bumpy
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| posted on 5/8/15 at 12:24 PM |
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MX5 as donor car
I am looking to be a bit creative and perhaps assemble a JBA Falcon type of car using an MX5 donor.
Now the JBA Falcon has its own chassis and the challenge is to integrate that with the MX5 parts.
If I understand it correctly the MX5 has a front and rear chassis joined by a central 'rail' that offers little or no integral strength to
what would be a complete rolling chassis.
My first thought would be to isolate the front and rear chassis assemblies and then integrate them into the JBA chassis to make a single structure.
This would solve most problems of mounting the various major components of engine, gearbox, diff and lower suspension brackets.
My first question is "do the MX5 front and rear chassis have enough inherent strength to make this a possibility"
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micksalt
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| posted on 5/8/15 at 01:01 PM |
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quote:
Originally posted by bumpy
My first question is "do the MX5 front and rear chassis have enough inherent strength to make this a possibility"
Don't see why not, MEV have been using the full sub-frame assembly for years. As long as your pick-up points
are OK, no mither.
 
RollingSubframe
[Edited on 5/8/2015 by micksalt]
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adithorp
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| posted on 5/8/15 at 01:06 PM |
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Notquite right.The MX5 has front and rear sub-frames mounted to the body shell but also joined to each other with a beam between them. The sub-frames
are substantial enough and the beam is there to add ridgidity, as the shell (being a soft-top) alone would have too much flex.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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bumpy
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| posted on 5/8/15 at 01:20 PM |
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Excellent news guys so far.
Next question
The donor car will almost certainly be an MOT failure due probably to rust problems.
I know rust affects wings, cills etc, but do the subframes escape the ravages. If not where to look for damage for problems.
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coozer
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| posted on 5/8/15 at 01:22 PM |
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Just to add, the beam connecting the subframes also acts as gearbox support/mount.
1972 V8 Jago
1980 Z750
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bumpy
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| posted on 5/8/15 at 01:23 PM |
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quote:
Originally posted by coozer
Just to add, the beam connecting the subframes also acts as gearbox support/mount.
Useful to know. Thanks
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micksalt
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| posted on 5/8/15 at 01:28 PM |
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Sub-frames do succumb to metal rot. I've just stripped an MX-5 with not a lot of meat left on the rear sub-frame. Front sub-frame was fine and
of course, the aluminium PPF linking the two was untarnished. What you might want to consider is retaining part of the PPF to act as gearbox support.
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bumpy
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| posted on 5/8/15 at 01:34 PM |
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Good suggestion about the PPF (whatever that stands for  )
If the rear subframe is shot can (new) replacements be bought, and if so where?
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micksalt
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| posted on 5/8/15 at 01:41 PM |
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Power Plant Frame, the bit that links the two subframes together. However, if you do this, you'll have to ensure that the chassis gives good
alignment between the gearbox and diff. The PPF guarantees this. Many kit cars do not use the PPF, my future Vortx included so just follow whatever
practice they use.
[Edited on 5/8/2015 by micksalt]
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prawnabie
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| posted on 5/8/15 at 01:52 PM |
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The ppf doesn't link the subframes together with any strength, it links the gearbox casing to the diff casing, both of these are rubber monted
to the subframes (well the engine is, not the gearbox)
[Edited on 5/8/15 by prawnabie]
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Slimy38
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| posted on 5/8/15 at 02:40 PM |
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quote:
Originally posted by bumpy
If the rear subframe is shot can (new) replacements be bought, and if so where?
Not that I know of, but there are literally hundreds of MX5's being binned that have perfectly good rear subframes. Take your pick.
One thing to be aware of (if you don't know already), the subframes don't hold the upper suspension mounts. The car itself has to bridge
the gap between subframe and top mount.
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bumpy
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| posted on 5/8/15 at 03:01 PM |
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quote:
Originally posted by Slimy38
quote:
Originally posted by bumpy
If the rear subframe is shot can (new) replacements be bought, and if so where?
Not that I know of, but there are literally hundreds of MX5's being binned that have perfectly good rear subframes. Take your pick.
One thing to be aware of (if you don't know already), the subframes don't hold the upper suspension mounts. The car itself has to bridge
the gap between subframe and top mount.
Thanks for that.
Regarding the suspension, I am picturing a lower wishbone on the subframe and single point upper fixing that will be engineered to fit to the new
chassis. I guess that adjustable coilovers can be used to replace the existing springs.
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Slimy38
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| posted on 5/8/15 at 03:56 PM |
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MX5 already uses coilovers, they're just a bit big and not adjustable. Reusing the same design upper mounts should give you plenty of options,
although depending on the weight of the car you might need to reduce the spring rates?
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maccmike
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| posted on 5/8/15 at 04:03 PM |
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Chassis rails rust
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bumpy
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| posted on 5/8/15 at 04:42 PM |
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quote:
Originally posted by maccmike
Chassis rails rust
Yes, bit worrying when you read about that. From what I can tell the construction of the chassis rails was changed from the Mk2 onwards to introduce a
crumple zone but it also introduced rust.
[Edited on 5/8/15 by bumpy]
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Slimy38
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| posted on 5/8/15 at 05:31 PM |
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Surely that works in your favour though? It just means that Mark 2's end up as MOT failures in bits of the car that you're not planning to
reuse?
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bumpy
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| posted on 5/8/15 at 05:38 PM |
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quote:
Originally posted by Slimy38
Surely that works in your favour though? It just means that Mark 2's end up as MOT failures in bits of the car that you're not planning to
reuse?
It certainly works if the rails are not part of the front chassis. So, are the rails part of the front chassis or part of the bodywork structure?
[Edited on 5/8/15 by bumpy]
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l5tuy
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| posted on 15/9/15 at 12:02 PM |
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My mk2 was terrible with rust, including the chassis rails. The rails crumpled during jack up, as for the floor pans, literally crumbled to touch.
Typical Jap cars, good running gear but poor corrosion protection on bodies.
Stu
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Mr Whippy
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| posted on 15/9/15 at 01:56 PM |
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Totally agree, I was all up for buying an mk3 mx5 until I just gave up finding a rust free example, some of the undersides were horrific and looked
like the whole car had been parked in the sea even though the upper bodywork looked immaculate. Even suspension and subframes looked in dangerous
condition.
Tbh I’m seeing this with quite a few jap cars and my Almeria, Corolla and Avensis where all basically written off due to terminal underside rust that
was just not worth fixing yet they all looked great from above. Hence I’m back to the Volvo’s which are really well protected underneath.
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