Model A chassis- tubing size VS. horsepower- flex?

this thread kinda incorporates a few questions relating to model A chassis builds. ive messed about with various terms in the search and i cant find the info im looking for. its more theory than the nitty gritty 'cut here and weld this' stuff im after.

my major question relates to what kind of power a 2x3 frame can take compared to a 2x4frame. the thing is, im buying a unfinished '29 sedan proect, that has a 2x3 box tube chassis. it has a rover V8 with 4 speed manual, kicking out about 200hp, but my mate is saying ile regret it if i dont fit the 455 BBO with tunnel ram and comp cam kit, backed with a TH400 in that i have here. that will more than double the horsepower (quoted figures for similar motors are 450hp and 500ft/lbs). simple question is, will i twist this thing in half if i drop that motor in?

also, how much does Z-ing the chassis effect the torsional regidity of the frame? surely, the more it is Z-ed the more it will twist as it will move the propshaft up away from the chassis rails, creating a turning moment on them? or am i worrying about nothing again?

the rest of the rear setup isladder bars, coilovers and a chevy 10 bolt (id probably swop that for a 9" i have). VCW drop beam, TCI hairpins, posies reverse spring, flat crossmember. it has no Z at the front, and 8" at the back. channelled about 3.5" to cover the rails.

im 6'3" so cant channell it any more than that as i plan to chop it 6" as well. this means to get the ride height i want, i will need to kick up the rear end more, as realistically i want the top of the tyre level with the swage, its currently just past the top of the arch. will a 14-15" kick make the back of the chassis flexy? also i plan to kick the frame 4" at the firewall and slope the rails up slightly, and mount the spring to the hairpins like on the shine '34 pickup.
i want this thing to sit low, but id dont want to do the mods to the 2x3 chassis if ile need a 2x4 one for the bigger motor.

and finally, whats the best style of crossmemebers to use for big power like this? will a simple straight crossmembers above the rear axle and for the front spring, and a K-member suffice, or should i be looking to add a big 'X' in there under the foor?

all help/advice greatly appreciated

 

go read frank costins book. save yourself the trouble and learn something new.

 

sorry, do you have a title i can search for? ive googled his name, but thats only given me some information on who he is, i cant find anything about titles hes written. his history sounds like the book should be a good read though

 

The amount of Cross bracing and diagonal support Depends on "how you mount the Motor and transmission".

If you do like the racers with a Solid front plate and a solid bell housing plate... you'd only have to brace the rear part of the frame.

But I'd say... build it as though you're going to run a "Blown Hemi" in it and you'll never regret it...! Same principle as "killing a mosquito with a sledge hammer"... Too much is "Just right"...!!

 

Why don't you not worry with the old frame, sell it and make enough to buy new tubing and build your own, that you know will be strong enought for the Big Olds... Then you can get it all set up and still drive it while buiding the new chassis. Then probably swap everything in a weekend with a couple cases of beer. That's what I'd do! Then there's no question. Also depending on the tires you'll probably won't be able to make em stick with 450 hp... Lot's of smoke!! SWEET!

 

i kinda agree on the 'over kill is the best option' method. but, conversely i dont want to go loading it up with so much bracing the frame becomes a boat anchor....
i have been reading the buildup thread of the littleman coupe 'deaths doorstep', and see he has a LOT of bracing in there. i beleive that frame to be the kind of thing i need to aim for but with a more conventional motor location. i beleive it would be sensible to have a prop loop on a very powerful car?

i do not want to solid mount the motor or trans. i want to be able to put some miles on it without it sending me deaf or rattling me and it to bits. conversly, i want to make sure it gets the power down without ripping the mounts off. a friend can get me nylon re-enforced rubber discs from work, about 1" tall and 2" diameter, they are quite stiff/hard and i was planning to use those to make motor mounts like these on the bass coupe-


as the rubber is used as a vibration dampener rather than a positive mounting point, as the bolt up the middle does that.

 

you could be right. run it for a while as is then i can take measurements around whats there to get it how i want it. i dont plan on ever building another 'rod, just spending the time to get this one exactly how i want it. ive been planning and saving for this project for over a year now, but now its coming down to crunch time its getting scary!!

i dont expect the tyres to stick a lot either, but, if they do on a sticky track, i dont want to end up with a pringle shaped frame!

 

ttt

 

Take a look at the advertisements in magazines pertaining to hot rods to get a good idea of how to lay out crossmembers.

Imo you need more than a front, middle and rear crossmember to adequately resist flex.

If you want to do a little reading on the subject, the two books listed below are a couple of the better ones.

"Engineering Street Rods" by Larry O'Toole.
Excellent book, printed by Australia's premier hot rod magazine publisher.
Lots of detail, oriented toward the home builder and the nice part for you is that the steering is on the correct side for the UK and Oz.

The SoCal Speed Shop's "How to Build Hot Rod Chassis" is also a good reference with lots of good, clear photos at various stages of frame construction.

Both of these books are available from Amazon.
There is a UK Amazon and I'm guessing both these titles will be in stock there.

Do yourself a favor, get the books, study them and follow what they have to say....

 

thanks for the info. i have just ordered the first title you suggest, but the second is out of stock so ile have to wait for that one.