# Sidecar Design Formula - IMPORTANT!

Discussion in 'Hacks' started by Get Back, May 25, 2009.

1. ### McCardiganhappy Budda

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Yep got that Andy.

No wucking furries.

Any chance you can predict next weeks \$40m Tatts Lotto numbers?

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2. ### ClancyLong timer

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oser oser oser
3. ### brockolibuild of cool stuff

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Ask "Getback", aka Shaun, about his sway bar on his super narrow enduro rig with lots of travel. Wouldn't have worked with out one. When are we going to start using electro-magnetically adjustable spring units like Corvette's and Cadillac's ? That would change the ball game all over again too! Glad to see this post has brought about change for the better, lets keep it going!
4. ### Andy-GadgetAny bike can go anywere

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The influence of rider and passenger mass on the static centre of gravity.

Adding the rider to the vehicle changes the centre of gravity of the outfit.
This change is towards the added mass, the rider, and can be calculated relatively easily, but as the rider is "dynamic", that is the rider centre of gravity can be moved intentionally through the application of "body English" to assist the handling of the outfit.
This means that the calculated new CG is of limited use practically, but the rider&#8217;s knowledge of the change in CG is helpful, and the cornering dynamics of the outfit can be calculated to at least give some performance parameters for the outfit.

Having used the beam reactions system to calculate the position in three dimensions of the static CG of the outfit, there are a number of way of integrating the two CG's together, the easiest being the use of vectors, so that is what I propose to use.

Some notes on vectors first, there are two ways of representing a force, a scalar (a magnitude but NO direction) and a vector (both a magnitude AND direction).
All groups of forces acting on something can be resolved to one vector, and any single vector can be broken down into any number of separate forces, so long as all directions are known.
These vectors can be added and subtracted using simple rules.
Marine navigators use vectors constantly to find the resultant direction and speed of a vessel when acted on by a current.

If the addition of all the vectors acting on a point makes a closed loop, then there is no resultant vector and the point is stationary.

Tony Foale has looked into the solo motorcycle version of this in his book "Motorcycle Chassis design", and I will borrow some of his research and apply them to sidecars.
The first assumption I will make is where the CG of the rider is located, and this is slightly ahead of the riders lap, and just above the thighs.

To use vectors to calculate the new CG, the riders CG needs to be broken down into vertical and horizontal components, this sounds scary but is easy.

The vertical component is the difference in height between the static CG and the riders CG, so measure the riders CG from ground level and subtract from that the static CG height, if the result is positive then the riders CG is above the static CG, if negative then below the static CG.
The mass of the outfit is the sum of the three reactions from the calculations for static CG, remembering to apply the multiplier appropriate to the second-class lever proportions used.

The horizontal change in CG is on the line between the static CG and the riders CG and the distance it moves is again proportional to the difference in masses and the horizontal distance between the masses (the vertical difference has already been calculated).

The calculations so far have been for one additional mass, to add more than one mass is relatively simple.
The horizontal components for the masses can be added together to make one resultant vector that the above calculations are used with.
The horizontal components are measured from the static CG as follows.

The vertical change in static CG is calculated in a similar way, with the differences in vertical CG now being a single mass (the sum of the two masses) acting at a point along the difference in vertical height between the masses proportional to the two masses.
Here it is assumed that the sidecar passenger&#8217;s CG is lower than the rider&#8217;s CG.

The result of all this is a CG for the outfit with added masses, but as stated earlier this doesn&#8217;t take into account any &#8220;body English&#8221; involving these masses.
5. ### claudeSidecar Jockey

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Okay so we have a center of gravity that is really influnced on a sidecar rig by driver and passnger. It (The CoG )can also be moved around by body english of the driver and passenger.
Wider track widths will create more stability as would a lower CoG.
I think all of these are pretty much 'givens'.
The thread began with info on sidecar wheel lead. I think tip ovcer lines were mentioned also.
We have writen about rigs with swingarm suspensions and it was stated that the Roll Centers on them will remain at ground level.
So with this being said what do we have to work with?
Suspensions on all wheels, rake a trail and it effects in cornering apart from just steering, front end designs (teles, leading links, center hubs), swingarm lengths, swingarm orientation on sidecar (front or rear pivot) and of course antiswaybars.
Of course there are rigs with a frame type suspensions on the sidecar and even on the rear of the bike which can open up more discussion when and if we wish to get into it.
Where do we go from here ?
6. ### SidecarjohnSidecarJohn

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Surely, the beauty of sidecars is the relative unique flavour of each and every rig. This lack of uniformity is part of the attraction. Whilst some degree of science and mathematics potentially offer some solutions to severe problems such as mishandling and inadequate engineering, there is much to be said for the outcomes of personal preference.

However, even if a sidecar outfit doesn't appeal to my needs, I still admire and respect the vast array of vehicles in the sidecar world. Long may it continue for it represents something that no "Volksrig" could ever satisfy.
7. ### Nemo DeNovoBanned

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All this engineering is an interesting diversion for the winter days when there's no riding (and you're stuck in the house with the female nattering away about something or other), but if the road is free of ice I'd rather just get out there and tweek the rig & ride it. Seems the "seat of the pants" method satisfies the masses best. "If it feels good, do it!" If it weren't for that, the human race would have become extinct long ago....we're all to damned lazy to fuck if it didn't feel so good
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8. ### Andy-GadgetAny bike can go anywere

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Bringing it all together, after all the maths.

The goal I had in my head at the start of all this was a set of operating parameters that would describe the way a given outfit would behave, like standing ¼ mile times for an indication of the performance of a car or bike.

But before I could do this, pertinent information about the outfit, such as the location in three dimensions of the Centre of Gravity, had to be determined, hence the maths.

The Physics is needed so vectors are understood, as it all comes down to vectors in the end.

Now that we know roughly were the centre of gravity is, its height above the ground and the distance to the wheel contact triangle horizontally can be measured.
The height above the ground determines the scale of the vectors, and this scale is then applied to the horizontal component to determine how much cornering force (lateral acceleration) can be applied before the sidecar wheel will lift.

As an example, for turns towards the sidecar, if the CG was at 1 meter height, and the bike pivot point was 500mm from the CG, as the acceleration of gravity is 1 gee, the lateral acceleration of ½ a gee will cause the sidecar to lift, 1000 / 9.81 = 101.9 mm per 1 meter per second squared.

This lift is much more controllable as any acceleration / deceleration vectors are along the axis of the tyre contact patches, and so have very little affect of the stability, hence the fun of flying the chair.

The other lift, the back wheel in the air, pivoting on the line between the sidecar wheel and the bike front wheel, requires more lateral acceleration to achieve the lift, but when lifted, is much less stable, as the acceleration / deceleration vectors are off the line of pivot.
An example, the bike is acceleration around a turn towards the sidecar, the CG moves outside the tyre triangle and the back wheel lifts, the wheel cant drive in the air, and so the acceleration vector disappears, this moves the CG even further outside the triangle, and generates the rapid flip some have experienced.
I have taken the distance from the CG to the sidecar to bike front wheel pivot line as the shortest distance, rather than as it actually is, being much closer to the sidecar wheel, and hence longer, as I wanted the value arrived at from the two lateral accelerations to be the worst case.
I would like the towards the sidecar turn lift point to away from the sidecar turn lift point to be a performance figure of the sidecar.
If we take the away from the sidecar lift lateral acceleration vector to be 1and the away from the sidecar lift vector to be the multiple of the first vector, this figure is a direct value of the lateral stability of the sidecar.
The above example would be something like 2.5, so the back wheel lift vector is 2.5 x 0.5 so 1.25 meters.
As the centre of gravity is moved towards the sidecar, through intelligent placement of fixed components such as extra batteries, then the figure will reduce, indicating that the sidecar wheel lift point is closer to the back wheel lift point, it is debatable as to how good an idea this is, but the rider would then know an important parameter of the cornering capability of the bike.

The simple example I have shown is to show how it all works, but is slightly more complicated in practice.
The vertical height of the CG would be divided by the value of gravity, 9.81 for we metric people, being 9.81 meters per second squared.
The metric challenged can apply whatever value they are happy with, it will all work out in the end.
The result is the scale to be applied to the horizontal, mm / meter per second squared, or whatever.
This scale is then applied to the horizontal distance to the pivot point, and the result is the lateral acceleration required to reach this point in meters per second squared, or whatever.

I find that I tend to be symmetrical in my cornering speeds, if I have compromised clearance on one side, it is applied by my mind to both sides.
I ride sidecar outfits sort of the same way, it is the sidecar wheel lift point that is determining the cornering speed in both directions, unless you are much more bold than I, which is very much possible.
9. ### DirtyDRDana

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10. ### grafikfeat-Out There-

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How does suspension, rider weight, sidecar weight to motorcycle weight ratio play into that math? :huh

I see a lotta guys w/ Harleys and a velorex plastics tied on.
Clearly too light a tub for the bike.

Me? I ride within my limits.

I know jets fly. I'll even get in one to travel. I don't care how it does it... Just so it does it w/ out crashing!
11. ### Andy-GadgetAny bike can go anywere

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Less weight on the side car wheel, in proportion to the total weight of the outfit.
It moves the C of G closer to the bike centre line, with the ease of flying the chair that that entails.
It also raises the C of G as well, generally not a good thing.
12. ### dbarnes180Been here awhile

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I KNOW Nothing! And to prove that I have a question. Can the sidecar be hinged to keep it on the pavement instead of being lifted into the air?

I have never owned, driven, seen a sidecar in person. So I just have to show my ignorance and ask this.

take
care
a
friend

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15. ### grafikfeat-Out There-

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I understand that...
I meant all that calculation works in a perfect world/set up.
Although... It does establish a good baseline.

There are too many variables. IE a Road King w/ a plastic velorex.
That's just plain dangerous.
16. ### Andy-GadgetAny bike can go anywere

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The only fix in the type of case you describe is to make the chair heavier.

This is the situation where you get clever with just where the weight is added.
Two bags of cement powder on the chair seat will work, but one bag behind the chair wheel will work better, This is something drummed into us in Marine stability, it isn't the weight alone that is important, it is the distance of effort that matters from centre of boyancy that makes it all work.
This is the classic lever thing, the longer the lever arm, the less effort needed.
Better to be humping around one bag of cement powder, than two, unless you like cement powder of coarse, and it leaves the seat free.

17. ### claudeSidecar Jockey

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Adding weight (ballast) to a sidecar can make a difference as can mounting the sidecar farther away from the bike. The 'variable' here, in some cases, can be related to whether the sidecar frame,wheel, spindle, mounts and suspension are up to the task of carry the additional loads. There are limits. To load down a very small or light sidecar in an attampt to make it work on a very large bike can be inviting troubles in other areas. Ballast may add stability in turns into the sidecar but it can also impose a ton of stress onto other things in turns away from the sidecar. We have modified a few smaller sidecars to allow them to work well on larger bikes but the return of the effort and /or expense to do this can be questionable in many cases.

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Hi,
If you could send me the PDF of the shetch I would apprieate it.
Thanks,
TT
19. ### Center-standLong timer

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I "think" this Citeroen suspension

is what spawned the research and developement of the Bose electromagnectic motor suspension. The way I understand it, Dr. Bose had a Citereon, loved the car but hated the way it handled.

http://www.bose.com/controller?event=VIEW_STATIC_PAGE_EVENT&url=/learning/project_sound/suspension_components.jsp

There are some real cool video's of the Bose suspension if you care to look for them.

I have found this thread interesting, though sometimes too complicated for my brain. I am reading trying to determine if I really want to hack one of my bikes. For a novice this thread doesn't exactly paint the rideability of a hack in the best light.

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Lots of interesting information here.

Glad to see that the location of an outfit's roll centre (assuming solo + sidecar geometry) is agreed to be at ground level. :)

Turning into the chair and lifting the inside wheel is simple geometry. The relationship between the height of the CG and the lateral offset of the CG gives you a cornering rate limit in "G's". If the limit of traction in "G's" is lower than the G force required to lift the inside wheel, the outfit will slide before it rolls.

i.e. if the CG is 1 foot above the ground and offset 1 foot to the inside of front and rear tyres, it will take 1 G of lateral acceleration to lift the chair.

The subjects of sidecar wheel lead and steering geometry when limited to solo type suspensions (single plane) are very interesting. If you want to be able to corner at the same speed into and away from the chair, you have to look at the forces each wheel generates. The cornering force from the front wheel can be taken as a constant turning either way. It follows that in a maximum turn into the chair the inside tyre is adding near zero lateral force (since the weight has transfered to the outside tyres). In a maximum turn away from the chair the rear wheel is unloaded and the chair wheel does all the work.

With the contribution of the front tyre a constant, if follows that the rear and sidecar tyres have equal contact patches and to set the sidecar wheel lead such that the dynamic load is equal whilst turning.

If the CG is perhaps 55% of the front/rear wheelbase ... the CG should be 55% of the front/sidecar wheelbase to load the tyres equally on both turns.

The last outfit I built (in the late 70's) had 4" track centre offset front/rear (rear wheel contact centre 4" outboard) and equal "wheelbase" between the front tyre contact and both rear and sidecar contact. The idea being that the dynamic wheelbase was equal on right and left turns.

One trim setting that is largely ignored is chair wheel "camber". Lean-out is a camber adjustment to balance the drag of the chair. Chair wheel camber can help too. Chair wheel lean-in can reduce toe-in and lean-out requirements. With lean-out, the camber hurts turns into the chair, you would like to have lean-in. With chair wheel lean-in the driving and steering wheels are more upright for turns into the chair the lean-in is "good" camber for turns away from the chair.

I'm reading this thread with great interest as a street kneeler is not on the option list and I want to learn about tall outfits with skinny tyres and small HP. My R75/5 and Bingham outfit was NOT fun to ride. I'm hoping a Ural will be. If I can get a test ride to see if I "fit" the bike, the V-Strom is going ... a Ural in Mexico sounds like just the right choice.

Randy
(I was the passenger in this shot, taken in the esses at Riverside)

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