Round tube & chassis mods... a world of pain?

[TQ_uk]

Ok, to set the scene, I can now nearly see the garage floor post house move & seriously thinking about starting a build.

For perverse reasons best known to myself I fancy increasing some of the dimensions of the 'book' chassis & materials.

Can't help thinking the standard tubing looks a bit 'weedy', aesthetically as opposed to physically, so I'm considering going for deeper rectangular base rails, and swapping much of the remaining tubes for larger OD tube (say 38mm) but of a similar final chassis weight (ie tube wall thickness may come down a bit).

I'm also considering increasing the wheelbase slightly.

Now ignoring the fact that roadster bodywork won't necessarily fit (I've a few ideas of my own here), my questions are this:

a) am I making life (too) difficult for myself, and should just stick to the book plans?

b) what effect on steering geometry be if I stretch the wheelbase say, another 100mm or so?

c) Will seamed tube (ERW?) be an ok replacement for square section rather than the expense of CDS?

I realise in areas where stressed brackets (eg wishbone) are attached the wall will have to be of a suitable thickness.

Any opinions from your experiences? This is my first build, so I expect some 'keep it simple' response but not too phased by a bit of experimentation for the cost of some steel & welding supplies ;)

[AshG]

if its your first build I would keep it to book spec tbh. obviously i dont know your background but the chassis was designed using cad which simulated the stresses on the design. if you start messing around with material thickness's and tube sizes you may think it looks stronger to the eye but may be actually weaker.

[dogwood]

Have to agree with Ash here.
Having built to the book, and now doing a full rebuild.
If you alter just one thing, it throws out everything else.
So you will end up reworking every measurement.

As for the tubing you're planning, unless you intend to fit some megga engine.
You are just adding unneccacary weight to the chassis, which it don't need.
As for a 100mm wheelbase extension...You would also need to widen it significantly, and prob raise it some. Otherwise it would look out of proportion.

IMHO of course

[HandyAndy]

I too agree with David & Ash, as this is your first build, stick to the book, get it on the road & then maybe change things to how you,d like them (obviously not the chassis design) but other area,s where you can "put your own mark" on your car,

or

build another, from the knowledge you,ll have gained by building a book spec car then your next project won,t be so daunting.

As for using round tube, just think of all those "fish mouth" joints :eek: try keeping 2 bits of round steel at the correct angle with a welding magnet on them :eek:

whatever you decide to embark on , i wish you good luck with your build:)

cheers
andy

[twinturbo]

Can't help thinking the standard tubing looks a bit 'weedy', aesthetically as opposed to physically,

I cant remember seeing the tubing when looking at a finished car, so asthetics of the tubing can't be a factor.


I think if you deviate you will never finish or end up with something that has no advantages.

IMO stay book.

TT

[Talonmotorsport]

I build rollcages and grasstrack specials in round tube which needs to be fully welded together (rule book says no bolt together cages) and the parrot beak/fish mouths are a pain to do with out the right machine(s). An angle or die grinder will become very tiring to use after the first 4-5 goes at the tubing, not to mention a little dangerous at times. If you've got the right kit and know how it could be a nice project but if you hav'nt got the fabricating capability it will cause you nothing but agrovation.

[TQ_uk]

Thanks, all for your input - I think tbh you've kind of confirmed my inner thoughts & I should stick with the tried & trusted route (at least on the first build ;) )

Think there's a phrase along the lines of 'my aspirations exceed my ability' or somesuch:o

[Davey]

Whilst I wholeheartedly agree with all comments so far made I'm still going to throw a funny shaped ball into the park. I'm rebuilding an existing car, not a Haynes or Locost I hasten to add. I'm using the Haynes book for inspiration and general ideas but while my finished car will hopefully look something like the Hanyes there are going to be major differences. My wishbones for example will be quite different in dimensions than the Haynes, my chassis is made from a mixture of 80X40X3, 60X40X3, 25X25X3 box and 25X25X3 angle. Although a lot of the dimensions are going to be similar a lot are going to be quite different. I still wouldn't go for round tube for the main chassis structure though. If you want to build a Haynes then go by the book, if you want to build something else then fine but you need to be pretty sure of your own abilities. The point about CAD design and stress assessment is a good one and not to be ignored lightly.

D.

[TQ_uk]

Hi Davey,

thanks for the curved ball ;)

Its not so much a deviation from the design I have in mind (ie in tube placement), but a substitution of 'larger' materials (bigger OD tube of comparable strength to 'book square'). Think of just replacing the top rails
(sorry don't have book to hand to refer to part numbers) with bigger OD tube.

Re TT's point regarding aesthetics, I'm considering alternative body treatment that may leave top rails etc exposed ( see http://www.mevltd.co.uk/missilegallery.html for an example) hence thinking of all this in first place...

[twinturbo]

Quite likning them MEVS.....

NExt time me thinks.

TT

[Bonzo]

I have kept quiet on this thread untill now !!

Not knowing your engineering background it would be hard to avise on the merits of what you hope to achieve.

I can see that you have had some very sound advice indeed :cool:

Yes I too like the Merves ..... In my view the Merve is a whole different ball game .... If that's what the heart desires, trawl the tinterweb, I am sure you'll be able to find some plans or at the least, a solid sense of direction ;)

Personally I am not a qualified chassis designer but from my many years of structural fabrication.
I am fully aware that any alterations made at one point of the chassis will most likeley have a secondary effect elsewhere !!??

For me personally, my build has given me so much more than a well proven rolling chassis sitting in my workshop.

The build, as outlined in Chris's book has given me the confidence & the desire to design & scratch build a rear wheel drive 7 esque car.

Well if I can complete the Roadster build before I draw my pension :o

I wish you well for when you start a build, great to see another Roadster on the go & equally great to follow the progress of a whole new concept