering Design'. All of these titles *should* be available from Amazon or the SAE book store.

If you're really stuck with the Maths, then see if you can't get some info of the guy who built the system above. Either that or look at cars of a similar power/weight/grip and have a word with the bearing manufacturers.

Hope that's some help. If you need any help with the Maths let me know and I'll see if I can't dig out some formulae for you

In my own personal opinion I would imagine that 20mm may be somewhere in the right ballpark. The lowers on our Formula SAE car are 5/8th in (about 15mm) and the car is a 220kg slick shod BEC with around 80bhp. It pulls some pretty high G loads of 1.7 though so the forces are fairly high. A lot of what size you can get away with will be down to the inboard spacing of the A arms. The wider they are the lower the forces will be.

[Dennis de Rooy]

Hi Mike,

Thanks for replying. I've been reading Gorans'page for years now and check up every now and then on the Nordic Supercar. Your link on the article from Pat Clark was a good eye opener. I'll stay clear of the rod-end. Adding two clevises on the chassis isn't allowed in my class, I'm afraid.

"The position of the rotational axles of the mounting points of the
suspension to the wheel uprights and to the shell (or chassis) must

remain unchanged."

Or am I wrong? If I could change, I could design something like Goran did on his pantera. Could someone shed some light on this? My english isn't good enough to properly interpret the regulations on the suspension (FIA annex J article 255 - gr. A touringcars).

Cheers, Dennis

[MikeHart]

Yeah, it sounds like you're interpretating that right. I can understand why they are not allowing movement of the wishbone bases since doing so would encourage people to completely optimize their geomtry which would result in cars that aren't really in the spirit of the originals.

You can get over that though, by having a clevis at the upright. It requires either modification of your existing upright or a new upright, but fabricating a new one from sheet metal with a turned bearing cage isn't *too* much work. Sorry to use Formula Student as an example again, but here is an illustrationn of what I mean:





Those aren't fabricated uprights btw....they have a sponsor that casts stuff for them!

If new uprights are off the options due to rule restrictions, then I think it would be fair to say it's a good justification to perhaps using a threaded rod end on the top link which isn't so bad. It's not loaded as heavily as the lower link, and it would give you the adjustment you need which will outweigh any weight penalty you'll have from a large rod end.

If you've got the time, the rest of Pat's colums on the Formula Student Germany website are a good read. I think there are about ten or so now. Just search through the news blog and you'll see them

I do like Gorans solutions as well, and that's another option. The thing to do would be to compile a few pictures of various designs and then see which is going to be the most suitable for your application given the size of wheels you have, the layout of the existing geomtry and the upright design. At the end of the day, if you designed some wonderfully engineered upright that avoided using any threaded rod ends at all but was a complete pig to adjust and resulted in you just leaving your settings the same between sessions, then you may have well have just gone for a slightly less elegant but more more easily adjustable solution. Good engineering is as much about compromise as it is about good design!