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 :confused: .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]

That is a very good idea :)

http://www.roadrunnerracing.net/rr-galleries/sr2-3d-cad-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

[Airhead]

a) Yes :)

b) Yes absolutely correct but the steering rack issue is relating to bump steer rather than fit. (the mazda front subframe is 727 and 654 respectively between mounts)

I have just got off the phone with Talon who already have prototype suspension mocked up, their setup uses a book build with regards to mounting points, his figures tie up with the ones you posted above so my build will have a fractionally narrower track - about 2" overall.

[3GE Components]

What does the MX5 upright measure in comparison to the Sierra upright? Unless they are exactly the same in dimension across the ball joints then the front geometry using "book" mounts probably won't work.

There's far more to suspension design than just making it fit the car. Apart from getting the camber & castor right there are a host of other factors in suspension design to consider, some of them quite critical. My advice would be to get yourself a good suspension book ( i can recomend a few) and read it thoroughly.

With Chris's new book he has spent a lot of time getting all these factors correct, so much so that we have had to make custom uprights for him to get exactly what he wants, as nothing was available off the shelf.

Kind regards

John

[Airhead]

Yep, I appreciate that there is a lot goes into it which is why I am asking here rather than botching it. I will post up the measurements for the front later :)

As far as the rear is concerned if I replicate the mazda layout on the vertical and widthways with variation only longitudinaly I shouldn't run into any problems should I?

Matt

[Big Vern]

As 3GE says,
There is a lot to getting the suspension geometry right and you'll need to draw it out on a suspension design program to understand exactly whats going on especially with regards to the geometry designed into the MX-5 uprights.
You won't be able to use wishbone lengths the same as the MX-5 ones as they're totally unsuitable for the Haynes roadster.
I would put the haynes design into a suspension design program to see how it works then replace the sierra upright with the mazda one to see how it changes Akkerman, TOOT, and caster. The steering rack will need mods to bring bump steer to an acceptable level. (some is desirable for stability under heavy braking).
Or you could cheat......Find a MX-5 based westfield and get your tape measure out!!!

BV.

[Airhead]

Thanks for the input BV.

When you say that the MX5 wishbones aren't suitable are you talking front, rear or both? The only reason I ask is that the rear mounts arent a million miles off the book anyway and in order to retain the track (and the halfshaft lengths) the rear wishbones will end up a similar length to the book and Mazda have already done the maths with regards to their rear uprights. I appreciate that the weight is different tho'...

[TQ_uk]

What does the MX5 upright measure in comparison to the Sierra upright? Unless they are exactly the same in dimension across the ball joints then the front geometry using "book" mounts probably won't work.

There's far more to suspension design than just making it fit the car. Apart from getting the camber & castor right there are a host of other factors in suspension design to consider, some of them quite critical. My advice would be to get yourself a good suspension book ( i can recomend a few) and read it thoroughly.

With Chris's new book he has spent a lot of time getting all these factors correct, so much so that we have had to make custom uprights for him to get exactly what he wants, as nothing was available off the shelf.

Kind regards

John

Hi John,

I'd be most interested in good suspension book recommendations as would like to know more about this area

Cheers

Tarquin

[TQ_uk]

Airhead - don't know if you've got/seen this, its a handy Excel file/program someone sent me this a while back that shows the effect of geometry changes. Worth getting your head around:)


EDIT - attachments not appeared (too big) PM me your email & I'll send it to you

[spud69]

Just my 2p worth, sound like HandyAndy now......;)

Why not ditch the Mx5 front uprights and buy a new set of sierra uprights from Rally Designs, about £95, and use the Haynes designed front suspension to run the car on. The suspension layout and geometry was meticulously designed, on Lotus own design software, to give an ideal ride between road and track use and is a major design in the Haynes to give it the excellent ride and handling that it has. Obviously would need discs, £18, and callipers, ???, to go with it but i believe this would be the best solution. We are just about to star putting a car together based on the Rx-8 and will be doing it this way.

All the Best....AndyH

[TQ_uk]

Where's the fun in that?! ;)

Seriously though, I'm way off starting a build, and also following the Midlana project (which uses MX5/Miata uprights) so much of my quest is to collect knowledge & info before getting stuck in, ensuring I have a clear idea, so (hopefully) the project won't stall.

I don't want to make things (any) harder for myself than necessary, so in all likelihood will stick to book at the end of the day.

Just exploring my options :)

Appreciate all the feedback and expertise this forum provides from those who've been/seen/done!

[Airhead]

Just my 2p worth, sound like HandyAndy now......;)

Why not ditch the Mx5 front uprights and buy a new set of sierra uprights from Rally Designs, about £95, and use the Haynes designed front suspension to run the car on. The suspension layout and geometry was meticulously designed, on Lotus own design software, to give an ideal ride between road and track use and is a major design in the Haynes to give it the excellent ride and handling that it has. Obviously would need discs, £18, and callipers, ???, to go with it but i believe this would be the best solution. We are just about to star putting a car together based on the Rx-8 and will be doing it this way.

All the Best....AndyH


Do you know what? I was thinking about just that this morning in the shower, the only issue would be the rear track but I could use spacers to compensate.

[3GE Components]

It makes sense to me to use a single donor, as 9 times out of 10 you'll have all the parts you need in the donor, I cant see the point in buying an MX5 then throwing away uprights designed for wishbone suspension to replace it with parts that have been adapted to wishbone suspension. The MX5 is good for this that's why so many are going down that route.

There's other annoying things too like different PCD for the wheels, etc etc.

Matt, I've looked at the pictures, I'll sit down over the next week and put something to paper (well CAD) :) I've got to do a couple of things for Chris's single seater first.

Kind regards

John

[Airhead]

Matt, I've looked at the pictures, I'll sit down over the next week and put something to paper (well CAD) :) I've got to do a couple of things for Chris's single seater first.

Kind regards

John


Thanks :) I look forward to seeing what you come up with.

Can anyone confirm that my logic is good or otherwise with regards to copying the MX5 setup on the rear?

[spud69]

Doesn't help anyone in the meantime but there is going to be an Mx-5 version of the bible coming out in the new year.

AndyH

[TQ_uk]

It'll probably help me, as unlikely to have started by then! Thanks for the heads up.

Just realised, isn't Keith Tanners book based on a Miata(MX5) build, ie hasn't he done all this already??

[Big Vern]

Re: wishbone lengths. The rear of the chassis could be altered to use the MX-5 location dimentions so from the geometry point of view that would work but I don't think you'll be able to use the actual MX-5 'bones.
The front is totally different and the 'bones, which are on show, both look awful and won't comply with radiused edges etc.
Re: single donor. For those wanting an age related plate then you'd probably need to use as much as you can from the donor to amass enough points to get the age related plate, also the benefit in my view is in using what you have by the way of hubs, brakes and wheels, all of which came with the donor.
RE: MX-5 version of the bible. Is it worth contacting CG himself? two heads better than one and all that.

As I'm still working through the haynes manual for "building a large workshop for you kit" :D I will probably buy the MX-5 version when it comes out.

BV:)

[Airhead]

There was never any plan in my mind to use the Mazda wishbones, yuk! Only the dimensions. :)

M