Suspension - the important ratios?

[Airhead]

Hi all

My build is cracking on and although I've not upadated the build thread yet I have been busy. I am at a point where I have planned the rear suspension mounting points by taking the measurements off an MX5 and replicating them on the rear frame. The wishbones will be the same dimensions as the donor ones so my track should be spot on; as a pleasant aside I am still within book dimensions so the rear tub will fit, fingers crossed. I have drawn the rear in my CAD package and then on Sketchup to prove that it works.

The front is a bit more complex...

Book has the dimensions between the mounting brackets as 670mm for the top, 494mm for the lower and 182mm between, the MX5 is 727, 667 and 178 respectively (there are some weird angles off the top tube as well with it being 4.5 and 2.5 degrees off true on two of it's three planes). I am not worrying about the exact geometry of the donor as I am using bespoke wishbones.

Question:

a) Should I be worrying about the variation on the top tube?
b) Moving the top mount down by 4mm and lengthening the top wishbone accordingly should be fine, shouldn't it?

Thanks for any constructive input.

Matt

[ozzy1]

Personally as the mx5 is a fine handling car i would try to replicate the wishbones as accurately as possible and i dont think you would have any probs.

[Airhead]

The MX5 front wishbones are huge, there is no way they, or a facsimile would fit a roadster.

The mounting points should be good enough though. As far as I can see it is only the distance between the mounts vertically that is going to alter the geometry, as long as the ratio between the spindle and the pivots (and the angle of the upright) is maintained it should be fine. Or am I wrong?

[ozzy1]

My train of thought is the same as yours on this.On a slight tangent there are a few companies using the mx5 as a donor,maybe it would be worth buying a set of wishbones from them if poss and save any probs .Not as cheap as making your own i know.

[Airhead]

That is a very good idea

[TQ_uk]

Hiya, am following your build as just picked up a complete MX5 front end (it was local, cheap, and Sierra's appear to be getting rarer).

Do you intend keeping the MX5 front track? I would've thought it best, to match MX5 rear end, but you mention the rack as being too wide?

Looking at the front (stock) wishbones the lowers do mount a long way back at the rear, if you're going to make new wishbones can't you ('just') bring the mount further towards front (put in the same pivot plane) for more aesthetic
lower wishbones?

Seems its the position the rack is mounted on the MX5 subframe dictates the 'awkward' lower wishbone mount.

Also will the difference in wheelbase between MX5 and Haynes roadster (5" ? never have determined the book wheelbase) have any effect on steering geometry?

[TQ_uk]

Originally Posted by Airhead

That is a very good idea

http://www.roadrunnerracing.net/rr-g...d-gallery.html


Front shocks look like they're at quite a shallow angle

[Airhead]

Hi

Yep hoping to keep the track et all the same. From what I can find out on the internet the track width on a Sierra is 1450 front and 1470 rear which is narrower than the MX5 at 1490 and 1495 respectively - can't see it myself but there you go.

The Roadster is quite a lot shorter than the MX5 so if the track is narrower I can't see it adversely affecting the handling.

The steering rack is too long due to the reletively short front wishbones on the donor, the Escort rack is the right size and can be modified to work with the MX5 spindles.

I am in esscence doing what you suggested by keeping the dimensions from the donor apart from the longitudinal distance between the mounts.

I noticed that Talon Motorsport have claimed to have done some work with MX5 suspension; I'm just waiting for them to return my call...

Matt

[TQ_uk]

Sorry, think I've confused both of us with my previous post....

Am I right in:

a) you're keeping front & rear track as per MX5
b) the wishbone mounts on chassis will be book 'width' (or thereabouts), ie 670mm top, 494 lower, thus new wishbones will be need to be longer (than
standard MX5) to retain MX5 track, and as a consequence MX5 rack will be too wide

Think thats rephrased my previous post in a clearer fashion....

[TQ_uk]

Btw, info I've got re track is:

Sierra: F 1452mm/ R 1468
MX5: F 1405mm/R 1430

from http://www.carfolio.com