New Roll Bar -

[Davemk1]

Time to start ticking off "winter car work" boxes and one of the first is to build a taller and lighter roll bar for my Westfield. I had the standard three piece bar (a bit taller and with the two rearward struts with rose joints at the end.....is it called the "RAC" bar maybe?) and I wanted something taller to keep my race sanctioning body happy while at the same time taking some weight off.

 

I built this using 1.500" x .120" DOM tube and it's fillet brazed together. It's 2 3/4" taller that the old bar and 9 lbs (4.1kg) lighter. Now it sits far enough above my helmet that no inspectors are going to give me a hard time. I'm 6'4" so it's hard to find a bar that's tall enough and that doesn't add weight.

 

Tomorrow it goes off to the powder coater to get some color on it.

 

Next it needs some rubber fuel hoses replaced....less fun that bending tube and brazing it together but no less important.

 

Dave

[Ian Kinder (Bagpuss) - Joint Peak District AO]

Very purposeful 

[NigelO]

That looks to be very well made and very neat

 

However, I'm slightly surprised that you've brazed - I would have thought that for motorsport use, it would require welding. Isn't welding significantly stronger than brazing?

[AdamR]

Beautiful work, I like that a lot!

 

Dave is the man in the know, but as far as I'm aware they're similar strength - or perhaps brazing is stronger on certain materials due to reduced heat affected zone.

 

Also welding is often used 'in industry' for speed reasons. There's a lot of work in tidying up brazing, whereas a half decent MIG or TIG weld looks just fine with some powder coat over it.

[Dave Eastwood (Gadgetman) - Club Chairman]

The strength thing seems to be a really contentious one; the only conclusions I can draw from studying its real world application, (as opposed to what was taught in materials science sections at Uni), is that a Mig/Tig joint seems to be reasonably consistently strong over a narrower failure range, (unless done to an appalling low standard, whereas a brazed joint can have much more of a failure range, depending on the quality of workmanship in the joint.

 

The one thing I know in this instance, (following Dave and his world renowned cycle frames on FB), is that a brazed joint by Dave is likely to be the equal of any, and not something I'd even think about, just like I wouldn't on a brazed Caterham of Ariel Atom chassis from Arch.

[Jim RS]

We used soft silver brazing in re-tipping diamond drill bits and if correctly done the supporting steel would fail first before the braze. Weakness in brazing comes with high heat applications and reduced resistance to rigidity so I would have thought this is ok, but it is up to the rules to decide.

[Dave Eastwood (Gadgetman) - Club Chairman]

Yep, it appears, (empirically), brazed joints that do fail, don't tend to fail at the fillet/joint, but an inch or two out where the heat affected zone meets the normal steel.

[Davemk1]

Good morning -

 

The question of which is stronger, welds or fillets, is an old one. Some will say that a welded joint is stronger than a fillet (assuming both are done properly) and this is hard to argue....but....one needs to look at potential failure of the completed structure and not just the joints and this is where fillets come out on top. The reason is, as stated above, the heat affected zone will typically cause a welded structure to fail adjacent to the joint.

 

One needs to remember that a welded joint requires that the base material be heated to its melting point.....which in the case of steel is typically about 3500°F. This lowers the strength of the base material in a big way. If one does either ultimate strength or fatigue testing of a welded assembly it will typically cause the tube to fail adjacent to the weld in the dreaded heat affected zone (HAZ).

 

Brazing on the other hand does not require that the base material be brought to its melting point. Instead it only needs to be brought up to the melting point of the filler being used. In my case here that is about 1750°F. This typically does not have a negative affect on the strength of the tube so when testing an assembly one can expect it to out perform welded joints in both strength and fatigue resistance.

 

There is an additional issue - the temperature gradient of a welded joint is very short. The weld itself got up to 3500° but just a short distance from the weld it will be much cooler. This results in a stress riser of sorts in the steep gradient area adjacent to the weld. The brazed joint on the other hand isn't as hot but since it took much longer to form the joint the heat runs further down the tube away from the joint. This makes for a long gradient and much less of a stress riser.

 

So why aren't all joints brazed instead of welded? The simple answer is time. A welded joint takes very little time compared to a brazed joint and when it's done it's done. A brazed joint requires more cleaning and prep and then one needs to remove the flux from it when done. It takes easily 3-4 times longer and time is money.

 

But I have time and I'm a better brazer than I am a welder so it's my joint of choice. Fun stuff.

 

Dave

[Dave Eastwood (Gadgetman) - Club Chairman]

And probably too modest! Dave's photos of his hand made bike frames have the braze equivalent of the sort of (tig) weld porn that many of us petrol heads drool over. (gear Heads for those that side of the pond!!)

 

The heat element does have other benefits on more complex structures too, like less heat distortion meaning less heavy jigs needed and potentially less stress built into structures.

[Davemk1]

A quick update - I got the bar back from the powder coater and bolted it in place today and figured I share a few photos of it now that it's ready to use.

 

Life is too short to not use some fun colors!

 

dave