- [Instructor] The next section of our frame to build
is the rear triangle.
And this is a little bit more complicated than the front
because it actually consists of two triangles
that are mirror images of each other.
In terms of real life parts,
the sides of these triangles are the chain stays
and the seat stays.
And these come out from the front triangle
and hold the rear wheel in place.
These parts need to be wide enough
to hold the hub of your rear wheel
and have clearance for the wheel to turn.
But they also need to be narrow enough
that they can come in and join the rest of the frame here.
So the tubes are often a smaller diameter
and they also usually have a bend or curve in them.
To visualize and arrange these
we're going to add another layout sketch.
And this might seem a little bit confusing at this stage
but it will make more sense
when we actually make the rear frame parts.
If you're starting fresh from this point
and using the exercise files,
then unzip the chapter three files to a new folder
and open the assembly file for chapter three.
And that's the same as this assembly
that I've got open here.
So now we're in our assembly
and we know from the first layout sketch
where the rear axle we'll set.
The first thing that we need to decide
is the width of the rear frame.
This is how far apart the two rear dropouts
are going to be, the parts that hold the wheel axle.
So there's a number of standard sizes for this.
I'm going to go with 142 millimeters.
It's fairly large, but it's quite common on touring bikes.
To add this dimension to the assembly
first we need a new plane.
So let's go to assembly,
reference geometry plane.
For the first reference
let's click the end point of that wheel base line.
So where the rear axle will be.
For the second one let's click the other end
of the chain stay line.
So down here.
And then for the third reference
let's just expand this little feature tree
and let's choose the right plane of the assembly.
Then we should be perpendicular here.
It should automatically select.
So you should be adding a plane that looks like this.
Press Okay.
And let's call that chain, stay plane.
Now let's start a sketch on that new plane.
And select a mid-point tool.
You can find this midpoint line
on the dropdown next to the line tool.
So there is midpoint line.
Now we want to draw a line starting at the end point
of the layout sketch here.
So start there, go out to the sides, like this
and draw a midpoint line something like that.
If you go to a top view, it should look like this
going out to both sides.
We can then set the size of this as 142 millimeters.
Then we can select that line
and we can make it for construction here on the left.
So this line represents the width of the rear dropouts.
Next, we have to decide what kind of tire clearance we want.
We'll stay in this sketch
and we'll select a center rectangle.
So get the rectangle tool and then choose sensor rectangle,
and then go normal two.
And let's draw a sensor rectangle
from the sensor point of that line, going out,
something like this.
And then when you've made it choose for construction
here on the left.
This rectangle is going to represent the maximum tire size
that we can have without the tire snagging on the frame.
So we can set this to anything we want really.
If you're making a mountain bike
with really knobbly wide tires, then you want more clearance
than if you're making a road bike with skinny tires.
Tire sizing of wheel sizing
could be an entire course in itself,
but for our model I'm going to make this tire 2.5 inches wide.
So I'm going to set 2.5 inches here.
So this is pretty wide.
It's probably wider than most mountain bike tires.
And I'm going to set at 720 millimeters long.
So many wheels are 700 millimeters
and this gives us a bit more space
so we have a bit more clearance of the frame.
Now we can exit that sketch and we can call it,
change stay layout.
Now we need to do a similar thing for the seats stay.
So firstly, let's add a new plane
go to assembly, reference geometry plane.
Then for the first reference
choose the end point of the line.
For the second reference choose the other end point.
And then for the third reference
as before, let's choose the right plane.
And make sure you're on perpendicular.
So we should be adding a new plane like this.
Let's call this one seat stay plane
and then start a sketch on that plane.
Then you can go normal two,
and let's draw another rectangle similar to that first one.
So get the rectangle tool, get a sensor rectangle.
Start from the midpoint of that line
and it's going to be the same size.
Again we'll make it for construction.
So it should be 2.5 inches wide.
And it should be 720 millimeters long.
Then you can exit this sketch.
And let's call this one seat stay layout.
So at this stage, this might not make that much sense
but maybe you can visualize what we're doing here.
These two rectangles are the maximum extent of the tires.
So when we make our rear triangle,
the chips have to go from here to here,
but avoiding this rectangular section.
And then the one up here has to go from here
to the frame here but it has to stay out of this rectangle.
If you're not completely following at this stage
don't worry too much,
'cause this will make a lot more sense
when we make the chains they part in the next video.
But for now you can save your assembly.