I'm embarking upon a totally new build and so ditching my Champion based chassis and going with the Saturn MX5 variant of the Haynes Roadster crossed with both Aussie mods and a few of my own for good measure.

However here's the tricky bit....500kg with a car engine!

I plan to get as close to the 500kg mark as possible. The Caterham superlight manages a skinny 515kg so I see no reason why I can't trim a further 15kg from my build. My power plant is a 1.8 mx5 VVT complete with matching gearbox, prop and hopefully lsd diff if I can find one. I imagine I'll use uprights from the mazda but I'm on the lookout for lightweight calipers as I understand the mx5's are cast iron. I plan to employ a minimalist approach including my own design seats which weigh less than 1kg each, carbon dash, tiny custom made digital display under 0.5kg and the lightest alloys I can find at the right price.

So with this in mind what are my chances of hitting my target of 500kg and with that a nippy 300bhp per ton

GM

Also has anyone ever tried to modify the passenger foot well so the battery can be placed lower in the car? I hate seeing such a heavy thing so far off the ground.

GM

My Mojo is 523Kg (with a Ford ZetecSE engine) which is mainly due to it being a mid-engined layout and tiny in comparison to a standard 7. I'd imagine it would be possible to achieve 500kg in the standard layout without too much hassle though?

Keep us posted!
ed.

Why would a 5kg battery cause you problems when a 150kg engine and gear box are In there? You will struggle to build a cec at 500 kgs but would you notice a difference between a 550kg and a 500kg car? I would opt for a higher powered cec or a decent lightweight bec!
I would love to get my build under 500 kg but realistically I will be over 600 kgs. If you do find any weight reduction mods then please let us all know. Especially the 1 kg seats. Cheers

My Haynes with a zx9r engine in it is 520 kg with a full tank. I think you will struggle to get near 500.

Good Luck, We are building our duratec car with the 500kg aim, but we are going to extremities and no corners are being cut on weight....

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and no corners are being cut on weight....

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OK serious options/opinions

First try to get/give weights to some of the fixed things you’ve decided on i.e weight of a finished Saturn Chassis, chosen engine/box, uprights front & rear, prop, steering rack etc.

Start an excel sheet totalling these fixed items, next add the “upgrade stuff you’ve already mentioned i.e. put a weight for standard mx5 wheels and as you look into differing better options either enter a new weight or estimate the saving. Likewise callipers and/or discs etc.

Some practical suggestions:

If you’re fabricating the chassis (or having it made especially for you) go one gauge lighter on all tubes that are part of proper triangulation i.e. aren’t subject to bending loads – this should save 20% per tube, replacing some with equivalent round tubes can save another 20-25% per tube. The only caveat I’d add to this is that the lighter tubes you use, the better welder you need to be!

Bodywork, even though there isn’t much on a 7-type vehicle there is no reason you can’t save at least 50% over standard panels. They don’t even need to be made in anything fancy/expensive like carbon or Kevlar, just regular glassfibre but with more emphasis on using thin cloths rather than thick matt will make a massive difference.

Wheel/tyre choice, don’t go bigger than you need- 15” max, 14”s are a possible but limit tyre choices and your chance of finding light rims second hand. 13”s are often quoted as the “right choice” for a 7 but might not go over your mx5 brakes.

Just to add one final though… any 7 is a light car, getting down to your target won’t come via one or two big changes, going for this style of car has already made those changes for you! Saving these last few kg’s will come from lots of little choices; use the shortest bolts you can, drop a size, gauge, thickness on anything you can. Basically don’t over engineer!

Anyway, good luck and keep the forum updated on your progress



[Edited on 31/12/11 by orton1966]

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Originally posted by Gear Monkey
Also has anyone ever tried to modify the passenger foot well so the battery can be placed lower in the car? I hate seeing such a heavy thing so far off the ground.

GM

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One more area that people don't pay attention too is simplicity.

I have seen cars where the brake lines are a mess, wirring going up and down through the body etc...

I am sure, if some time is spend on designing these,a couple of kilos could be gained.

I would be careful about concentrating weight loss on the chassis (sprung weight) and ignoring unsprung weight. The suspension works best when then ratio between the two is at it's greatest. That's when the least inertia is transmitted through the suspension to the chassis and upsetting the balance of the car. Think of Newtons Balls!

Using MX5 uprights hubs, calipers and discs will be overkill as they are designed for a much heavier car. I would use Cortina pattern alloy uprights and hubs, wilwood calipers and modular brake discs, perforated discs with separate alloy bells.

On the subject of reducing gauge on chassis tube. I agree tringulation tubes are not subject to bending moments, but the are subject to compressive loads and they will be considerably weaker.

Instead of using a conventional battery and putting it in the foot well, why not just use a PC 630 type battery.

Lay it flat on the bulkhead very small and light.

Have a look at this old thread for reference:
http://locostbuilders.co.uk/viewthread.php?tid=157212

I think you're going to have a hell of a job.

Does seem a fairly tough call but I reckon it's possible from a quick totting up of weights. As said above, I'd concentrate on unsprung mass to begin with - 520 with light unsprung mass will be a lot better than 480 with heavy unsprung and a twisty chassis.

aren't mx5 engines steel blocked??

If so I'd say no chance.

solid brake discs at the front also , dont need to be vented .

road registered ?

not road registered , you only need a rev counter .....

you could of course just use a stack module or something , but when i was building darrians the lightest ones were the simplest ones . the less you have to put in the better ....

with an mx5 engine and diff you havent got a chance hitting 500kg. also the haynes roadster is a much much bigger car than a caterham superlight. infact the superlight is dwarfed by a roadster.

here is a picture of my roadster next to an r300

Why not narrow the body and reduce the overall lenght by a few inches, to the exact engine, seating area and tank size as thats what you are set by.

Surely the chassis is pretty much the last worry on this list though? You're talking a few hundred grammes, maybe a few kg in chassis and body weight, maybe another in driveshaft length if shortened. You're needing to drop tens of kg?

Yeah but as John Tesco once said "Every little helps"

In fact didn't Mazda go to the lengths of shaving 22g off the rear view mirror for the latest version?

[Edited on 4/1/12 by Ninehigh]

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Originally posted by TimC
Have a look at this old thread for reference:
http://locostbuilders.co.uk/viewthread.php?tid=157212

I think you're going to have a hell of a job.

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U2U sent (to OP)...