Pritfire chassis cracks - suggestions for repair

[Escadrille Ecosse]

I am in the process of rebuilding my Mk1 sprint/hillclimb car and have found a small crack on the main rails on the bottom outside corner just in front of the front outrigger. There's a crack on each side of the car in exactly the same place at the toe of the weld.

The rest of the chassis and outriggers are in excellent condition so I'm reluctant to start cutting too much off so I wondering about options. This is probably the most highly stressed point of the whole chassis given that the car has a full cage and stiff suspension so in many ways it's not surprising. Photo of the car at Harewood shows the sort of twisting loads.

Do I just weld it back up? or add a small external reinforcement between the outrigger and the chassis rail at that point? I don't want to start adding cage bracing etc as that's just going to open a whole new can of worms and ultimately I'm looking at making the car more road oriented.

Thoughts appreciated

 

[Nick Jones]

Interesting,  Suspect you are correct that the cage is partly responsible, concentrating twist at that spot when it would normally be spread over a longer length.  Stiff suspension and sticky tyres do the rest...…  (looks great!).

I'd clean the area up carefully and weld it.  It's tempting to add reinforcement but given that it's a twisting force I'm not quite sure what to suggest.  Only other thing is to suggest checking that the area where the outrigger overlaps the main rail is not bulging at all.  You sometimes get rust between the two skins and it usually perforates the main rail first, unseen apart from some bulging.

[zetecspit]

I had similar with my spitfire. But that had been de-lipped along the rails to fit the ford engine/box. And my cracks were a bit extensive by the time I found them.

I went a bit OTT and ordered some 3mm wall 50x75mm box section. I cut the chassis rails open and slid about a 600mm length in, it went beyond the front edge outriggers and fowards as far as I could get it. That ought to solve the issue, but I do worry in case it has just moved the stress point.

In your case a good clean up and inspect the area, weld and maybe a bit of reinforcing?

[Escadrille Ecosse]

Thanks gents for the replies.

Like you say I'm concerned about simply moving the problem somewhere else. The cracks are very small, maybe about 5mm either side of the bend in the rail.

There's no evidence of any bulging to suggest corrosion inside the rails and looking inside the outriggers everything is clean so I'm reasonably confident that is ok.

Maybe the answer for now is to just weld it back up and keep an eye on it. Then if they reappear I get a bit more radical with some internal reinforcement. Certainly save a lot more dismantling at the moment as I'm just keen to get the car back on the road for next year.

[JohnD]

I'm sure your right, but whatever the cause, there is excess stress there, and IMHO it needs strengthening, however good your welding is!

I'd weld up the cracks, drilling them out at their ends, and then weld a plate over each, extending it onto the outrigger, forwards towards the suspension mounts, and over and under onto the other faces of the chassis rail.   To spread that load, as much as to strengthen the rail.

JOhn

[Escadrille Ecosse]

So. Took the front suspension off to get a better look and discovered a crack under the lower wishbone rear mount on one side. While I'm fixing that I'll have a think about how I could spread the load as John says away from that corner.

I'll let you know how I get on once I've got on as it were!

[Mark]

The original Vitesse/Herald chassis has  a star shaped fillet reinforcement added at this point, front face of outrigger to side of main chassis rail. Ive re-made them when replacing front out riggers, but don't think it would make any difference to the chassis flex on the main rail.

Edited November 12, 2019 by Mark

[Nick Jones]

More food for thought, IIRC the front part of the chassis is double skinned and has internal reinforcing sections until just behind the  rear wishbone mounts.  Sounds like the short unreinforced section between there and the front outrigger is suffering?

[zetecspit]

As above it must be the most stressed area. The front outriggers bolted to the tub must keep the chassis from that point back pretty stable. Then a couple of sections of approx 3x2" 2mm wall (I think) boxed C sections spot welded on flanges up to the suspension towers. 

I don't think the chassis is double skinned around the front wishbone mounts, but there is a zigzagged spacer/crush tube between the inner and outside faces. On mine I replaced that with a short length of thick walled tube and spreader plate on the outside face, and recessed the inner face so I had some clearance for my exhaust manifold. (yes, after eventually getting my manifold made, a lot of swearing, it being thrown out the door a few times etc it fitted OK.Then I discovered I had about zero clearance when I fitted the wishbone bracket, I had to cut the thread down so it was flush with the nyloc, and even then it was still too close for comfort.)

[JMH]

Do you have a picture ?

Agree with above!

Then: I would open up the outrigger where its welded to the mail rail (underside) - although looking good from outside, I have never seen one without rust there.

[Escadrille Ecosse]

In the process of repairing the crack under the wine mount I've opened up the main rail and can see what's going on inside.

The crack at the wishbone is because the bottom of the christmass tree shaped inner reinforcement was basically unsecured to the inside of the chassis rail at the bottom. It looks like there were four spot welds applied top and bottom to fix these into place. However the welds that were supposed to fix the bottom of this one only caught the very edge and must have broken off at some point. That allowed the rail to flex and crack along the bottom of the wishbone bracket.

See the crack running just above the lower pencil line. You can also just make out the shame of the christmas tree inside where I've sanded away a bit of the paint.

Having had a look inside now the news is good. There is very little corrosion inside and no sign of any holes where the outrigger joins the main rails. All I need to do here is weld the bottom of the 'christmas tree' to the bottom rail and weld in a patch. Got some 16g steel ordered for this and it'll be good to go. Onnce everything is finished I'll get the insides sprayed with wax to keep the corrosion at bay.

The lack of corrosion is quite remarkable. The history of this car is a bit hazy as although it was bought in London in 1963 it was imported at some point before I got it as it was unregistered. So presumably it was exported early on to somewhere a bit drier although you wouldn't have thought so from the condition of the bodywork! Anyway, very fortunate.

You can just make out the location of the crack at the outrigger, bottom left of the second photo where I've sanded away the paint forward of the weld.

As for reinforcing this section, thank you everyone for your thoughts on this and I have come up with a plan along the lines of the Vitesse 'star' plate which will wrap round onto the bottom of the main rail and outrigger. I'm only going to put this on the bottom of each side as this is the direction the loads are applied. And hopefully it will look like it was made at the makers.

Once the material has arrived I'll take some more pictures.

Colin

 

 

[Escadrille Ecosse]

The 16 gauge arrived today and I've made up the repair patch for the main rail and the two reinforcing plates. I also discovered that the 'christmas tree' inside the main rail is slightly narrower than the inside of the rail so I also made up a small plate to make up the difference. Now I just have to weld it all together. Overhead, urgh.

Photo of the various bits

[Escadrille Ecosse]

Got the repair done. Now just needs some paint and all putting back together...

 

 

[Nick Jones]

Good for another 50 years 

[Escadrille Ecosse]

[JohnD]

Nicely (=exactly) done!

[Escadrille Ecosse]

Cheers John