Bent frame quizz

**Agemax** · 06-23-2013, 09:04 AM

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** · 06-23-2013, 11:20 AM

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** · 06-23-2013, 11:32 AM

Originally posted by 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.

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** · 06-23-2013, 11:58 AM

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** · 06-23-2013, 12:04 PM

Originally posted by John Kat

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** · 06-23-2013, 12:25 PM

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** · 06-23-2013, 12:34 PM

Originally posted by posplayr

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** · 06-23-2013, 12:37 PM

[QUOTE=Agemax;1886739
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)
[/QUOTE]

You are on the right track

See my comment to Posplayr.

**John Kat** · 06-23-2013, 12:40 PM

Originally posted by 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?

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

**Agemax** · 06-23-2013, 12:47 PM

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** · 06-23-2013, 12:54 PM

Originally posted by John Kat

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** · 06-23-2013, 01:24 PM

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** · 06-23-2013, 03:42 PM

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** · 06-23-2013, 04:04 PM

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

Bent frame quizz

Bent frame quizz

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