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Arguments For "the Rear Squats Under Acceleration"


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#1 Rishi

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Posted 16 November 2012 - 01:27 AM

I've recently been in a "discussion" with some friends about what actually happens to the rear under acceleration. It started like this:

A discussion about trail braking included this line:

On shaft-drive bikes it's particularly usefull as the arse-end of shafties rise on acceleration, rather than squatting.



So I replied, as per the advice in the Twist DVD:

Chain drive bikes also rise on acceleration, the same as shaft drive.



Well, that created a furore. Here were some of the responses:

Only some do. Most don't. It depends on the angle of the chain pull


Still believing that the world is flat Rishi?


Stand beside a chain-drive bike.
Imagine a triangle formed by three points- the top of both sprockets , and the swingarm-pivot.
As you apply power, the top line (the chain) tries to shorten by pulling the top of the rear sprocket towards the top of the front sprocket.
Most of this results in the wheel turning, but some will shorten that line causing the swingarm to go up, and the back of the bike to squat.

All chain-drive bikes with suspension will do this.
Trailbikes try to minimize it by running the chain close (it not on) the swingarm.
BMW/Husky try to minimize it with their weird co-axial swingarm-pivot/counter-shaft.


But it is always there.


Like much of what is taught in Code's stuff, it is simplistic. And not necessarily correct.

The issue of rear squat has been explained by a number of people, including Tony Foale. Chain pull is just one of the factors. The location of the COG of the bike also comes into it, as does the position of the rider and his impact on the COG. Suspension settings also come into play.

Much of it has to do not only with chain pull, but also the correlation between the rear contact patch and the swingarm pivot, and how that relates to the COG. Therefore wheelbase and swingarm length also play a factor. As does rider weight.

Watch a video of a bike on a dyno (any bike) and the rear will rise under acceleration. That is because it is a static test and some of the real world forces, such as weight transference and rider position are not at play. Put the same bike on the road and it will most likely squat.

There is a reason Ohlins invented the Rebound Separator Valve, and it was to reduce the impact of rebound damping when driving out of a corner, or in other words, to help stop the rear squatting under power, which makes the bike run wide as the front end geometry is lengthened. They would not have invented it if it wasn't necessary.

Terry Hay fitted the valve to the rear shock of my TRX when he overhauled the suspension to help limit the squat under power. If you want to argue with one of the best known suspension experts in the country, then go right ahead.

If you look at bike racers, they will get their weight as far forward as possible under power when exiting a corner. This is not just to reduce the bike's want to lift the front wheel, but to try and compress the front (and stop the rear squatting) so the bike still turns well and holds its line.


Using any sort of static test for this situation is meaningless.


I know Code has done a video that supports his assertion, but my point is simply that rider position and suspension setup have a major influence, so the assertion that all bikes will rise at the rear under power is simply wrong. Some will, but most chain driven bikes will squat, and not necessarily simply because they are chain driven.

For example, my Aprilia hardly squatted at the rear at all under power. In fact if it did, it wasn't even noticeable. It was even hard to wheelie as it had so much weight over the front. Consequently the weight transfer under power was minimised and the rear didn't really squat that much. This was all due in unequal parts to the suspension setup, my body position on the bike and the geometry of the bike. The TRX squatted considerably, but much less so after Terry Hay installed the Rebound Separator Valve. The new Ninja1000 squats more than all of them. Respringing the rear end will help it, but it is a bike that will always squat, even with a new shock, purely due to its weight distribution and the riding position which limits where I can place my weight.

Truth be told, I've never ridden a chain drive bike that noticeably raised its rear end under power, and I've ridden a LOT of bikes in the last 20-plus years. Some of that will be to do with my weight, which is higher than the average rider, and the effect it has on the bikes COG (namely raising it and moving it rearwards).


Suspension engineers in race teams spend a lot of time working with rebound settings to limit the bike wanting to run wide under power, and it does this because the rear end is squatting. If Code is right in all cases, then they are all obviously wrong?




It is beyond my knowledge to refute or judge any of these claims. And with a wealth of mis-information on the web I won't be using Google to answer the question. Can any of the coaches or engineers comment on the claims above?

#2 Hotfoot

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Posted 16 November 2012 - 03:28 AM

A bad setup, such as too-soft rear suspension, especially if it has a lot of rebound damping that can cause it to 'pack' down, can result in a bike that squats and wants to run wide. Weird geometry could cause it, too - that "triangle" mentioned above is a real factor in what happens and a bad swing arm angle could cause squatting. Per your "friend's" comments above, if rider weight was way heavier than what the bike was set up for, the rear spring could be overweighted and I would classify that as a "bad setup".

The CSS material is intended to educate riders who have the misconception (as many do) that the rear suspension will compress under acceleration on a sportbike and that is generally not true. Sportbikes are built with anti squat properties and generally rise (both ends) under acceleration, increasing ground clearance - which is the part that is important to know for throttle control in a corner!

There may be a perception issue, also - most people don't really notice the feeling of the rear end rising, the writer above may think the bike is squatting when it is really rising - telemetry would be the way to find out for sure! The bike running wide does not necessarily prove that it is squatting - other things can cause that, like tension in the rider's arms.

Tony Foale or Dave Moss are good resources to explore all the nuances of geometry and suspension, there is a lot to learn and an apparent infinity of adjustments that can be made to change the handling of bikes.

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#3 Rishi

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Posted 16 November 2012 - 06:22 AM

Thanks Hotfoot! I will give your ideas a go on my local forum and see what response it gets.

#4 ktk_ace

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Posted 16 November 2012 - 07:08 AM

there are always exceptions to Mr Keith code's teachings imho...

BUT

If you can work out where His teachings work (90-95%) and where it doesn't (exceptions, ard 5-10% of roads in my case) by sampling...

its still going to be the most useful tool in your riding tech toolbox

#5 Brad VanHorn

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Posted 16 November 2012 - 01:49 PM

The CSS material is intended to educate riders who have the misconception (as many do) that the rear suspension will compress under acceleration on a sportbike and that is generally not true. Sportbikes are built with anti squat properties and generally rise (both ends) under acceleration, increasing ground clearance - which is the part that is important to know for throttle control in a corner!

This is point on increasing ground clearance is very observable. When we do our ARTD/AMOS training (both CSS developed programs) for the Marines, they ride a small road course we setup for them. At the beginning of the day you can see riders who have reduced ground clearance in large part due to being off the gas. Off the gas you can see the bike sagging. Once we work with them and get them using good throttle control you can clearly see the bike lift instead. It is a very noticeable difference in the amount of ground clearance they have. The most dramatic examples usually are the big cruiser guys who begin the day dragging just about everything through the corners. By the end of the day many of them are going faster but without dragging anything at all.

I need to watch some of my recorded races. My recollection is you do not see squatting from pro race bikes (WBSK, MotoGP, etc.) I do recall the front end often gets very light, which I suppose for the unitiated could be construed as the back end squatting. I need to watch for this specifically though, and in the analysis it may/may not be truly relevant. At least it'll be fun watching the races again :)



#6 Eirik

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Posted 16 November 2012 - 09:33 PM

If the rear axle sits higher than the swingarm pivot, you get pro-squat under acceleration and anti-squat under trailing throttle - just the opposite of a conventional shaft drive set-up. If the rear axle sits below the swingarm pivot, you get anti-squat under acceleration and pro-squat under deceleration.

How big these effects are depend upon the angle of the swingarm and by the amount of power delivered to the chain. So if you have an anti-squat configuration, the rear of the bike may still squat under acceleration if the weight pushing down is greater than the lifting force. Pro racers will tune the anti-squat effect to a point where the bike behaves in a manner they prefer, one that suits their way of riding. You can see that they all use a pretty acute angle of the swingarm that gives them substantial anti-squat effect.

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#7 Lnewqban

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Posted 18 November 2012 - 03:32 PM

It is beyond my knowledge to refute or judge any of these claims. And with a wealth of mis-information on the web I won't be using Google to answer the question. Can any of the coaches or engineers comment on the claims above?


I agree that it is as difficult to understand how all the forces act as it is to explain it.

The posters of your forum are correct regarding the description of the internal forces that involve the chain, sprockets and swing-arm.

However, they fail about quantifying those forces, which are not as strong to make the pivot of the swing-arm sink toward the road as they believe (mainly due to lever lengths).

The external forces and related levers are greater than those internal forces, resulting in that pivot to move up from the road, simultaneously with the steering column.

As a matter of fact, the second rises up at a higher rate than the first, pitching the nose of the bike up, which could be confused with a rear squat down.

Those external forces are generated by the forward acceleration of the combined center of gravity of the bike-biker plus the rotational inertia of the rear tire.

Exactly the opposite things happen during deceleration (braking): both forces act on the swing's arm pivot, the external forces win again, the pivot is pushed down, and the rear of the bike squats (even if clutch is in and there is no tension on the chain).

If our motorcycle were powered by an electric motor directly coupled to the rear wheel's axis (no chain or belt drive), it would be a flywheel hanging at the free end of a pendulum.

If we accelerate the motor, the end of the pendulum (where the wheel is) moves from the vertical in one direction and remains there as long as the acceleration is happening.

If we decelerate the motor, the end of the pendulum (where the wheel is) moves from the vertical in the opposite direction and remains there as long as the deceleration is happening.

If torque is lever times force, a lever (the swingarm) and a torque (the angular acceleration/deceleration or angular impulse of the wheel) generates a force (on the pivot).

Have you ever wondered why the A-shape frame of the Ferris wheels?

When the huge wheel starts or stops rotating (accelerates or decelerates), huge forces parallel to the ground appear.

This video shows an application of that principle (skip to 1:40):

http://www.youtube.com/watch?v=j9RDpmamRRQ

Although the force that is explained above or the torques applied to the swingarm are not mentioned in this article, it is great the way it explains how the remaining forces work:

http://www.sportride...ction_geometry/

Code's stuff is nor simplistic neither incorrect, it is simply a description of reality.

He/she, who has been riding long enough, knows from feeling them, that these things happen this way.

#8 Dylan Code

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Posted 21 November 2012 - 03:06 AM

Here's a picture of a rider accelerating. We can see his forks extended. You can look at the rear and draw your own conclusions. This Ducati has about 260 rear wheel horsepower.

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#9 mugget

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Posted 30 November 2012 - 06:27 AM

Haha Rishi, I saw that furore as well...

I think the most simple thing to do is to tell the doubting person to go out to their bike, place the front wheel against a solid object (or hold the front brake on I suppose), then gently apply throttle and watch what the rear of the bike does

No mystery, problem solved.

Can't remember where I learnt that, but it makes sense to me!

#10 Raakile

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Posted 14 March 2013 - 07:20 AM

Haha Rishi, I saw that furore as well...

I think the most simple thing to do is to tell the doubting person to go out to their bike, place the front wheel against a solid object (or hold the front brake on I suppose), then gently apply throttle and watch what the rear of the bike does

No mystery, problem solved.

Can't remember where I learnt that, but it makes sense to me!

Much easier to ask them to watch this :) http://www.youtube.com/watch?v=DVYgUGGaH5w&feature=player_embedded

#11 Cobie Fair

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Posted 14 March 2013 - 11:22 PM

One could see this easily at the track watching bikes change gears on short straights, like between Turn 5 and 6 at Laguna.

#12 Kruizen

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Posted 24 March 2013 - 05:31 PM

So if under hard acceleration the rear rises, I can make the assumption that weight transfers forward?


-Eric

#13 Eirik

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Posted 24 March 2013 - 06:47 PM

No. The weight will go rearwards under acceleration, but the lifting forces are greater than the weight transfer, giving anti-squat. You can still wheelie with a rising rear.

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#14 Kruizen

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Posted 24 March 2013 - 07:36 PM

just rewatched the section in twist 2 about suspension and acceleration. I was associating rear tire slipping under hard acceleration with weight transfer forward, whereas it was because of the stiffened rear suspension-due to rise-that caused the slipping.
-Eric




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