Chassis Strengthening - Sills. Opinions Please!

So I'm tackling a job that Ive been dreading for a while now....replacing the full length sill panels, my dog legs were shot like most cars but the sills in front of the dog leg and even the section under the front guards are rusted through in places so I wanted to replace the whole piece. Bought the reproduction sills from Warren and they're OK, far from perfect but way better than patching the old ones. Got the old sill off much easier than I anticipated and was surprised by how little strength there is in the sills both inner and outer combined. Mine is an early 240 and any wonder they are light weight....theres very little in the way of structural rigidity, the chassis rails only extend 3/4 of the way under the floor so really don't do much and this means that the sills are the major structural piece that ties the front of the car to the rear and there ain't much to them either!

My car will need to be engineered for rego and with over 300HP I'm sure the engineers will want to see some chassis strengthening....but I really want to keep the chassis as original as possible as its a lowish build # and Ive got the original engine so I want the ability to return it to original if values go through the roof.

So once I had the sills off Ive had a good look and wondered if I should do something to strengthen the chassis while I'm replacing the sills. At the front and the rear of the rocker box (more correct term for the inner sills) there is a strengthening piece between the inner and outer but for the rest of the sill, which is the basically the length of the doors, there is nothing just the inner sill panel which is punched full of holes and the outer sill panel and both can be flexed by hand with little effort. Appreciate that once they are welded together they are much more rigid but I'm wondering about the merit of running a full length middle reinforcement piece that will connect to the front and rear reinforcement pieces? Took me all of 20 minutes to mock up a piece and it should be pretty easy to fit...will make the fitting of the outer sill a bit more complicated but I end up with 3 layers to the sill for the full length which has to be better than standard.

So I'm after opinions....has anyone else done this....any down sides you can think of. Also thinking it would be stronger again if I get a bead rolled into it probably 2 beads actually.

Comments please!!!

As long as it doesn't trap water & allow the metal to 'sweat' Dave, then go for it.

I would however like to see a bead or fold added to that, as a bead or fold has quite a bit of inherent strength in it.

You could also put a cross hatch in it if you are really keen.

I had thought of using the space left in the rockers on another project for the tube chassis that I had planned for it to hide in, and still allow me to keep the floor at or near stock location.

What I was going to do was build new front frame rails, to replace the stock ones, actually I did get that far, I'll also mention this was FWD to RWD conversion. So with the new front frame rails in place, my plan was to tie the rear sub frame (once I had bult it), to the front, by using some round tube, probably would have been 1 5/8 DOM mild steel, bent in the appropriate ways to mate the rear sub frame to the front subframe, but run through the rockers. I'll likely end up doing something similar to my Datsun, I'm just not at a chassis building stage with it yet.

On an S30, the front of this rocker tube would have to run inside the wheel well from the front of the rocker box to the frame rail, so it will be visible, which would probably bring down the value a bit, if you ever did return it to stock.

I don't really see a sheet of metal welded in between like that doing much, yes it will strengthen it a bit, but I'm not sure the effort to go that route will yeild any appreciable results. Like I was getting at earlier tieing the rocker box into the front and rear frames will be needed to get any real benefit from rocker strengthening.

Why not just get thicker chassis rails and extend them further back?  That would be the most common and simplest way of strengthening the chassis.

Why not just get thicker chassis rails and extend them further back?  That would be the most common and simplest way of strengthening the chassis.

+1 that is what have done

Loui

I'm not sure it's worth the effort and the risk of having more metal to rust. If you do go ahead, I agree with Lachlan that you need to roll some ribs into the panel. You really should focus on the frame rails as has been suggested and to that end you might want to give more consideration to full length rails - which can be very easily removed at a later date. Any strengthening of the sills should be in respect of side intrusion (e.g. P Brock crash) where a tubular component would be more suitable.

I don't follow the market all that closely but the idea of ever increasing prices for early Zeds is a bit over-hyped in my opinion - never consider them a financial investment.

Hi Kato ..

I considered this for my 'brown hornet' too .. and aside from adding a lot more welds to the new chassis rails, we did not add any additional structural items. We could and might later add a little half cage which itself tightens things up somewhat.

We did make some changes in the engine bays though that I thought are worth passing on, that I learned from James Flett and the Group SC guys. These guys all tend to add a lot more welds onto the front strut towers, where they meet the engine bay (body). We added a lot more spot welds to the joins between the front strut towers and the body, a modification that BMW also does with their 3 series when 'making' am M3, for example. Other manufacturers also to this. Also of interest, if you do the welding under the car, in the actual wheel well, then you get the benefit of the extra rigidity, without the weld itself being visible from the engine bay, that is the reinforcement becomes largely invisible. Once you seal the joins with the drip check or whatever and then use the stone guard, the weld kind of disappears.

Well worth doing mate .. if you have the patience. I also note that on Tomitaku's web page, if you study the photos of his build, there appear to be a whole stack more spot welds in the same location as I was mentioning. Of course, you can add this treatment along the chassis rails, and in the rear strut towers ... It DOES add weight though, of course so use the technique wisely.

I hope this helps, I know the Group SC cars, that are beating all manner of 911's do these tricks (without any form of strut bracing might I add) legitimately might I add to keep their cars handling.

I've added a photo of my car where we strengthened mine. Note the welds are under the car, around the perimeter of the front strut towers... with the undercoat and stone guard they are virtually invisible .. but they are there !

Cheers

Adam

Thanks Adam and everyone for the input, much appreciated.

More for the sake of appeasing the engineers I'm probably going to go ahead and add this strengthening inside the sills as I see more upsides than downsides and its easy to do while Ive got it apart. Just need to find someone who can roll some beads for me.

Peter Allen, price of early Z's may or may not ever be significant but I still cry when I think of the genuine and unmolested HK 327 "Bathurst" Monaro that I once owned....wish I still had it now$$$$$. Hence I'm conscious of destroying value with unreversable modifications.

more torsional rigidity to be had from the sill panel guys, not the rails.

by sleeving inside the sills with cage bars or by adding an inner skin as being done above, but again with ribs and pressings or dimples. have a look at any modern car and this is how they are done (factory anyhow), but aftermarket is done with round tube/pipe as in similar to that from a rollcage addition.

As the sill panels are furthar apart than the seat/floor supports (which is all that they are) they can withstand greater torsion caused by the engine, lateral cornering and combined lateral and longitudinal forces as seen on the race track, ie braking and turn-in.

Your approach for a mid layer is very good for increasing strength in a vertical bending plane for the sill panels but you will also need to add reo from the fron of the sill sections back up toward the strut towers and across to the other side of the car, ie through the dash or under the trans, or through a hoop around the dash under the windshield.

Think of this its great to have 2 solid beams in parallel, but you can still twist them easily about each other, this is what you are trying to minimise is the twist not the bending, grab 2 rulers or pens and play with them, try bending when holding both ends, the try twisting.....think of these as chassis rails ie close together, then think of them as sill panels ie furthar apart, it is easier to restric when furthar apart but you need to brace in an upward or downward maner to be able to resist what I and others call eccentric loading, ie loading off to the sides of the members which cause twist. the further they are up or down the more resistance to the twist.

can I say twist 1 more time?

triangles ar what you need to try and make as it is the strongest means of reo', but this is quitete hard with the monocoque shell of the Z so it requires some heaver sections where this cannot be achieved with some form of gusseting req'd to achieve strong intersections.

Roll cages are purely design for occupant safety ie a safe strong shell around the driver, so do not be confused when we talk about making a car more rigid, and roll cages, yes they do add stiffnes to a chassis but not and nearly almost never in the right areas to aid in what the s30's suffer from, yep, need i say it........

My recommendations are continue with your mid skin, and maybe think about a round 38mm tube from a box inside the front of the sill up through toward the base of the a pillar, and a through connection under the dash, ie bolt in bolt out with the dash.

Ideally what are you trying to achieve and for what purpose.

cheers Nat0

Without any doubt, a fully welded roll cage with multiple cross bars and multiple attachment points to the shell (ie A pillars, roof, strut towers, etc) will significantly enhance the strength of the shell and prevent twisting. Simply put, the more bars and touch points to the shell, the stiffer the shell becomes.

Without any doubt, a fully welded roll cage with multiple cross bars and multiple attachment points to the shell (ie A pillars, roof, strut towers, etc) will significantly enhance the strength of the shell and prevent twisting. Simply put, the more bars and touch points to the shell, the stiffer the shell becomes.

And heavier.  HMAS Melbourne was a very stiff ship and could turn inside 12 miles and could stop in 8 miles.

And heavier.  HMAS Melbourne was a very stiff ship and could turn inside 12 miles and could stop in 8 miles.

Thanks Nat0.

Car will need to be engineered to get regoed due to the engine swap I'm planning so the primary reason for the stiffening is that the engineers have said they would expect to see some sort of chassis mods to improve strength to handle the added horsepower. This rules out roll cages etc so however I meet their criteria will need to be both subtle, within the law and still effective. I can do the extended chassis rails easy enough but I agree with you about the effectiveness of this vs adding strength to the sills. I'm hoping that upgrading the sills will be sufficient to satisfy them, if it isn't then I will probably have to do more but will wait until I'm directed by them to do so.

Whilst a cage obviously adds weight, so too would any of the suggested work to the chasis stiffening. A well designed fully integrated cage not only strenghtens and stiffens the car overall it actually does it in the correct spots where body flex occurs. That's the point of having a properly designed cage nowadays is to first stiffen the car and secondly provide protection (and not the otherway around as suggested!) The added weight in relative terms is neglible for the handling benefits it provides. Testing has always shown that all other things equal, a car with a proper cage is much faster around a track than a car without one. The suggested chasis strengthen alone compared to a proper cage will not do as much to improve the cars handling/performance.

Agree 100 % Graham and if it was a race car then I wouldnt hesitate but car has to be legal so a cage is not possible.

Whilst the new Vic Roads roll cage reg's a lot tighter than the old regs (due to adoption of the Commonwealth Standard), 4 point cages (ie half cages) are permissable for road use. Roll cages installed before Nov '09 that comply with the old reg's are still OK.