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MG Midget and Sprite Technical - SWA measurement required please

Evening all.

I am in part way through building my K Sprite and changing the rear suspension. However, I forgot to measure the distance between the wheel centre and the arch before I took it all apart and now I don't know if I'm building in enough travel or not.

So can anyone either please nip out and measure from the rear wheel centre point vertically up to the wheel arch, or does anyone already know the measurement? I'll also need to know if the car is stock height or lowered.

Many thanks in advance!!
S Overy

23CM but mine is low.

I assume you are building an IRS.
Why not check the front sill to floor height and get the rear level.

Now you can just wait for 237 other dimensions
Onno Könemann

deleted my post after seeing Onno's and then realising that yours must be SWA


out of interest mine are 345 at rear and 325 at my too low front
Nigel At

Onno,

I'm building a De-Dion set up based on the back end of a Caterham. Only thing is, I need to cut the boot floor a little, and I don't want to over do it. As the car is currently on it's side, levels aren't doable at the moment.

When you say yours is low, how much is it lowered by?

Nigel,

The clue's in the title! ;)
S Overy

Simon.
Mine measures 245 at the rear, 335 at the front. Front is lowered with spacers but rear is not intentionally lowered, although it does sit quite low. It was fitted with new Clive Berry springs about 2 years ago, and they have flattened out a bit. But to me it looks OK. See photo.

(photo also demonstrates the use of a zip-out rear window in action; this was discussed on another thread recently)


Guy

Thanks to all so far. Does anyone know the measurement for a standard height car? All seem to be lowered so far!
S Overy

How low?
low had a lot of trouble with the exhaust touching the road.
Fixed that but now it touches the floor of the car ;)

If you are going to to it this radical why not take the time to mock up and measure?
Onno Könemann

270 mm NS and OS.

Standard build, standard rear springs and blue Polybushes from Welsh MG about 7 years ago, but (shamefully) only about 15k miles since.
Steve Clark

Hi Guy,
What wheels are those on your midget?
(Sorry to hijack the thread S Overy)
Neil
Neil K

Mines 300mm both ns and os. Totally standard.
M Adams

Onno,

I am mocking up, that's what I need the measurements for! Only problem is that the top of the De-Dion arm, where a longitudinal Watt's link attaches lies in line with the edge of the boot floor/chassis rail. I've made a cut into the corner of the boot floor, but I don't want to build the lot up only to find it needed to be 1" taller, which it's starting to look like it will need to be.
S Overy

But as you see from the measurements there is a big amount of variation.
70mm can be fitting easy or cutting a lot.
That is why you need to do it in relation to the desired front rode height.
And with a K and assumingly mods there too I'd realy want to check it.
Unless your dion back end can easly drop 70mm with out affecting geometry.
Not realy at home with dion back ends other than knowing it was used in a lot of 70's cars iirc
Onno Könemann

I'm with Onno....work out the desired front ride height against the sill and aim to get the sill level-ish. How will you know where the suspension will settle though (whatever ride height you're aiming for) ?
Dean Smith ('73 RWA)

Neil,
They are Cosmics. Period ones from the 1970s. I understand they are very rare in 13" size.

Guy
(with apologies to Simon)


Guy

Guy never apologise for pretty wheels

however Simon, sorry for the interruption


as for sagging rears (oops missus 8) )

they all sag sooner or later

do the parallel sills bit rather than look for a non-existant ideal
Bill 1

S Overy,
The factory height should be 43" (1.09 m) from ground to the top of the top of the windscreen.
Jon Saylor

>>Nigel,

The clue's in the title! ;)<<

Sorry Simon, my eyes saw that but my brain didn't, which is fair enough as it's often the other way round

I'll see if it's in the workshop Manual but as that on a DVD it hogs the rest of the computer
Nigel At

sorry no joy anywhere
Nigel At

Simon,

I understand where you are with this and my advice would be to make as much room as possible as in my opinion not having enough suspension travel is a real problem, whereas having too large a cut out to clear everything is less of an issue (Although you probably want to minimise how much you cut out). If in doubt - cut it out!

Personally I have been using sill heights as my datum to gauge ride height, the good thing about sill height is that you can harvest information from all Spridgets irrespective of RWA / SWA and it is still relevant. 150mm seems to be a relatively common sill height (measured to the flat part of the sill / floorpan) and depending on what tyres you are using your tyre static loaded radius will be in the region of 250 - 260mm, therefore implying the rear axle centre should be about 100mm (minimum - I have been using 110 - 115mm) above the sill height at your std ride height condition. From this position I would be looking at about 70mm of wheel centre bump travel as a condition which should still not foul anything at full bump (Full bump being metal to metal condition).

What are you using for lateral axle location with the De Dion?

Cheers

Spencer

S Deakin

Spencer,

The Caterham set up uses a lower A arm link which locates at the centre of the De-Dion tube and underneath the forward Watt's links for lateral location.

The main problem I have at this moment is that sill height isn't measurable a the car is on it's side, hence the need for a fixed on car distance. With that I can gauge how much to cut out. And it's looking like it's a good job I asked, as I think I'll need to massively extend the cut upwards.

Si
S Overy

Simon,

I understand you can't measure to the ground that's why I converted it in to a wheel centre from sill measurement. As long as you have a sill / floor to measure to then the 110 - 115mm I am using or the 100mm minimum I quoted would give the wheel centre height?

The lower 'A' arm setup gives a very low roll centre compared to the live axle it replaces, I assume the De Dion Caterham uses a RARB as a result? Are you going to fit a RARB

Spencer
S Deakin

Spencer,

There is a RARB kit for the Caterham, but it's an optional extra. I'm not sure as yet if I'll fit one or not, as I'm having to narrow the De-Dion tube to make it fit so the off the shelf option isn't viable anymore. It's something I intend to play with once I get the car on the road, as there are enough things to do as it is, and I'm already behind with where I want to be.

Using the 100mm minimum you've quoted puts the wheel looking right. However, with the current cut out, it only gives 1" of bump. Had I realised this before, I could've saved myself hours of work. Ho hum, you live and learn. It also means my big shiny, and newly made, suspension brackets are no use. But it does mean that I can simplify them as the front Watt's links will need a separate bracket to move them upwards by 2.5" so I can weld in their own brackets.

The only worry I have is that the cut out is almost entirely inside the chassis rail, and will remove most, if not all, of the boxing. This may not be much of an issue if I weld, rather than bolt in a cage as I can give it some form of rear legs to carry the loads and tie the shock absorber towers in too. Any one got any thoughts on that, or can think of an alternative option?

Si
S Overy

Simon,

TBH I think most things on a Caterham were listed as optional extras ;)

As you know (from having to narrow everything) a Midget is narrower (and heavier than a Caterham) and as such I would think a RARB will be more necessary than on the donor Caterham. A narrower car will roll more than than the equivalent wider car, combine that with being heavier...

If from the quoted wheel centre positions you only have 25mm of bump travel then it is good you looked in to this.

Cutting in to the chassis legs is a problem if you are using longitudinal Watts linkages as the rear mounts will become disconnected from the rear bulkhead and flex badly, as you mention you will need to take the roll cage back to this point to try and get some stiffness recovery.

The alternative to a RARB is running stiffer springs but if you do the calcs the increase in spring rate to include the equivalent RARB rate makes the road spring quite harsh as far as ride goes.

You could try and make the longitudinal mounts narrower (closer to the car centreline) and miss cutting in to the rear chassis legs hence leaving this structure intact, but you need to be careful with roll steer effects if you have to narrow things (but I am sure you already knew that).

Are you planning to use a Sierra rear diff, I have mocked one up already and the mounting flange to prop face is quite far forward and inside the Midget tunnel making fitting the prop a real problem without cutouts in the floor pan.

Post some pics up and others may be able to better offer alternatives

Cheers

Spencer

S Deakin

Spencer,

The problem with narrowing the linkage mounts is that I'm not entirely sure what it will do to the geo. But since they're Watt's link and no trailing arms, they should still limit/eliminate roll steer.

Actually moving them is just a case of cutting the plates off the De-Dion tube ends and moving them inboard 2-3", which may well be the better option, and one I hadn't thought of, so thanks for that!

Doing this would leave the rear most mounts sitting on the boot floor though, and not attached to the chassis legs as they currently are. Would some spreader plates under the boot get round any flex that this might incur? The rear mounts don't take as much force as the front arms do since there main job is to stop the axle rotating.

As you say, I will almost certainly need a RARB, but given that mounting one should be fairly simple, I can do that over the winter once the car is hopefully on the road.

Yup, I'm planning on using the Sierra diff, mainly because it's what I've got. But since I'm going to need to get new, shorter drive shafts made, I could always get the inner ends cut to a different spline to accept a different CV joint/diff. When you say it sits in the tunnel, how far in? I'm contemplating removing the tunnel and replacing it with a backbone/space frame section to aid with strengthening/stiffening the car as my tunnel already has a fair few dents and a large hole in it anyway. If I extend it backwards, it would give the diff a nice strong mount too.

Si
S Overy

Simon,

Yes moving the Watts mounts inboard on the De Dion tube was what I was suggesting when I said:

"make the longitudinal mounts narrower (closer to the car centreline) and miss cutting in to the rear chassis legs"

Hence leaving this structure intact altogether. I would move them inboard just enough to clear the chassis leg, that way you can gusset / triangulate back to the chassis leg otherwise if you move inboard too much it will be harder to keep stiffness of the structure it is mounted to.

I can't visualise your installation but I had thought packaging an ARB would be quite tricky as fitting one underneath will hurt ground clearance and above means cutting more holes but without conpromising the stiffness of the areas which the Watts mount to. Even if you don't make the hardware now I would play with a few wooden dowels stuck together just to mockup the space one will take up.

I have some sketches I made of the 7" Sierra diff, I will try and dig it out and remind myself how much the nose of the diff was inside the tunnel - not a big problem but one better considered now as access to the bolts which mount to the prop was restricted, but might be solved by preassembling the diff and prop together before bolting the diff to the chassis.

The idea to replace the tunnel with a tubular backbone would in my view be one of the biggest gains in torsional / bending stiffness of the chassis (short of a properly welded in / attached full cage) - I just can't bring myself to do it (yet) I have tried to see if I can hide some sort of structure inside the existing tunnel (or something which looks like it) but around the gearbox in particular is challenging with this self imposed limitation.

As far as the use of longitudinal Watts linkages goes, do you know the specific reason they use them on the Caterham? as I was intriguied when you mentioned using them. The Watts linkage could make the top mount move near perfectly vertical (depending on the pivot and the link lengths / angles) during bump / rebound but the lower 'A' frame still moves through an arc so the diff nose will still 'nod' up and down with vertical axle travel. I need to play with a model as I can't visualise it but it might be used to deliberately to cause some roll steer effects - need to do a bit of reading up on De Dion and maybe that will remind me...

Cheers

Spencer
S Deakin

Sorry ignore the comment about diff nose moving up and down I meant De Dion tube which just controls wheel precession / recession in vertical travel.
S Deakin

Edit:

Yes the longitudinal Watts seems to be used for a roll steer reasons, it is supposed to reduce / negate the roll steer that would normally be experienced by a short upper longitudinal link
S Deakin

Spencer,

Having measured up, I have taken the possibly brave decision to move the Watt's linkages inboard by 5". I didn't want to move them so far, as it leaves a 2.5" gap between the cut out and the chassis rail, but the coil over needs to stay mounted as near to the outer edge as possible, so they have to move inboard enough to clear it. What I intend doing is creating an additional box across the car to link the cut outs, which will have large boxes over them, together with both chassis rails. This should more than replace any lost stiffness.

As for fitting an ARB, with the lower A arm being mounted below the floor level, I will have space on the rear face of the bulkhead to create the mounts when I get round to it. This link shows how it should fit:
http://www.flickr.com/photos/exfordy/502475508/

My idea for the tunnel seems quite similar to yours. I don't want to change the look of the tunnel if I can help it, so I'm thinking about trying to cut it out as a whole panel and then fabricating a frame for it to bolt back on to.

Si
S Overy

Simon,

WOW

I just looked on the Caterham website (online store) at the De Dion parts and there prices, I think the De Dion tube alone was over £300...

Being too lazy to add it all up, roughly how much does it cost for everything that you have to buy from Caterham, such the ears, hubs, A frames, Watts links etc...

I guess it will require a custom fuel tank moving the longitudinal mounts in that far.

Where do the coilovers come through the boot floor? Do they go through chassis leg area.

Will the exhaust go into the boot area or is there a clear path through the area with the De Dion and the driveshafts.

Sorry I know it's twenty questions, just interested ;)

Cheers

Spencer
S Deakin

Spencer,

Don't worry too much about the prices, I've not paid anywhere near those that Caterham charge even though all the Caterham stuff I'm using is/was brand new! There are also a few cheats built into my set up, such as using the disc brake hubs that I got off a full Sierra rear end that I only paid £70 for. The ears I need are therefore also much simpler, although I haven't had them cut as yet. The drive shafts will also be custom, but still much, much less than Caterham want. Although over all it's not the cheapest set up, it should still come in at less than £600, plus coil overs, for the whole lot, which is a fair bit less than a full Frontline/Peter May Midget axle and will be capable of much more power. It will also leave me with an adapted Caterham rear set up, which may be worth more, or inspire more confidence, should I ever need to sell the car.

Yup, a custom tank will be required, but that was always going to be the case. But with moving the Watt's links inboard the tank may now have to be a funny shape to get me the volume I want of roughly 30 litres and still use the standard fuel filler location.

The plan is for the coil overs to come up through the chassis rails where the bump stop boxes are. The rail will be boxed and probably heightened to give me the right range of movement and sufficient strength. I'm sort of copying Tasmin's set up for that part.

Not 100% on the exhaust route yet, but it should be able to go under the whole lot. I know there won't be space for an over axle option as the De-Dion tube will be to close to the boot floor to allow it to come back down.

This is how far I've cut out so far, and I've got to go slightly further back yet....


S Overy

This thread was discussed between 29/08/2011 and 01/09/2011

MG Midget and Sprite Technical index

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