NA MX5 ground clearance

[Phil Bradshaw]

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Further to my post on suitability of a MX5 front end for a hot rod project what I need to know next is the ground clearance. I'd like to know the height of the front and rear guard lips from the ground with the car at normal ride height ideally without any extra weight onboard. I'd like to know the tyre size as well please. If you can measure it I'd really like to know the distance from ground to the centre of the front suspension lower arm subframe pivot bolts (both front and rear bolts on the bottom arm). This will help me work out the correct plane the suspension needs to be in. Thanks. Phil

[project.r.racing]

That is the long way around when it comes to suspension geometry.

All you need to know is the length of the shock from top hat to lower bush when when car is parked. And the angle of the lower and upper A arms when parked.

But unless the hotrod you are building has the same weight on all 4 corners like the MX5 does, then not of these measurements will be useful. As they will are be incorrect for your application.

[Phil Bradshaw]

I will be using a coil over so shock length for mx5 is of no help. I want to set suspension up so arms etc are in stock positions relative to each other at the normal ride height. In effect set the suspension to the stock ride height/neutral travel position and then attach coil over with suitable mounts so that there is a 1:2 ratio of droop to bounce. Use load cell to determine load on coil over to help with spring selection, and use adjustable platform on coil over to set final ride height. Car will weigh similar to mx5 and weight distribution not too far off. Standard anti dive geometry will be about right to make car handle better than most hot rods. Thanks. Phil

[93_Clubman]

FWIW, 1990 NA6 factory manual doesn't provide a ground clearance figure. NA6 tyre factory spec was 185/60x14 with 26PSI front & rear.

[project.r.racing]

I will be using a coil over so shock length for mx5 is of no help. I want to set suspension up so arms etc are in stock positions relative to each other at the normal ride height. In effect set the suspension to the stock ride height/neutral travel position and then attach coil over with suitable mounts so that there is a 1:2 ratio of droop to bounce. Use load cell to determine load on coil over to help with spring selection, and use adjustable platform on coil over to set final ride height. Car will weigh similar to mx5 and weight distribution not too far off. Standard anti dive geometry will be about right to make car handle better than most hot rods. Thanks. Phil

Well if weights and distribution is similar to an MX5. Then why attempt to find this info. If everything is similar and you a purchasing coilovers, then you can adjust as needed as you just described. I assumed you weren't using coilovers, hence the requirements in the questions as adjustments couldn't be made. Or I have missed some important detail?

But to answer your question. I think from memory that he lower bushes on the front a arms were 115mm off the ground.

[Phil Bradshaw]

The key is I want the suspension to be in the correct geometry at normal ride height. Without knowing the ground clearance to a suitable reference point I could in an extreme example set the car up at normal height with the suspension displaced by say 50 mm From stock - which would be as bad as lowering the car by that amount with subsequent impact on camber change, roll centre, bump steer and so on as the suspension moves through its travel. Or worse if it was higher than stock. I have seen so many hot rods with appalling front end geometry and bad handling that for the sake of knowing a couple of dimensions I may as well do it right. General the only benefit from lowering a car is to reduce the centre of gravity- something as well set up as an MX5 often handle worse when lowered - in effect I will channel the body over a custom chassis to achieve the correct since but with stock front end geometry. Phil

[smy0003]

You could measure the angle of the lower control arms relative to the ground and use that as a reference?

Mx5 suspension is hugely adjustable, I dropped my car somewhere between 35-45mm and the alignment shop didn't have any trouble bringing the geometry back to normal.

[Phil Bradshaw]

I can't measure as I don't have a car - I will just buy the front end components. It is not about getting the camber and toe at factory spec - it is about the change in them with suspension travel. The suspension is optimised around the stock ride height - lowering the car etc starts getting the suspension into bad camber change and in worst case as the bottom arms go 'over centre' you can start getting into positive camber change as opposed to negative with travel. Best dimension for me is the centre of the inboard pivot bolts on the front suspension (both front and rear bolts) to ground with the car at factory ride height. From this coupled with tyre size I can calculate where the subframe needs to be in relation to the ground. Front tyre squish will be about 15 mm. Phil.

[project.r.racing]

The suspension is optimised around the stock ride height - lowering the car etc starts getting the suspension into bad camber change and in worst case as the bottom arms go 'over centre' you can start getting into positive camber change as opposed to negative with travel.

Are you confusing double wishbone suspension with macpherson strut suspension?

[plohl]

I have the na6 suspension geometry in a solidworks file if you want that?

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[Phil Bradshaw]

Yes please to the solid works file - 33woodie@gmail.com please.

And yes, you are correct re me thinking of McPherson strut (too many projects) but similar issues apply to unequal length a-arm suspension as the arcs of travel are designed to increase camber change with position change.

Phil