Practical limits of lowering

Technical Area Steering & Suspension
2

I am currently gauging the practicality of a kit car project and would appreciate any advice that experienced MX5 engineers can offer.  The kit will be a Jaguar D-Type bodyshell applied to a Mk2/2.5 MX5 - an engineering project delivering good-looking fun.  A lot of bodywork will be cut away to fit the shell, but overall I anticipate no significant change in overall weight, with a possibility of an overall lightening.  However, whilst I have cleared the bodywork planning, MX5 suspension remains an unknown quantity.

Having analysed a number of photos and measured a friend’s Mk2, the MX5 sits substantially higher than the original D Type, so the lower I can get the car, the better the overall look.  The D will also need larger wheels to fill the arches.  15" TR6 or ex-BMW Mini One steel wheels are planned given their close resemblance to the original Dunlop steels worn by the Jags in their heyday; I’ve yet to work out what rubber will grace them, but I will be aiming for as large an outer diameter as practicable and a relatively narrow tyre; I expect the tyre to contribute to the overall suspension.  However, if one was to try and match both Jag ride height and wheel diameter, I’d have to drop the MX5 by 150mm - which I anticipate is rather beyond the limit of practicality!

Can I ask therefore for some general limits on lowering the MX5 Mk2 - what’s practical in terms of overall ride drop, where are the limits of practical/comfortable driving, and what degree of suspension travel can be expected in lowered configuration (as I’ll need to leave room in the wheel arches!).

Something different to consider - grateful for any views/advice, even if it’s to tell me I’m bonkers.

Thanks in advance!

3

Hi

i would have thought in general no more than 80mm 

otherwise your altering suspension geometry so much its going to be nasty 

would you not be better too lower the body of the kit more .

4

I’m no engineer and appreciate you’ve taken some measurements, but having just looked at some photos of D-types they look to me to sit considerably higher off the ground than a standard MX5 and not vice versa  so I just wonder whether the suspension really does need to be lowered…? Is it more about making the larger wheels sit correctly in the wheel arches rather than making the body fit the chassis? Very interesting project though. Would be keen to see pictures as you progress the build. One last thought, why use a MX5 chassis? Is that the best solution or would another be a better fit and potentially less hassle?

5

TAS

I have never looked at a D type suspension. I have only seen one in the flesh.

From what I can gather they used torsion bar suspension and that usually is bars that are twisted but fixed to the underneath of the car.

The MX5 has a springs and dampers that finish well above the wheel.

Therefore if it is as I think, then the MX5 base suspension apertures may well be too tall to fit under the D type wings. In saying that I had a quick look for D type details but then decided that you would be a better person to do the research.

6

All good questions and very happy to expand.

Why am I even trying this?  I am looking for something different to do and a kitcar is for me a great idea.  I don’t like either the Caterham 7 derivatives or some of the more extreme kits such as the Exocet, but I came across a couple of companies making kits based on much older cars.  Some of these are rebodies of older chassis, others are full kits.

The D Type is a stunning looking thing and I have decided - I want one!  However, all the examples I have found so far (that are affordable) are based on older donor cars such as Heralds or Spitfires.  I’ve had an MX5 before and loved it, and moreover I want something modern and reliable underpinning the kit, and have no intention of rebuilding an ancient Triumph.

I’ve given myself three options:

-  A full kit including body made by ANC Sportscars (http://ancsportscars.co.uk/).  This is actually an Aston Martin DBR1 bodyshell (well, nearly) applied to an MX5 PPF and a bespoke spaceframe.  Advantages - very easy to build and a great result, plus the shell comes with a polished gelcoat meaning I can hold off on a paint job for a while.  Disadvantages - requires an IVA/SVA approval, and it’s not the D Type!

-  A rebody of an MX5.  Tribute Automotive make the body (http://www.tributeautomotive.co.uk/dtype.html) but I’d have to retain the donor bodyshell.  Advantages - much less costly, no IVA/SVA required.  Disadvantages - means relying on the tub being sound, it’s less neat than the spaceframe solution (which the D’s had), needs painting (not supplied with finished coloured gelcoat) and makes the vehicle heavier overall.

-  A third way might be to merge the two - buy the spaceframe from ANC and the body from Tribute and combine.  That’s the most expensive route to market - easily £4000 for the spaceframe plus the IVA, but it would be the most elegant and complete solution.

The scale measurements are taken from a particular series of drawings of the D Type; although there are plenty out there, having measured and scaled I’m happy that this particular image is as representative as I’m going to get.  Matching that to an MX5 drawing was relatively easy, and the attached image (crafted in exquisite PowerPoint!) shows the differences in ride height and wheel diameter - however, overall body height is about the same.  I hope the image link works…  The body outlines are representative - I can cut a lot away from the rear especially although I plan on incorporating a useable boot space.

I have estimated the base MX5 ride height to be around 190mm (floorpan to ground on standard wheels), and the Jag sits at 140mm so that’s a 50mm drop.  Any increase in overall wheel diameter will need futher lowering IF (and I say again, IF) it’s practical and necessary.  I have no intention of compromising driveability - it’s meant to be a fun car as well as good looking.

In terms of containing the suspension within the bodyshell it’s not a problem; the ANC kit shows the fit nicely (see here: http://ancsportscars.co.uk/gallery_gen/656c85754b9f4c2db766a1b4dcd104c9.jpg), the Jag bodyshell has large bulges over the wheels anyway, and the suspension pillars will fit under the shell even when the body is fitted closely to the donor.  The body will not really go much lower than the front scuttle as the engine gets in the way anyway - I’m not up for trying to fiddle the engine mounts as well.

Assuming I lower by 50-80mm, how severe is this likely to be - can I safely assume this will need adjustable coilovers, or is that the gold-plated route?  If this becomes the limit, then it will drive the wheel size I end up with.

Hope that’s vaguely interesting!

MX5-Jaguar Ride Height Comparison