Recently, I had the privilege of diving into the depths of this intricate relationship between braking and suspension on the "Demystifying MTB" podcast, where I had an enlightening conversation with the brilliant minds behind Brake Ace, Matt Miller and Rohan Martin.
So let's embark on a journey into the nexus of braking and suspension, where I step into the role of a curious investigator rather than a frame designer. Therefore, I wholeheartedly welcome constructive corrections from those who hold expertise in this field.
Let's start 'simple'
I'm going to start by looking at a single pivot design as this is relatively simple, I use the word 'relatively' very deliberately!
Let's bring in an image of a bike to give us something to relate to:
I've shown in green the link between the main pivot above the bottom bracket and the rear axle. The position of the main linkage has been chosen for a number of reasons including; how it affects chain length growth and therefore pedal kickback, and, the rear axle path and perhaps most importantly, the leverage curve. These are topics in their own right and I don't want to go into them, but, at the same time, they cannot be totally ignored.
I'd like to introduce you to my pizza cutter and sausage fingers in an attempt to explain my understanding of the interaction between the rear brake and the single-pivot suspension system.
This little clip tries to demonstrate this; the blade represents the wheel obviously and the handle the swingarm of the bike. The blue tack represents the brake fully on and therefore preventing the wheel from turning. At this point, the rotational force of the wheel starts to cause a rotation at the pivot rather than the axle. In my video this causes the handle to move clockwise. On your bike this causes the suspension to compress.
Again, I'm happy to be corrected but as far as I can see, this effect is impossible to prevent. It's worth saying at this point that it is not all bad either. As we brake hard our body weight shifts forwards - like when you have a bottle on the passenger seat of your car and you jam the brakes on causing it to fly off into the footwell. On the bike when our body weight shifts forwards like that it causes the bike to pitch forwards as gravity effectively tries to throw us over the bars. The dominant force counteracting that is to shift our body weight backwards in anticipation and response. If the suspension dips in a bit at the rear though it will supplement that effect. As the bottom bracket moves down our centre of gravity is slightly lower and we are slightly less likely to go over the bars. The effect of the suspension dipping into the travel as a result of braking is referred to as anti-rise.
Importantly to the rest of this blog though, there is also interaction between the tyre and the ground and the position of the wheel (and therefore brake rotor) and the frame. Again, possibly oversimplifying things but I'm bringing back the pizza cutter!
This time the video attempts to show how the wheel rotates with the suspension and therefore how it interacts with the ground. I've drawn black dots to help show this. As the swing arm moves up the wheel effectively rotates forwards almost a quarter of a turn. Again, I apply the blue tack to simulate the brake being jammed on and therefore preventing the wheel from moving relative to the frame. You see that the brake makes no difference to the wheel movement relative to the frame.
I'm not sure that this is the correct use of this value but it's a useful way of seeing it in your mind and thinking of it in practice. The anti-rise of this bike is 100% down to the effect demonstrated in the first video. There is no further interplay caused by the way the wheel moves relative to the frame as the suspension compresses.
Now things get more complicated as we try to mathematically calculate these effects. Again, I'll place a diagram for reference.
We have a few more lines to consider now. The first is questionable. The horizontal black line represents the centre of gravity of the rider. I say this is questionable because this will vary from rider to rider and is a constantly moving value as the rider moves back and forth and side to side as they ride. Let's go along with it though.
The second line we draw is a theoretical line running from the contact point of the tyre through the pivot point of the rear suspension. This is shown in green.
A third line is drawn vertically through the front axle. Shown in red. Where the red and black lines cross over is the point at which the anti-rise value is 100%
What we see on this bike is that if the rider's centre of gravity is in fact where we have predicted then the green line passes through the red line at the same point as the black line. The anti-rise value is 100%. This quantifies the principle I outlined earlier and my way of thinking is that 100% of the interaction with the brakes is the unavoidable force on the swingarm. The interaction of the wheel/rotor movement is insignificant.
It should be noted that the value will change as the bike moves through its travel. The gradient of that green line gets shallower. But, as the fork compresses the red line moves back also. Overall, if this bike compresses equally, the anti-rise stays close to 100% meaning that there is minimal interaction between the brakes and the suspension.
You would think this was pretty ideal then and there is no need to deviate from this design. The problem is that as a single pivot swing arm moves upwards it also moves the wheel forward - towards the very bump it is trying to absorb. The leverage rate is also pretty much the same throughout the travel (the bike is linear). Most modern full suspension frames prioritise the axle path and aim to have the rear wheel move directly upwards or backwards and upwards simultaneously, and a progressive leverage curve over this braking interaction. Frame designers may also aim to build in either positive or negative anti-rise for reasons we'll get into next.
More complex designs
Now things get a little more complicated, stay with me!
We're going to look at the Nukeproof Megawatt as that's the bike I used for the test I'll write about in the Part 2 of this blog and that I speak about on the Demystifying MTB podcast.
We now have multiple pivot points in the rear triangle which allows the frame designer to control the leverage curve and the axle path. The key feature to this post is the axle path. On this bike the wheel wants to move in a more 'upwards and backwards' path. Bring back the pizza slicer!
Now we see that when the wheel wants to move upwards directly it is actually rotating backwards a little in relation to the frame and the ground. The blue tack represents the brake calliper. When the brake is off the rear axle moves in a more desirable pathway as it moves up as it hits a bump rather than towards it as the single pivot did. But, if the brake is fully on there is a tug of war where the suspension movement wants the wheel to rotate backwards relative to the frame and the ground but the brake caliper does not allow it. This results in the suspension movement being restricted somewhat and therefore the suspension does not move into its travel as it would have without this interaction and its value is less than 100% (or at least by my way of thinking). The bike has an anti-rise value of less than 100%.
Is this bad? The jury is out because bike brands find ways to market these values differently. If the anti-rise value is less than 100% it is argued that the suspension stays higher in its travel than it might have and therefore remains on the top of its travel where the suspension is more supple. Most four-bar and horst link designs do this.
This can be shown with this diagram for contrast with the single pivot design:
The black horizontal is once again the rider's centre of gravity. The red a verticle line from the front axle.
This time because the rear triangle is moving on multiple linkage joints so the green line is not so simple. This time the point that the rear triangle is moving around is a virtual pivot point or instant centre. To find this point we draw lines through the pivots of the linkage. So I have a blue line from the pivot at the rear near to the axle and through the main pivot near the top of the front chain ring. I have a second blue line through the two linkages joints at the toptube and near the bottom of the shock. The point where the two lines cross over is the instant centre and I've placed a yellow spot on it. Now again we draw the green line through the instant centre and connect to the contact point of the rear tyre on the ground. As you can see the green line intersects the red line at just below the 100% anti-rise point (where the black and red lines intersect). So, at the top of its travel, this bike will want to raise its rear end slightly under braking due to the brake interaction (remember the dominant force is still to try and compress it though).
The situation is further complicated because the instant centre is not static. It changes as the bike moves through its travel. This graphic from Biketechspert on Instagram shows the movement of a bike with a similar (though not identical) linkage design.
Watch the instant centre (IC) move as the bike compresses. That causes the green line (anti-rise) to move down also. The deeper into the travel this bike goes the more the brakes will try to make it stand up in its travel.
At this point, I'll keep this addition brief but we should note that the opposite effect can also be stimulated. If the anti-rise value is over 100% then the suspension will be pulled into its travel as the brakes come on.
Optimising anti rise
On the podcast, I speak about the Forbidden Druid V2 so I'll start with that. Forbidden's frame designers seem to have given a lot of thought to optimising anti-rise for this bike. I've estimated where the virtual pivot is and shown it with a yellow spot again. Again, I've drawn on the anti-rise with a green line and by now you know that where it intersects the red line is important. The intersection of the red line is above the horizontal line so the anti-rise value is above 100%. This means that when the bike is at the top of its travel the braking effect is to cause the rear to move into its travel under braking. The logic here is that it counteracts that forward pitching caused by the rider's weight shifting forwards under braking. In addition, as the bike moves into its sag under braking the head angle slackens which is an advantage on steep sections of trail - where the brakes are likely to be on hard.
As the bike moves into its travel the instant centre moves and the anti-rise value of the bike moves through 100% and gets a value below 100%. At this point, the suspension starts to act like it did for the Megawatt where it tries to lift the bike out of its travel. So, the closer to bottom out the rear goes the more the braking is trying to pull the wheel back towards its sag point. This is shown by this graphic shared on their website:
I've added a couple of lines; The red line shows the 100% anti-rise point where braking has minimal interaction with the braking and the green line shows the recommended sag of the bike. They intersect perfectly showing that if the bike is running at its recommended sag the interaction of the brakes is minimal.
The Druid is by no means the only bike to do this. This graphic shows a 2019 Saracen Myst doing the same thing:
Hopefully, if nothing else, you can take from this that frame design is a complex process and a lot of thought has gone into designing the frame you ride. More and more brands publish their anti-rise data and it is certainly something to consider when you are looking for a new frame/bike. In addition, learning more about your braking habits is something worth doing. If you are a very rear brake-biased rider then the anti-rise value of the bike you chose is going to be quite critical. If you are front dominant then you may prioritise another feature of the frame. Or, maybe you need to change your braking habits in order to suit your bike. You certainly need to consider the anti-rise when working out optimum suspension settings.