Bent frame quizz

[John Kat]

I've been fighting for a long time to get my GS 1000 ST to handle correctly...
I had the frame straightened but I was still unhappy with the behavior
In the end, I decided to use a small laser beam to determine if the wheels had really been aligned?
The procedure is a remake of the string method but far easier and faster
It consists of shining the laser beam parallel to the rear wheel towards the front in order to set a reference mark on the ground at the level of the front tire.
You repeat the procedure for both sides of the rear wheel.
You then measure the distance between both marks to the front tire with the tire parallel to the marks.
Guess what? My front wheel is offset by 10 mm to the left of the bike's centerline
Now the quizz
To what side of the bike do I need to put my weight for the bike to go straight?
In other words do I need to steer the bike to the right or the left to go straight?
Second question: justify your answer
Third question: how come an offset of only 10 mm on a wheelbase of 1500 mm can be felt that much?

 

[Agemax]

i wouldnt know the answer on how to steer straight but i would take a whole load of measurements from many different points and see if i can determine exactly what point of the frame is bent and get it rectified. or buy a straight frame!

 

[posplayr]

I actually think the string method is more accurate than a laser, because if the beam spread at 8-10 foot exceeds that of a string. I posted an analysis of this s few years back.
Anyway if the frame was "straightened" then what makes you think it is still bent? The front wheel can be out if you did not set the rear axle adjuster correctly.
Given the great variety in opinions on counter steering I full expect a great variety of answers to you poll.

 

[John Kat]

I actually think the string method is more accurate than a laser, because if the beam spread at 8-10 foot exceeds that of a string. I posted an analysis of this s few years back.
Anyway if the frame was "straightened" then what makes you think it is still bent? The front wheel can be out if you did not set the rear axle adjuster correctly.
Given the great variety in opinions on counter steering I full expect a great variety of answers to you poll.

Given that my rear wheel is set correctly then the frame must be bent for the front wheel to be offset by 10 mm.
It's because I wanted to correlate this measurement with the riding impression that I developped the theory to back it up
It might also be usefull for all GSR members to immediately be able to state how the front wheel is offset just by riding the bike?

 

[Crankthat]

Would it be the upper and lower sections of the triple tree are tweaked?
Or possibly one of the frame legs around the headers resulting in the stem being angled?
Looking at the welds in the older bikes I doubt many were spot on to begin with.
For someone to straighten one of these frames would be very difficult for it to be spot on and for it to remain that way upon reassembly.
You would need to add a couple of welds to pull the metal into place as the heat would warp it in or out.
Add some shims to keep your wrenching from pulling and pushing?
I am not a engineer, this is just speculation!

Upon more thought, could you bubble level various bolts to come to a conclusion?

 

[posplayr]

Given that my rear wheel is set correctly then the frame must be bent for the front wheel to be offset by 10 mm.
It's because I wanted to correlate this measurement with the riding impression that I developped the theory to back it up
It might also be usefull for all GSR members to immediately be able to state how the front wheel is offset just by riding the bike?

So who straightened you frame a blacksmith?
My point is if you used anybody competent then I would trust their equipment more than a $10 Chinese laser.
You mention the rear is done proper but that is really not a technical term so how was it done?

 

[Agemax]

probably a silly question but are the spacers on the front wheel axle fitted in the correct order so the wheel isn't off set to one side?

also, if the frame is bent it is almost impossible to bend it with out the headstock twisting at an angle.
with the front wheel dead straight, put a straight edge on the side of the wheel and put a decent accurate spirit level against it. if the frame is bent then the front wheel will not be perfectly vertical, (90 degrees to the floor)

obviously you will need to be on a dead level surface. you could always do the same test on the rear wheel to make sure that is vertical too

 

[John Kat]

So who straightened you frame a blacksmith?
My point is if you used anybody competent then I would trust their equipment more than a $10 Chinese laser.
You mention the rear is done proper but that is really not a technical term so how was it done?

I suppose even a pro can make a mistake
The rear swingarm is from a bike that ran straight so are the fork tubes and the triple trees.
The rear wheel is aligned as per the marks on the swingarm.
If I had to correct a deviation of 10 mm at the front wheel, I would have to skew the rear wheel by over 2 mm at the axle level and the tire would touch the swingarm...
To put you on the right path, remember what makes our bikes go straight in the first place.
Then think of the consequences of having the front wheel deflected to the left

 

[John Kat]

You are on the right track
See my comment to Posplayr.

 

[John Kat]

Would it be the upper and lower sections of the triple tree are tweaked?
Or possibly one of the frame legs around the headers resulting in the stem being angled?
Looking at the welds in the older bikes I doubt many were spot on to begin with.
For someone to straighten one of these frames would be very difficult for it to be spot on and for it to remain that way upon reassembly.
You would need to add a couple of welds to pull the metal into place as the heat would warp it in or out.
Add some shims to keep your wrenching from pulling and pushing?
I am not a engineer, this is just speculation!

Upon more thought, could you bubble level various bolts to come to a conclusion?

It definitely coming from the stem that is no longer vertical but which way will the bike tend to turn?

 

[Agemax]

so if the front wheel is off vertical with the bottom being 10mm to the left, then the wheel would be lent over to the right, in effect the bike is trying to turn right all the time.
so to counter it you would have to lean to the left?

 

[posplayr]

I suppose even a pro can make a mistake
The rear swingarm is from a bike that ran straight so are the fork tubes and the triple trees.
1.) The rear wheel is aligned as per the marks on the swingarm.
If I had to correct a deviation of 10 mm at the front wheel, I would have to skew the rear wheel by over 2 mm at the axle level and the tire would touch the swingarm...
To put you on the right path, remember what makes our bikes go straight in the first place.
Then think of the consequences of having the front wheel deflected to the left

It is often quoted that Rear wheel adjustment marks are "notoriously in error" , I have no statistics; I simply aligned rear wheel with front . Apparently drag races align the rear wheel by measuring the axle distance to the pivot bolts.

As you apparently realize there is a 5:1 multiplication factor for the axle mark uncertainty and movement in the front wheel so being off by 1 mm will be 5 mm at the front wheel.

I would look for a way to check your rear axle is square in the frame (something other than the swing arm Marks)as that will wholly screw up your alignment with the front wheel if not.

 

[GregT]

There are several questions here...and i build and repair frames...

Agemax has asked re wheels being vertical, this is the easiest one to check - just set the rear vertical with a spirit level and go check the front.It's also the most common evidence of damage as when a bike is dropped there's a fair bit of weight swinging on the end of the forks which are going to be fully extended thus having the most leverage on the steering head.

Alignment...when these things came off the line at the factory,the chain line was pretty well parallel to the frame centerline. The wheels may not have been in alignment - can be anything up to 5mm out - but they did try to get the chain line right. My quick check is to pull the chain guard so you can see the chain and check it's parallel to the wheel.

On all the racebikes I build I set up the frame and wheels on the bench, align them as best I can and trim the adjuster bolts so the they are equal lengths when the wheel is aligned. Take a small steel rule to the races with you and you can realign quick and easy. And yes, I never trust to standard alignment marks...

 

[John Kat]

OK guys, I understand everything you are saying but I've tested various front forks and two different swingarms with different wheels and the bike still pulls to one side.
There is no escape to the fact the frame is bent
Now to the more theoretical question I was asking: with the front wheel to the left of the centerline will the bike steer right or left?
Agemax is the only brave man here so far guessing(?) it's to the right.
Let me give you one more hint: what is the main factor that keeps the front wheel centered when the bike is moving?
Next in what direction will the wheel go if you sit straight on the bike without touching the handlebar?
Now what happens if the steering stem is no longer in the vertical plane?

 

[Agemax]

surely if the wheel is still vertical relevant to the rear wheel, and pointing straight relevant to the rear wheel, then the bike should still travel in a straight line.
even though it is 10mm to one side, they are both pointing straight ahead.

also known as crabbing, in the car world

 

[GregT]

If the steering stem is not vertical (or to be accurate, in the same plane as the rear wheel) then the contact patch is offset and the trail is biased to one side of the theoretical centerline of the bike...

Result....with the bars apparently centralised, you'll ride around in circles....

As an aside. a couple of years ago I was going to the track and got passed by an MV. I noticed the rider was sitting on it a bit oddly so dropped back and got a good look from directly behind. From directly behind the wheels formed an X.....It was nearly new too.
What caught my eye was the amount of body english he was having to use to get it to go in a straight line.

 

[John Kat]

GregT is right about the trail.
It's the trail that makes our bikes go straight as long as the frame is not bent
If the frame is twisted i.e. when the wheels are no longer in the same plane (and appear as an X from the rear) then the front wheel will try to follow the intersection point of the steering stem axis with the ground.
If the wheel is offset to the left of the frame's centerline, the intersection point is also offset to the left and...the front wheel will try to go left and...the bike will follow making a left turn
My first conclusion after measuring the front wheel offset was identical to Agemax as when you initiate a turn, you countersteer so that the CG makes you go in the other direction.
This is off course true but not in a steady state condition.
My last question was: how come you need so much correction eitheir by "body english" or by pulling on the bars?
I believe the answer is again related to the trail.
The 10 mm offset must not be considered in conjunction with the wheelbase of the bike but in conjunction with the trail itself.
So given a trail of 110 mm an offset of 10 mm represents an angle of over 5?at the handlebar.
No wonder one can feel it...
So in summary, if your front wheel is offset to the left of the bike's centerline your bike will pull to the left

 

[John Kat]

surely if the wheel is still vertical relevant to the rear wheel, and pointing straight relevant to the rear wheel, then the bike should still travel in a straight line.
even though it is 10mm to one side, they are both pointing straight ahead.

also known as crabbing, in the car world

You are definitely right.
If both wheels are in parallel planes the bike will travel in a straight line but...the rear wheel will create it's own trace.
The handlebar will be slightly turned with respect to the frame centerline by approximately 10/1500 radians or .38? only.
I forgot to add that this time the steering will have to go to the right!
This is another proof that if you feel that you need to pull a lot on the handlebars to go straight it's because your frame is twisted in an X pattern.
Offsetting the rear wheel to catch the front offset will NOT change the problem...
Skewing the rear wheel in the swingarm will not help eitheir...
The front wheel follows the intersection point of the steering stem with the ground and the bike follows!
That's it.

 

[Nessism]

How do you know the wheel is sitting "straight" in the swingarm. You can not wheel alignment based on the hash marks on the swingarm.

What happens when you use the string method to align the rear wheel parallel to the front? What do the hash marks on the swingarm tell you? Typically, they may be slightly off which is fine. The string tells the real story.

 

[tatu]

Years ago when I was a mech for Suzuki I remember being told that there had been a run of GS1000s (not necessarily only 'S' models) with bad swinging arms that were replaced under warranty. I never saw or had to do any, just heard the story. My first 'S used to wear the front tyre unevenly, more wear on one side than the other, I never did get to the bottom of it. In those days the bike used to get ridden much harder than these days