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Rake and trail change with braking forces, there is no way known that the steering head angle changes, physically impossible unless you have offset bearing cones as some race bikes have .
 

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Trail braking cannot steepen the steering head, the steering head is a fixed member of the frame !
Apply brakes = front end down, rear end up = steeper steering head.

You are talking about the angle relative to some fixed point on the frame (or relative to vertical when the bike is sitting still with sag at both ends at some predetermined setting, everyone else is talking about relative to the ground (or to vertical, if you prefer) under dynamic conditions.
 
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Measure all you like, the only thing that matters is what happens when you’re riding down the road. Measuring a static motorcycle is a reference for comparison. Raising the back, or lowering the front will change the head angle, rake , and trail. If you don’t believe it, go measure for yourself. Take one front spring out and sit on the bike to simulate weight transfer from using the front brakes, angle will get steeper. Visual proof , if you don’t want to do the math.
 

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The horizontal line/ground is the base of the right triangle, the vertical side (90 degrees to the base) is used for for all measurements, and the wheels are on the ground. Even though the steering head angle is fixed, rack and trail can change. Trial is the length of the side opposite the rack angle. The picture shows it best

This comes from thompsonchoppers.com/rake-and-trail-calculator

Rake and Trail Explained:
We know you were day dreaming about sex when the triangles were floating around the blackboard and you were doodling motorcycles while the teacher was explaining geometry and tangents. Now all these years later when you finally got the money to build your dream bike, all you can do is thumb through pictures of other people’s projects, searching for that “look” blissfully unaware of what all that trigonometry was for. We’re going to show you how to do the calculations. It might explain why shopping carts weren’t meant to go 200 mph.
Your front suspension geometry is defined by the following six variables which are:
OFFSET: Centerline of the top steering neck to the centerline of the top of the fork tubes.
RAKE: The angle in degrees of the steering neck from vertical.
FORK LENGTH: The distance between the top of the fork tubes to the centerline of the axle.
DIAMETER: The diameter of the front tire.
TRAIL: The distance defined by the vertical line from axle to ground and the intersection of centerline of the steering neck and ground.
RAKED TRIPLE TREES: In order to bring trail figures back into line, triple trees with raked steering stems can be used. Usually adjustable in 3, 5, 7 degrees of rake.
HOW TO MEASURE CORRECT TRAIL
Raise the bike to an upright position, using a tape measure, hold the tape straight down from the front axle to the floor. Put a mark on the floor at that point. Then place the tape parallel to the steering neck, following the angle of the steering neck all the way up to the floor. Put a mark here also. Now measure the distance between the two marks and you have your trail measurement. It should read between 2 and 4 inches. Note: If your bike is equipped with a rear suspension, have someone sit on the seat when you make the measurements to simulate your actual riding condition.
TOO LITTLE OR NEGATIVE TRAIL
With too little or negative trail (steering axle mark behind the front axle mark), the bike will handle with unbelievable ease at low speeds, but will be completely out of balance at high speed. It will easily develop a fatal high-speed wobble. EXTREMELY DANGEROUS!
NORMAL TRAIL
Normal trail is somewhere between 3 and 6 inches. The bike will handle easily at both high and low speeds. Flowing smoothly through curves without swaying or wobbling. If you use a very fat rear tire, you should keep the trail as close to 4 inches as possible.
TOO MUCH TRAIL
If the trail is more than 6 inches the bike will handle sluggishly at high speeds. It will seem almost too steady. You will have trouble balancing the bike at lower speeds or on winding roads. It will feel generally sluggish and clumsy.

To decrease trail you can:
-decrease the neck rake
-increase the triple tree rake
-decrease the size of the front wheel
-install shorter fork tubes
-raise the bike or trike in the rear only
-lower the bike or trike in the front only
the last two also happen under braking

sources, plus your trigonometry book
https://alexkpedersen.wordpress.com/rake-and-trail-explained/
http://www.thompsonchoppers.com/rake-and-trail-calculator/
https://www.cycleworld.com/2015/05/...icks-understanding-motorcycle-rake-and-trail/
https://www.rbracing-rsr.com/rakeandtrail.html
 

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Yeh well you better go back and repeat then
I've heard a couple of frame guys talk about "headstock angle" or something like that. It IS a frame geometry parameter that is established as the frame is constructed. The frame builders are usually measuring it relative to some reference in a frame jig. It may or may not be related to how they expect the bike to sit when on the road but is not the same as steering head angle when a complete bike is on its wheels with a rider aboard.
 

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The steering head is a F$%* fixed part of the frame and cannot change. I get what all you guys are saying bit I am at a loss to see how you cannot see rake and trail will definrately change but steering head is a fixed part of frame
 

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Yep it is the weekend and that is ride time, oh and I might try some trail braking, hang on I been doing that for forty years
 

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The steering head is a F$%* fixed part of the frame and cannot change. I get what all you guys are saying bit I am at a loss to see how you cannot see rake and trail will definetly change but steering head is a fixed part of frame
I thought I said that each time I posted that the steering head is a fixed part of the frame. The discussion I thought the was about the dynamic change of rake and trail.
Of course this whole discussion hacked the trail braking :surprise:.

The question now is do I ride with my peers (old farts) or my grandson and his friends this weekend. Who will do more trail braking - ??
 

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Wait a minute, the steering head is welded to the frame, like's it part of the frame? It doesn't float freely? So the forks are physically connected to the rest of the bike? Are you sure @Roasted or are you just messing with us? I'll pull my tank off and check for myself, but I think maybe a prank is being pulled here :suspicious
 

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I might try some trail braking, hang on I been doing that for forty years
So you been doing it but don't know how it works. Well that's cool. Most of us don't know how a computer works, but we know it does work, and we know to use one, and someone smarter than us knows how to make it. That's all that matters.

I've been riding for 40+ years and never trail braked as a deliberate strategy until the last couple of years. It was always brake in to a corner and accelerate out with no front brake in the corner itself. But then in the twisties you get the situation where a corner tightens on you unexpectedly or similar and you get the urge to jump on the front brake. This is bad of course and so taking in a bit of advice from actual motorcycle trainers on how to deal with that scenario seems like a good idea.
 

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I give up , you guys can ride your bikes with steering head angles that miraculously change in the welded position from the factory.:x
 

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I know how and why it works , done many track days. Just cannot understand how everybody thinks their steering head angle can change from grabbing front brake. Yes the whole front end dives and puts weight on the front tyre, but that does not change the factory welded angle. The whole geometry changes not the head angle.
 

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I know how and why it works , done many track days. Just cannot understand how everybody thinks their steering head angle can change from grabbing front brake. Yes the whole front end dives and puts weight on the front tyre, but that does not change the factory welded angle. The whole geometry changes not the head angle.
No one thinks the head angle changes and you know it. You are trolling.
 
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