Swing arm cross brace query.

[J-max]

Disclaimer: I am a full grown adult. Any decisions I make are on me & no one else, regardless of any advice I may receive here or elsewhere. I am asking other people, off forum, for advice on the same issue.

Brief recap for those who've haven't seen my other posts.

This was the crappy cross brace on the swingarm when I got it, inc the 1/2" of filler slathered over it to hide some of the crappiest welding I've ever seen.



I removed that and replaced it with this.



Following a couple of 30-40 mile shakedown runs one of the welds on the upper, smaller cross brace has failed (circled in blue).



I'm presuming this was from a shock load twisting force caused by a pot hole (or several). Certainly isn't from high speed cornering Given I'm running a full width English axel from a Mk1 Ford Escort that would be a lot leverage.

What I'm wondering is which of these two options might be best.

Re-welding & adding fillets across the joints to increase stiffness - or

removing it entirely.

Currently I'm thinking of removing it. My reasoning is that by allowing the arm to twist slightly this will lessen any loads on the swingarm at the pivot point, whereas a stiffer swing arm will transmit more load to that area. I haven't really been able to fully examine what the welding is like at that end but assuming its sound (enough) that wide axel could put some serious loads on the pivot itself if the swingarm is too stiff.

I'd be very interested in thoughts about this.

 

[irondad01]

This might be related, when the street rod guys split the wishbone suspension on the old model T to lower it that stiffened things up. Now the axle had to flex when hitting a pothole. The stock I beam axle was able to flex so all was good. Then they started to use custom round tube axles that would not flex so some saw some cracking on suspension parts. All this to say sometimes some flex is good.

 

[LarryA]

agreeing with irondad.. there is a large crossmember under the drive shaft that is providing a similar function..thinking the top brace is maybe a driveshaft saftey loop. You could replace it with a hoop of 1/8x1.5 inch flat stock that would take the twist. my .02

 

[Sidecarbill]

Did the weld fail in that short of a distance. Or did the failure occur from material failure. If the weld broke it wasn't a good one. It will never see the same torsional loads as a car. It's lighter and only has one wheel in the front.

 

[J-max]

Thanks for the input guys.

Since posting I've been for another test ride without fixing or removing that cross brace. Handling was good & rr suspension comfortable even on the crap bits of road. I've had a much closer look at where it cracked, removed paint & flexed with a crowbar & it's looking like it wasn't a good weld rather than material failure.

I've discussed this with 3 other people & like you, everyone is of the opinion that some flexing is a good idea so I will be removing it. Will have a better idea what failed once that's done.

I originally made this upper cross member as the sole replacement for that crappy original. Then found that nice shaped tube & decided to use that further back & underneath as well. I may well fit a safety loop later.

Sound comment about this being a light vehicle that effectively has an additional pivot of the front wheel.

Thanks again everyone.

 

[J-max]

Update:

Removed offending cross brace. The failed weld wasn't ever a proper weld. Hadn't taken at all in the 1st place, hence I noticed no dif in test rides.

All I can think of is that that side arm was somehow electrically isolated from the earth clamp but then was fine for the other cross brace ? The other end was a good strong weld. I can't remember where the earth clamp was put for the welding or if it was shifted at any point. It was all welded in the same afternoon.

Anyway it's off now I don't plan one back.

pictures won't upload will try later..