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Old 25th May 2020, 03:44   #1
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Default Suspension squat - Swingarm geometry effect on motorcycles

A few days ago I'd changed the rear shocks to taller ones on my CT100B, if anyone has been following the conversations in the Offroad Riding - Tips & Advice thread you'd come to know that I've had it in mind for a while now as I was facing the concern of the front wheel lifting off the ground when launching/accelerating on upward inclines an issue I'd not faced with any other motorcycle I've owned.

A quote of my post;

Quote:
A major challenge I face with the CT100B is that at times the front end is too eager to lift off the ground when encountering rocky uphill terrain and before the motorcycle does a 12:00 I chop the throttle and lay the motorcycle down most of the time......

.....I'm contemplating raising the rear by an inch or two by going for a different set of shocks hoping it would make the motorcycle more front biased as at times even when climbing aggressive inclines on tarmac with a pillion the front tends to come off the ground.
Having confirmed beyond doubt that the issue has been sorted, I did want to know if the feeling I'd had about raising the rear was really what resulted in solving the case or if it was merely me overgearing the motorcycle(Bigger Front Sprocket), so I did spend some time and came across wonderful but scarce resources regarding the same and thought it best to share what I've learnt as I believe there is an information gap regarding how the rear suspension impacts weight transfer under acceleration.

Now it is already obvious that changing the height of your rear suspension can affect weight transfer as well as your wheelbase as your swing arm is fixed at a pivot on the chassis.

But what we do not realize is that the angle at which the the rear wheel axle sits from the swing arm pivot determines how your suspension would react when you accelerate.

Squat

This is when your motorcycle's rear axle bolt is higher than your swing arm bolt, this results in Squat as the pull from the chain results in the rear suspension compressing and making the front end go up, the same is what sends you wide when accelerating out of a corner.

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Anti-Squat

This is the opposite of Squat as the name implies, in this case your rear axle bolt should be lower than your swing arm bolt, as a result the pull from the chain results in your rear suspension expanding forcing the front end down, and ensuring that you're in control when accelerating out of a corner.

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This is something that is incorporated in sports bikes;

Suspension squat - Swingarm geometry effect on motorcycles-antisquatgeometrykawasaki.jpg

I've drawn some lines to make it more obvious, also do note the placement of the front sprocket, it is slightly above the swing arm bolt to accentuate anti-squat geometry.

So what I've done on my CT100B is change the rear suspension to one that is taller and stiffer than stock to promote anti-squat under load which would prevent the front end from becoming lighter on an already rear end biased(weight) motorcycle, this in effect has returned favorable results.

Plus we've all seen videos on social media of several instances of the CT100 popping an unsolicited wheelie with a pillion on board, a few examples;





I'm no expert by a long shot coming from a non-engineering background but from my understanding promoting anti-squat should resolve such concerns as well.

At first I did consider that this was only effective in sports bikes but a little more probing and I got to know that the same is applicable even for bicycles, in which case it does make sense for commuter motorcycles as well.

Do share your thoughts on the same!

P.S. You can find pictures of my CT100B post suspension change by following the below link:
Bajaj CT100B - Utilitarianism Redefined!

This clearly shows Anti-Squat at work, do observe the rear and front suspension on acceleration.



When on the gas, it is evident that the rear wheel gets pulled in causing the rear suspension to expand which raises the rear end of the motorcycle and in turn mashes the front end downward in the process compressing the front suspension.

Last edited by Aditya : 26th May 2020 at 07:28. Reason: As requested
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Old 25th May 2020, 17:24   #2
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Default re: Suspension squat - Swingarm geometry effect on motorcycles

Quote:
Originally Posted by ashwinprakas View Post
A few days ago I'd changed the rear shocks to taller ones on my CT100B, ...
I could not control laughing after having a look at the second motorbike video. That man has skills, he knows how to drop his wife but not his bike and himself

Coming back to your point that a longer suspension would help and I do agree with your observation that if the Rear axle point is below the Swing arm pivot point in the vertical direction then it would help in the anti squatting mechanism. The force resolution you have tried to explain is not correct though I feel.

The force direction remains the same in both cases just that there is a greater reaction provided by swing arm pivot in the second case.

You not being an engineer, still able to come up with this thought is worth appreciating.

Regards,
Amit
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Old 25th May 2020, 20:06   #3
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Default re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by amit_purohit20 View Post
The force resolution you have tried to explain is not correct though I feel.

The force direction remains the same in both cases just that there is a greater reaction provided by swing arm pivot in the second case.
The direction of pull remains the same in both cases as illustrated in both diagrams shared in my earlier post, no doubt about that.

Only difference is the net effect, as the force center(point of intersection of line from direction of pull(chain), direction of movement(wheel), direction of slant(swing-arm)), is behind the wheel in the event of Squat resulting in front wheel lift and in front of it in the event of Anti-Squat which counters front wheel lift, a third case is that even on a motorcycle with anti-squat under extreme suspension travel(compression) the force center still remains in front of the rear wheel i.e there is reduction of anti-squat but no lift of front wheel, but yes this would momentarily make the motorcycle unstable.

The resource I've got this from is 'Motorcycle Handling and Chassis Design the Art and Science' by Tony Foale, it has a diagram for better understanding(Fig 9.10), I've tried to convey the same the best I could as the reason I didn't directly quote or share the info is due to uncertainty regarding copyright.

Quote:
You not being an engineer, still able to come up with this thought is worth appreciating.
Thank you for the kind words, Google and Tor sites(if I dare say so) do help a lot as not knowing the theory to complement or critically analyze the 'butt-feel' can be limiting at times.

Last edited by ashwinprakas : 25th May 2020 at 20:15. Reason: Removing Snippet from Book
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Old 29th June 2020, 11:49   #4
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

At the time of writing this post 'suspension squat' wasn't a much discussed upon topic that I had to refer to books on motorcycle chassis design to know in detail about it, but now the same has been picked up by Ryan from Fortnine and explained in detail with respect to the new Yamaha Ténéré 700, in his usual manner of detail that sets his reviews a class apart from the rest.



Cheers,
A.P.
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Old 29th June 2020, 14:21   #5
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

Have you been able to test the bike out thoroughly in different circumstances?

Im no engineer either and I could be wrong in my understanding, but comparing the information presented here, it seems to me that your issue may have been sorted by something unrelated to suspension squat.

IIRC, your issue was the bike front lifting off even at slow speeds or low rpms.

In Fortnine's video, the guy goes over how suspension stretch\squat does not come into play at low speeds which allows the T7 to get away with a rear weight bias at standstill, allowing a desirable amount of light and nimble front end feel for slow and technical riding.

The effect seems to be proportionate to torque being put out, which is what it looks like going by the video you linked to the bike on the dyno, where the suspension only markedly stretches after a certain point of running and upon hard acceleration. It seems to remain composed under low levels of throttle.

So, if your bike no longer lifts off unexpectedly during slow speeds or low rpms, i'm guessing it would probably not involve suspension squat, but would have something to do with correcting the power delivery via oversize front sprocket or correcting the weight bias of the bike with the suspension or a combination of the both. In any case, glad that issue is sorted.

Offtopic, the T7 seems to be an amazingly well thought out motorcycle. Ever since release, people seem to be having nothing but praises for the handling of the bike despite its weight. Nice to finally get an explanation on what makes it that way. No fancy high tech wizardry, just good, grounded engineering.

Last edited by drt_rdr : 29th June 2020 at 14:28.
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Old 29th June 2020, 17:22   #6
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by drt_rdr View Post
Have you been able to test the bike out thoroughly in different circumstances?
My most obvious measure was a steep incline right next to my place at TVM, where the front would go up with a Pillion if I crack open the throttle with load even at crawl pace. This time I was able to climb up without a care in the world, which was the first thing we tried when I'd returned to pick up some stuff.

As for other changes, I was running on 15T front sprocket compared to the stock 14T but reverted to stock as the CT100B has a 4 Speed transmission which is generally a long ratio gearbox compared to 5 Speeds which are short ratio gearboxes, so a concern I was facing is that the final gear has become completely overdrive as the motorcycle quite comfortably hits top speed in 3rd gear and changing to 4th only drops revs and nothing more. Convenient on the highways at least considering FE but not so much the rest of the time.

But I'll revisit the 15T at a later stage if all goes well, as I have quite a few mods stacked up for the motorcycle including a VM24 from the older CI350's as I recently got to know that the Honda XR100's engine is of similar spec to that of the CT100B's and there are enthusiasts running of 22~24mm carb's on it, the latter being with engine mods but since the 22mm was out of scope we settled for the VM24, the carb is not mine but lent to me to be dialed in right for a friends CT100 which is currently being restored, but once dialed in right I'll be able to get an idea if there'd be any gains I can squeeze out of the 15T in final gear, if present then I'll get myself one as well.

So back to point, with only suspension changed I've reached a reasonable conclusion that the front doesn't lift off under acceleration/launching as before, period.

I say period cause there is clutch wear and tear to be computed, the earlier judders(CT100B doesn't come with Judder Spring's from Factory) have mellowed down and the clutch cable slack seems exhausted which only means one thing, but there aren't any noticeable slips so far, even when I was running on the 15T, so thought I'd change after there's noticeable slipping cause like the rest I'm anyways not going on any long rides anytime soon.

The T7 is indeed a good motorcycle, do take note of its firing order.

Regards,
A.P.
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Old 29th June 2020, 22:46   #7
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

What happens in the case of a shaft driven bike?

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Old 29th June 2020, 23:31   #8
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by Sutripta View Post
What happens in the case of a shaft driven bike?

Sutripta
The same applies to chain driven as well as shaft driven motorcycles as per 'Motorcycle Handling and Chassis Design the Art and Science' by Tony Foale.

Spending some time on Google would lead you to the book, it has everything explained in detail with diagrams.
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Old 30th June 2020, 11:38   #9
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by ashwinprakas View Post
The same applies to chain driven as well as shaft driven motorcycles
Curious about the how and why. Esp. since in the previously given examples/ illustrations, what seemed to be important was the tension (force) in the chain, and the geometry of the system. The chain afterall is missing in a shaft drive.

Quote:
as per 'Motorcycle Handling and Chassis Design the Art and Science' by Tony Foale.

Spending some time on Google would lead you to the book, it has everything explained in detail with diagrams.
Initially at least I prefer asking a teacher (= expert willing to explain. Actually explains).

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Old 30th June 2020, 14:32   #10
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by ashwinprakas View Post
A few days ago .... suspension.
Mate, I am no engineer either but I think you are over analyzing this issue.
With my limited knowledge I can deduce your wheelie problems to combination of factors. Your big-ish build, an upright seating position and a really short wheelbase and ofcourse the short 1st gear ratio. You're basically as tall sitting on the bike as your bike is long, which is not a good thing. I can suggest a few cheap mods. Try a lower handbar set-up like the fury bars we used to fit in rx and shoghuns back in the day. This will bring your weight forwards. You can try dropping the forks through the clamps if it is possible. If none of these work then try a longer swing arm if there is any available that is a bolt on replacement. Again, if even that doesn't work then you've simply outgrown your bike and its time to get a bigger/longer bike.

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Originally Posted by Sutripta View Post
What happens in the case of a shaft driven bike?

Sutripta
Shafties rock from side to side due to the transverse arrangement of the engine. I have read that this effect is very pronounced in Moto Guzzis. They can still wheelie if enough torque is applied at the rear wheel.

Most Motogp bikes these days have a counter rotating engine to push the front down while acclerating, even the new panigale v4 has this setup. But they still pop the front due to the massive torque at the wheel.
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Old 30th June 2020, 14:44   #11
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by nitro.1000bhp View Post
Shafties rock from side to side due to the transverse arrangement of the engine. I have read that this effect is very pronounced in Moto Guzzis.
That is normal torque reaction. Should be there in the BMWs too.

If you check out a car with a NS engine orientation, and rev the engine, you will see the engine rock.

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Old 30th June 2020, 15:11   #12
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by Sutripta View Post
That is normal torque reaction. Should be there in the BMWs too.

If you check out a car with a NS engine orientation, and rev the engine, you will see the engine rock.

Sutripta
I guess what I am trying to say is that some of the force from the initial torque will be negated into side to side rocking motion than completely transferring the force to the rear shock like in a conventional engine arrangement and therefore a transverse engined bike may not wheelie as much as a normal bike with similar torque. But I have to admit, I dont know squat about suspension squat.

Most car engines rock in the engine bay regardless of their arrangement which is why they have suspended or damped engine mounts.
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Old 30th June 2020, 16:02   #13
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

On a side note the tech that Ducati have come up with in their current motogp bike to reduce wheelies is mind boggling. They have a manually adjustable ride height for both the front and rear suspension (electronic adjustment is banned) to overcome squat while accelerating. Apparently there is more to this than just squat. They effectively lower the bike which also makes the bike longer and makes it harder to lift the front. lowering the front ride height also makes the wings more effective it seems. And all this pops back to normal when the rider engages the brakes.

Just makes one wonder how much power that bike produces that it needs wings, electronics, a counter rotating engine and then this all to keep thst front wheel down. And imagine the brain overload on the rider. As though it was'nt enough that they had to brake from some terrifying speeds, coming down the gears and leaning at some extreme angles but now they have to manually adjust the ride height coming out of the corners to avoid wheelies while they gently accelerate!! Damn thats a lot to think about and do in the span of a few milliseconds.
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Old 30th June 2020, 19:53   #14
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Default Re: Suspension squat - Swingarm geometry effect on motorcycles

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Originally Posted by nitro.1000bhp View Post
Most car engines rock in the engine bay regardless of their arrangement which is why they have suspended or damped engine mounts.
Reason for mentioning NS orientation was to align it with that of shaft drive bikes. Remove a point of difference.

Nonrigid engine mounts in cars are for NVH control. Which is why the engine moves in the frame, and is very apparent. The body also rocks, but is nowhere near as apparent.

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Originally Posted by nitro.1000bhp View Post
a counter rotating engine
Counterrotating masses are normally used to handle gyroscopic effects.

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