Frame checking

T

Tigernut

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Jan 1, 2013
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Further to my 'Something's bent' post, I have removed the tank, cleaned the frame's spine tube of all crud / inuslating tape goo etc and put a straightedge on the top. It's straight from the steering head to the point where the tubes are welded each side (and the narrow tube from the steering head is welded beneath). From this point rearward, the angle changes slightly. If I put a 12" steel rule so that 6" is forward of this point and flat on the tube, the aft 6" has a gap of about 1/16" at the end of the rule.

I'm guessing the spine tube should be dead straight all the way along?
 
Tigernut said:
I'm guessing the spine tube should be dead straight all the way along?
Click to expand...

It often isn't.

Guido

Thread 'Bent backbone?'

I started a post about me not being able to get my swing arm spindle to slide into the bushings, that they were misaligned.
reaming-swing-arm-bushes-t12369.html
It showed the swing arm was bent, twisted.
The guy at the frame shop said the frame may also be bent. I am stripping it down now.
I noticed that the back bone has a slight hump to it. I checked out my other bike and it has the same slight hump in it's back bone.

If someone who has their bike torn down would check theirs with a straight edge to see if they also have the same hump in the back bone I would really appreciate it...
 
It should be straight but when made as its a fully welded frame it distorts from the heat, 1/16" is nothing to be concerned about and typical. I have a 750 frame that was in a front end collision and it has a bend in the top tube near where the 2 support tubes come in from the side and the gap is 1/4" at the front, it needs work to bring it back into spec.

Frame checking
 
Your arrow points to where the bend is on my frame. Looking at it again just now, the top tube is straight side to side. The front downtubes seem straight, as do the rear ones.

More encouragingly, I've jacked it up and put axle stands under the footrests. With the centreststand out of the equation, the rear wheel is sitting nice and straight - the tilt I mentioned in my other thread has gone.

The centrestand pivots are bad - must be at least 1/4" slop on the right-hand one.

I had wondered if the top tube could have become bent by welding heat distortion - there's a lot of welded metal at that point. Maybe all is not lost after all? And my 1/16" guesstimate is probably an overestimate - looks like a lot of light when you're hoping for none.
 
If you look at that top tube it has 2 triangles of tubes connected to it, the bend is where those 2 triangles meet so here is the weakest point.

Frame checking


Its also where any weld heat distortion will be concentrated.

So the frame starts off with a slight bend here and any front end crash will increase the bend.
 
Tigernut said:
Further to my 'Something's bent' post, I have removed the tank, cleaned the frame's spine tube of all crud / inuslating tape goo etc and put a straightedge on the top. It's straight from the steering head to the point where the tubes are welded each side (and the narrow tube from the steering head is welded beneath). From this point rearward, the angle changes slightly. If I put a 12" steel rule so that 6" is forward of this point and flat on the tube, the aft 6" has a gap of about 1/16" at the end of the rule.

I'm guessing the spine tube should be dead straight all the way along?
Click to expand...
Not always, and most case they are due to welding. Strangely, the thin wall spine tube is not the first place I look for damage on a frame, the down tubes from headstock and the area behind the gearbox are the places to look first. Get the frame stripped prior to sending it to be straightened as many miss the area behind the gearbox where the tubes can crack. We have recently had one frame in the workshop which was damage free but twisted, it looked off from the front as the forks were not upright. As for rear loops, wish I had a fiver for the ones I have seen that are bent and damaged.
 
Madnorton said:
Not always, and most case they are due to welding. Strangely, the thin wall spine tube is not the first place I look for damage on a frame, the down tubes from headstock and the area behind the gearbox are the places to look first. Get the frame stripped prior to sending it to be straightened as many miss the area behind the gearbox where the tubes can crack. We have recently had one frame in the workshop which was damage free but twisted, it looked off from the front as the forks were not upright. As for rear loops, wish I had a fiver for the ones I have seen that are bent and damaged.
Click to expand...
Can you recommend somewhere in England for checking & straightening, if I need it? Is this something you do? I'm in Norfolk so somewhere over this side of the country would be ideal. Thanks.
 
Tigernut said:
Further to my 'Something's bent' post, I have removed the tank, cleaned the frame's spine tube of all crud / inuslating tape goo etc and put a straightedge on the top. It's straight from the steering head to the point where the tubes are welded each side (and the narrow tube from the steering head is welded beneath). From this point rearward, the angle changes slightly. If I put a 12" steel rule so that 6" is forward of this point and flat on the tube, the aft 6" has a gap of about 1/16" at the end of the rule.

I'm guessing the spine tube should be dead straight all the way along?
Click to expand...
Nope. Every weld incurs distortion. That is normal.
 
Tigernut said:
Can you recommend somewhere in England for checking & straightening, if I need it? Is this something you do? I'm in Norfolk so somewhere over this side of the country would be ideal. Thanks.
Click to expand...
Motoliner in Kent have a good reputation, but ask the local young bikers who crash a lot who straightens their frames.
 
kommando said:
Motoliner in Kent have a good reputation, but ask the local young bikers who crash a lot who straightens their frames.
Click to expand...
I've used Seastar Superbikes for T140 frames and they've been excellent. I'll ask them, but I'm a little less than hopeful because they want the engine (or at least crankcases) bolted into the frame for setting up & checking. Given the Commando's unique Isolastic system, I don't think this would be practical. If Motoliner are known to have done Commando frames, I'll use them (if I need to at all) if Seastar can't help.
 
Tigernut said:
I've used Seastar Superbikes for T140 frames and they've been excellent. I'll ask them, but I'm a little less than hopeful because they want the engine (or at least crankcases) bolted into the frame for setting up & checking. Given the Commando's unique Isolastic system, I don't think this would be practical. If Motoliner are known to have done Commando frames, I'll use them (if I need to at all) if Seastar can't help.
Click to expand...
Bear in mind how the Commando frame works…

The crank cases are required to mount the cradle. And the cradle is required to mount the swinging arm, etc.

So anyone wanting to actually check that your frame is really true, ie that the wheels are in line, needs all of the above and more…
 
There are drawings in the workshop manual, check the frame against that yourself, there is no evidence its actually bent at the moment. In your other thread you are erring on the iso's being at fault which is much more likely. When you took the centre stand away the rear wheel righted its self, that says iso's.

The clue with my bike which came as a box of bits was the missing front rim, which was a bit of a give away.
 
The best way to check a frame, might be to put it in a frame jig. Somebody in England must still be making frames. The usual trick is to place rods through all the holes and measure.
 
Matchless said:
It may be worth giving Parallel Engineering a call on 07794 637159. They specialise in Commando frames. Based in Shropshire, Google them.
Click to expand...
I would check out his history on forums first, opinionated is being kind. Also he only checks the frame, so you will pay twice if it is bent as when you then get it corrected any frame fixer will want to check himself.
 
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Matchless said:
It may be worth giving Parallel Engineering a call on 07794 637159. They specialise in Commando frames. Based in Shropshire, Google them.
Click to expand...
Just looked at their website and they look very competent. If I lived in the UK I'd definitely check them out.

About 6 years ago now I followed the World's straightest Commando article to try and correct issues with my Mk2 850.

My bike had three issues with handling. Firstly at about 80 mph the front end started to feel nervous as if it was about to become unstable. Years ago I had found the beginning of a tank slapper but never pushed it hard to find out. In contrast for several years I had raced a featherbed 500 twin and never ever felt the slightest concern about front end stability.

Secondly if you let the bike drift without your hands on the bars below 20 mph it pulled very heavily to the left such that in order to keep it in a straight line I needed to slide off the seat to the right and hang my body way to the right to keep it straight.

And finally on a roundabout for example the bike felt like it was going around in a series of flops up and down and it was basically pretty much impossible to maintain a nice steady curve.

This was all on a bike with good tyres, properly adjusted isos and as far as I could tell good suspension.

I had the frame checked for alignment on a plane table. It seemed OK.

The swinging arm was badly twisted.

Both rims were well off the centre line of the main central frame tube.

The top iso was replaced with a Dave Taylor unit.

Both front and back isos were replaced with CNW units.

New Ikon rear shocks fitted.

Front forks checked for alignment and parallel and internals serviced.

Tyres were not changed as the were pretty new. Dunlop TT100s

After that the bike became a good handler and no more head shaking.

That Parallel Engineering place looks like they cover all the issues we worked through so I think they would be a good place to try.
 
johnm said:
Just looked at their website and they look very competent. If I lived in the UK I'd definitely check them out.

About 6 years ago now I followed the World's straightest Commando article to try and correct issues with my Mk2 850.

My bike had three issues with handling. Firstly at about 80 mph the front end started to feel nervous as if it was about to become unstable. Years ago I had found the beginning of a tank slapper but never pushed it hard to find out. In contrast for several years I had raced a featherbed 500 twin and never ever felt the slightest concern about front end stability.

Secondly if you let the bike drift without your hands on the bars below 20 mph it pulled very heavily to the left such that in order to keep it in a straight line I needed to slide off the seat to the right and hang my body way to the right to keep it straight.

And finally on a roundabout for example the bike felt like it was going around in a series of flops up and down and it was basically pretty much impossible to maintain a nice steady curve.

This was all on a bike with good tyres, properly adjusted isos and as far as I could tell good suspension.

I had the frame checked for alignment on a plane table. It seemed OK.

The swinging arm was badly twisted.

Both rims were well off the centre line of the main central frame tube.

The top iso was replaced with a Dave Taylor unit.

Both front and back isos were replaced with CNW units.

New Ikon rear shocks fitted.

Front forks checked for alignment and parallel and internals serviced.

Tyres were not changed as the were pretty new. Dunlop TT100s

After that the bike became a good handler and no more head shaking.

That Parallel Engineering place looks like they cover all the issues we worked through so I think they would be a good place to try.
Click to expand...
Good luck with Simon Ratcliffe
He's been thrown off of here for trolling and abusive behaviour
 
baz said:
Good luck with Simon Ratcliffe
He's been thrown off of here for trolling and abusive behaviour
Click to expand...
Interesting. I'm down in NZ so will never go there anyway.

But that's disappointing because the description on his site aligns with what I found works.
 
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