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Not quite actually. Offset does not effect the motion ratio. Again its counter intuitive but the offset just creates an overturning moment (positive or negative depending on the direction of offset) which is reacted as tension and compression loads in the control arms. It does not however effect the vertical load on the lower ball joint and therefor does not effect the motion ratio or vertical stiffness. If you were to keep the same wheels and increase track with control arms then you would be absolutely correct that it would cancel, in fact it would actually more than cancel because the motion ratio is squared in the vertical stiffness equation but the vertical stiffness is only first order in the roll stiffness equations.
That said I generally agree that the bias shift is going to be small, but I think its important for people to realize it will be in the opposite way than they might expect. We just don't want people to spend a lot of money on an investment like wheels just to make a problem worse.
I dont agree, lets not over complicate this if it doesn't need to be. Pushing the wheel further out via wheel center line (lower numerical offset, wider track width) or hub spacer, or even welding a custom extension arm to the lower control arm, all has the same net effect since its just a fixed extension of the existing lever aka lower control arm. Whether that increase "lever" is spinning or not, the coilover/spring rate doesn't know/care. If you increase the length of the lever (aka lower control arm) wile keeping the spring damper in the same location, you not only reduce the effective spring rate by increasing the leverage on it, but even increase the total net wheel travel. This is all very small in degree when your talking 30-50mm max per corner, but so is every other aspect of this conversation so im just throwing one more facet of influence into the mix for you guys to pick apart for the sake stirring up some fun. No one gets my satirical sense of humor comments so I figured I might as well join the more analytical conversation and hopefully cause more confusion, mostly for Rob .
Last edited by s2000Junky; Dec 10, 2016 at 07:28 PM.
I dont agree, lets not over complicate this if it doesn't need to be. Pushing the wheel further out via wheel center line (lower numerical offset, wider track width) or hub spacer, or even welding a custom extension arm to the lower control arm, all has the same net effect since its just a fixed extension of the existing lever aka lower control arm. Whether that increase "lever" is spinning or not, the coilover/spring rate doesn't know/care. If you increase the length of the lever (aka lower control arm) wile keeping the spring damper in the same location, you not only reduce the effective spring rate by increasing the leverage on it, but even increase the total net wheel travel. This is all very small in degree when your talking 30-50mm max per corner, but so is every other aspect of this conversation so im just throwing one more facet of influence into the mix for you guys to pick apart for the sake stirring up some fun. No one gets my satirical sense of humor comments so I figured I might as well join the more analytical conversation and hopefully cause more confusion, mostly for Rob .
It's not actually the same to increase track width with a long control arm or a lower offset wheel. Because of the ball join the lower arm isn't actually getting more vertical load from the lower offset wheel.
Think about if you put a hinge in the middle of a diving board, the deflection on the diving board will continue to increase as you walk out in the board but once you get past the hinge the only way to keep going is if you are holding a rope. Any addition moment past the hinge must be supported by you pulling on the rope. This is just like what's happening with the control arms, any addition moment past the ball joint must be supported by an external support in this case it's the upper control arm in compression.
It's not actually the same to increase track width with a long control arm or a lower offset wheel. Because of the ball join the lower arm isn't actually getting more vertical load from the lower offset wheel.
Think about if you put a hinge in the middle of a diving board, the deflection on the diving board will continue to increase as you walk out in the board but once you get past the hinge the only way to keep going is if you are holding a rope. Any addition moment past the hinge must be supported by you pulling on the rope. This is just like what's happening with the control arms, any addition moment past the ball joint must be supported by an external support in this case it's the upper control arm in compression.
I guess I still fail to see with your explanation how the upper control arm influences anything but wheel tracking/camber through the stroke, since the upper control arm is connected to the lower control arm albeit a hinge, its still a fixed hinge or the wheel/increased lever over the hinge would just be flopping around and in that case your right, it would not have any influence. Whether its hinged or not, its still supported like you said, the upper arm moves with the lower arm, just at different rates because of the lengths of each is different, but doesn't matter because the coilover is only attached to the lower control arm/lever which is therefor directly influencing the leverage/spring rate. Im just visualizing the set up and motion in my mind. The camber sweep would not change however, because the upper and lower control arms pre hinge have not, but again the increase in lever beyond the hinge would effect total leverage because its still effectively a fixed point when looking at the motion in totality.
Last edited by s2000Junky; Dec 10, 2016 at 08:51 PM.
Anyway, some really good input and food for thought.
Latest update is that I'm actually limited to a 255 tyre size if I stay 17" as that's the max that Toyo make the R888R (series control tyre)
I'm already running 255 on the rear and 235 on the front so the only advantage I'd really get changing wheels would be to get 255 on the fronts... unless I also go wider on the offset, then I'd increase the track and get 255 on the front.
I think a 9.5 inch wheel or maybe 10" with a good offset, and ASM 25mm rear arches and some Downforce front 30mm fenders. Difficult working out what offset would work under the arches, and what clears the Stoptechs. Needs some research...
Other option is to go to 18" but not sure I like that idea. Don't like spacers either.
I agree with Junky. Extending the contact patch outward with offset does add leverage to act against the shock spring but it doesn't change the motion ratio because that deals only with length between pivots and length to shock mount.
Anyway, some really good input and food for thought.
Latest update is that I'm actually limited to a 255 tyre size if I stay 17" as that's the max that Toyo make the R888R (series control tyre)
I'm already running 255 on the rear and 235 on the front so the only advantage I'd really get changing wheels would be to get 255 on the fronts... unless I also go wider on the offset, then I'd increase the track and get 255 on the front.
I think a 9.5 inch wheel or maybe 10" with a good offset, and ASM 25mm rear arches and some Downforce front 30mm fenders. Difficult working out what offset would work under the arches, and what clears the Stoptechs. Needs some research...
Other option is to go to 18" but not sure I like that idea. Don't like spacers either.
They make 315's in 17's
Maybe post in the wheel/tire forum and we can work out your fitment and pros/cons.
I agree with Junky. Extending the contact patch outward with offset does add leverage to act against the shock spring but it doesn't change the motion ratio because that deals only with length between pivots and length to shock mount.
SakeBomb, please correct me but this is how i'm thinking about it:
Rob, you are correct that it doesn't affect the motion ratio! Therefore it also does not affect the wheel rate. Regardless of offset, the wheel and hub move as a single unit, no matter the offset. So the offset does not change the force acting on the spring.
Where does that leverage from the high offset do then? Your mental model is assuming that the lower control arm and wheel are a solid unit, but they are not. Imagine the suspension if you removed the upper control arm, any force on the wheel would cause the hub to just fold inward since it is hinged on the lower ball joint. With the upper control arm there, the force of the wheel inward is applied to the upper control arm and so the wheel doesn't cave in!
If you move the wheel 1 inch, the hub moves 1 inch and you apply a set amount of force on the spring based on the wheel rate. If you have a high offset wheel and move it 1 inch, it is still a fixed unit with the hub and the hub moves 1 inch and the same force is acting on the spring. The extra leverage of the high offset wheel is applied to the upper control arm that keeps the wheel from caving in
SakeBomb, please correct me but this is how i'm thinking about it:
Rob, you are correct that it doesn't affect the motion ratio! Therefore it also does not affect the wheel rate. Regardless of offset, the wheel and hub move as a single unit, no matter the offset. So the offset does not change the force acting on the spring.
Where does that leverage from the high offset do then? Your mental model is assuming that the lower control arm and wheel are a solid unit, but they are not. Imagine the suspension if you removed the upper control arm, any force on the wheel would cause the hub to just fold inward since it is hinged on the lower ball joint. With the upper control arm there, the force of the wheel inward is applied to the upper control arm and so the wheel doesn't cave in!
If you move the wheel 1 inch, the hub moves 1 inch and you apply a set amount of force on the spring based on the wheel rate. If you have a high offset wheel and move it 1 inch, it is still a fixed unit with the hub and the hub moves 1 inch and the same force is acting on the spring. The extra leverage of the high offset wheel is applied to the upper control arm that keeps the wheel from caving in
Exactly correct ndogg! I'll draw a picture when I get home but yes, the lower ball joint is unable to support the moment from the wheel offset. Even though it doesn't rotate in this case doesn't mean that is is able to support the load.
I agree with Junky. Extending the contact patch outward with offset does add leverage to act against the shock spring but it doesn't change the motion ratio because that deals only with length between pivots and length to shock mount.
Yes, but you're forgetting something.
The hub does not have 1:1 motion with the lower ball joint.
That's why there's camber gain.
Push the example to a ridiculous extreme, imagine a wheel with several feet of negative offset. Jack up the tire. The tire will move further up than the lower ball joint. Each degree of camber gain will move the tire contact patch vertically.
The vertical motion of the hub center is not 1:1 with the vertical motion of the tire either. That is affected by the offset.
Changing the offset changes the motion ratio..
Also, I'm going to toss something else into this discussion, and I can explain it if people need it.
At their simplest, Anti-Roll bars do not affect side to side weight transfer. Physics does that. They don't move the roll center, and they don't move the CG. The arm from roll center to CG is what determines lateral load transfer, and you can't affect that with a bolted-on torsion spring.. Anti-Roll bars transfer load front to rear during body roll. Think about it.
Last edited by Spartarus; Dec 11, 2016 at 05:31 PM.
Well, this started out as a nice thread on the effects of track width. Thanks for your posts, thomsbrain. Well explained. To corroborate, after the addition of 15mm rear wheel spacers, I have noticed a slightly lighter steering wheel when driving near the limit which generally means the rear is a bit more playful. I also think the steering angle to yaw ratio the VSA is expecting is a little different because it seems to be a bit more nagging now. And to add to that, tricycles almost never understeer while the Morgan 3-Wheeler has it in abundance.
As for the talks about motion ratio, "The motion ratio of a mechanism is the ratio of the displacement of the point of interest to that of another point." Since we are talking about cars that touch the road via their tires, I would certainly consider the tire's contact patch to be a 'point of interest'...
You guys that keep using the equation that only takes the "distance from spring centerlines to control arm inner pivot center" and "distance from outer ball joint to control arm inner pivot center" into consideration are failing to understand that it is only an approximation because it is arguably easier to measure and unless talking about extreme examples, the numbers will be 'close enough' to determine spring selection. But you cannot take an approximation, extrapolate it beyond its bounds and then derive theory from it! That is ass backwards.
