Load transfer question
09-21-2005, 07:24 PM
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Re: (superpilun)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by superpilun »</TD></TR><TR><TD CLASS="quote">...Interesting, when the rear lifts, the car can only result in more understeer from the additional front weight transfer. </TD></TR></TABLE>
Not necessarily - the outside rear camber continues to change, affecting grip.
Scott, who...is gonna run out of these sentences some day...
Not necessarily - the outside rear camber continues to change, affecting grip.
Scott, who...is gonna run out of these sentences some day...
09-22-2005, 03:28 AM
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Re: (RR98ITR)
Exactly what magical effect causes the outside rear spring to lock once the inside rear lifts, stopping any further deflection or extension? What part of the lateral acceleration force split between the front and rear cg's disappears at the rear to stop all further load transfer there once the inside tire lifts? I don't see one spring talking to the other. How could the outside rear spring know it is to change its fundamental nature beacuse the inside rear has lifted? A spring is a spring, unless restrained by some greater force, such as a bump stop or a damper piston hitting the inside stop of the damper. Unless the outside rear spring has enountered such a greater force, it will continue to be a spring.
09-22-2005, 05:16 AM
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Re: (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR »</TD></TR><TR><TD CLASS="quote">
Not necessarily - the outside rear camber continues to change, affecting grip.
Scott, who...is gonna run out of these sentences some day...</TD></TR></TABLE>
assuming the rear decambers faster then the front. it is entirely possible that the rear in fact does NOT decamber faster or fast enough relative to the front. from what i can tell from photo's of my car, post rear wheel lift roll does not compress the outside front significantly, only increases droop on the inside front. both front and rear decamber at nearly the same rate despite the outside front's camber requirements increasing as cornering forces increase.
descartes, the outside rear spring doesn't compress further post wheel lift because there is no moment arm acting on that spring. the roll center is the contact patch (or a point near it) and as such the spring is now only acting in ride with zero effect in roll. if you hit a bump with the wheel the spring will still deflect.
nate
Not necessarily - the outside rear camber continues to change, affecting grip.
Scott, who...is gonna run out of these sentences some day...</TD></TR></TABLE>
assuming the rear decambers faster then the front. it is entirely possible that the rear in fact does NOT decamber faster or fast enough relative to the front. from what i can tell from photo's of my car, post rear wheel lift roll does not compress the outside front significantly, only increases droop on the inside front. both front and rear decamber at nearly the same rate despite the outside front's camber requirements increasing as cornering forces increase.
descartes, the outside rear spring doesn't compress further post wheel lift because there is no moment arm acting on that spring. the roll center is the contact patch (or a point near it) and as such the spring is now only acting in ride with zero effect in roll. if you hit a bump with the wheel the spring will still deflect.
nate
09-22-2005, 05:17 AM
#54
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Re: (descartesfool)
Here's the thread at FSAE.com: Roll Axis thread
Hopefully the right people bite. Look for responses especially from "Z", "Richard Pare", and "Kevin Hayward".
Hopefully the right people bite. Look for responses especially from "Z", "Richard Pare", and "Kevin Hayward".
09-22-2005, 05:40 AM
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Re: (GSpeedR)
Thanks Chris.
Claude - Nobody ever said the outside rear spring "locked" after the inside rear lifted. But unless lateral force input results in longitudinal weight transfer because of roll/moment center skew (which is the question) then once the inside rear lifts the outside rear is carrying all of the rear axle weight - so it's done moving unless the driver does something with the throttle or brake, or a surface irregularity is enountered, or a change in surface/grip is encountered.
Nate - once the inside rear lifts and the rear roll center is at the outside contact patch then it just rolls along with the body...the outside front, continuing to take load transferred from the inside front as the body continues to roll continues to run it's camber curve. So it's likely that at that point the front is decambering slower than the rear.
Scott, who is looking forward to some good reading interfering with all the work I have to do today...
Claude - Nobody ever said the outside rear spring "locked" after the inside rear lifted. But unless lateral force input results in longitudinal weight transfer because of roll/moment center skew (which is the question) then once the inside rear lifts the outside rear is carrying all of the rear axle weight - so it's done moving unless the driver does something with the throttle or brake, or a surface irregularity is enountered, or a change in surface/grip is encountered.
Nate - once the inside rear lifts and the rear roll center is at the outside contact patch then it just rolls along with the body...the outside front, continuing to take load transferred from the inside front as the body continues to roll continues to run it's camber curve. So it's likely that at that point the front is decambering slower than the rear.
Scott, who is looking forward to some good reading interfering with all the work I have to do today...
09-22-2005, 05:51 AM
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Re: (solo-x)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x »</TD></TR><TR><TD CLASS="quote">
descartes, the outside rear spring doesn't compress further post wheel lift because there is no moment arm acting on that spring. the roll center is the contact patch (or a point near it) and as such the spring is now only acting in ride with zero effect in roll. if you hit a bump with the wheel the spring will still deflect.
nate</TD></TR></TABLE>
I don't see where this concept comes from. The car can certainly continue to roll, as the outside rear spring can still deflect more or less than it had at the moment of "lift-off". The outside suspension has not become locked. Thus the roll center in the rear is not likely at the outside rear contact patch. Just assume that the rear suspension deflects more and the outside rear spring gets shorter. Then this means the car has rolled about the swing arm at the rear. The force at the rear cg can still cause a compression of the outside rear spring and the moment arm has not disappeared. Why would the moment arm disappear unless the centre of gravity has suddenly moved.
Just take a model car and try it out. It still compresses as you pull sideways on the chassis.
You can still compress the outside rear spring whether the inside is off the ground or not. Who said the rear roll centre moved suddenly to the rear tire's contact patch, and how does this idea come about? Can anyone show this effect using a suspension analysis program? Where is the magic that the outside spring knows the inside tire has no load on it, causing it to stop moving? Telepathy? I need an explanation to understand why the rear outside suspension would stop moving.
descartes, the outside rear spring doesn't compress further post wheel lift because there is no moment arm acting on that spring. the roll center is the contact patch (or a point near it) and as such the spring is now only acting in ride with zero effect in roll. if you hit a bump with the wheel the spring will still deflect.
nate</TD></TR></TABLE>
I don't see where this concept comes from. The car can certainly continue to roll, as the outside rear spring can still deflect more or less than it had at the moment of "lift-off". The outside suspension has not become locked. Thus the roll center in the rear is not likely at the outside rear contact patch. Just assume that the rear suspension deflects more and the outside rear spring gets shorter. Then this means the car has rolled about the swing arm at the rear. The force at the rear cg can still cause a compression of the outside rear spring and the moment arm has not disappeared. Why would the moment arm disappear unless the centre of gravity has suddenly moved.
Just take a model car and try it out. It still compresses as you pull sideways on the chassis.
You can still compress the outside rear spring whether the inside is off the ground or not. Who said the rear roll centre moved suddenly to the rear tire's contact patch, and how does this idea come about? Can anyone show this effect using a suspension analysis program? Where is the magic that the outside spring knows the inside tire has no load on it, causing it to stop moving? Telepathy? I need an explanation to understand why the rear outside suspension would stop moving.
09-22-2005, 06:36 AM
#57
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Re: (descartesfool)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote »</TD></TR><TR><TD CLASS="quote">The outside suspension has not become locked. Thus the roll center in the rear is not likely at the outside rear contact patch.</TD></TR></TABLE>
This is were we disagree. With zero weight on the inside rear (tire lift) the rear roll center is in fact at the outside rear contact patch. Because the roll axis goes through the rear contact patch, we are just rotating about the contact patch and no more weight will be transferred to this tire.
This is were we disagree. With zero weight on the inside rear (tire lift) the rear roll center is in fact at the outside rear contact patch. Because the roll axis goes through the rear contact patch, we are just rotating about the contact patch and no more weight will be transferred to this tire.
09-22-2005, 06:51 AM
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Re: (descartesfool)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by descartesfool »</TD></TR><TR><TD CLASS="quote">Just take a model car and try it out. It still compresses as you pull sideways on the chassis.</TD></TR></TABLE>
i don't need to take a model, i have a real life car with picture evidence of what is going on. the outside rear most definitely does not continue to deflect upwards due to chassis roll after the inside rear leaves the ground. this was the argument that chris and i had in the other thread. pictures of my car indicate the same. as such, the rear of the chassis is rolling about the outside rear tires contact patch or a point very close to it. ie, no roll stiffness contribution, ie, no further wheel deflection.
scott, agreed, the rear decambers at the same rate as chassis roll. again, picture evidence from my car seems to indicate that not only does the rear roll center migrate to the outside wheel, the front roll center migrates towards the outside wheel too, just not as far. the outside front still decambers less then the outside rear, but not as slowly as at least i'd like it to. add to that a not so great camber curve (it's good, but not "perfect" if there ever were such a thing) and imo we end up in a situation where the outside front isn't gaining enough camber as cornering forces increase to maintain balance. it's just my opinion and i'll concede that i might be off on my conclusions.
an interesting thought that occured to me while typing the above. IF what i say is happening truly IS happening, a zero droop front suspension could in fact increase front grip. by limiting the inside front from drooping, all further roll must happen about the inside front's contact patch, further skewing the roll center, but also forcing the outside front to compress with roll, utilizing the camber gain our suspension has, etc. with topeka moving to smooth asphault, this might be something i'll give more thought to for when/if i either return to ST or go for SP. gonna definitely need to trailer it though....
nate
i don't need to take a model, i have a real life car with picture evidence of what is going on. the outside rear most definitely does not continue to deflect upwards due to chassis roll after the inside rear leaves the ground. this was the argument that chris and i had in the other thread. pictures of my car indicate the same. as such, the rear of the chassis is rolling about the outside rear tires contact patch or a point very close to it. ie, no roll stiffness contribution, ie, no further wheel deflection.
scott, agreed, the rear decambers at the same rate as chassis roll. again, picture evidence from my car seems to indicate that not only does the rear roll center migrate to the outside wheel, the front roll center migrates towards the outside wheel too, just not as far. the outside front still decambers less then the outside rear, but not as slowly as at least i'd like it to. add to that a not so great camber curve (it's good, but not "perfect" if there ever were such a thing) and imo we end up in a situation where the outside front isn't gaining enough camber as cornering forces increase to maintain balance. it's just my opinion and i'll concede that i might be off on my conclusions.
an interesting thought that occured to me while typing the above. IF what i say is happening truly IS happening, a zero droop front suspension could in fact increase front grip. by limiting the inside front from drooping, all further roll must happen about the inside front's contact patch, further skewing the roll center, but also forcing the outside front to compress with roll, utilizing the camber gain our suspension has, etc. with topeka moving to smooth asphault, this might be something i'll give more thought to for when/if i either return to ST or go for SP. gonna definitely need to trailer it though....
nate
09-22-2005, 06:56 AM
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Re: (solo-x)
fwiw, playing around with lapsim agrees with my conclusions about roll center migration. post wheel lift the rear roll center is the outside rear tires contact patch. front rc migrates towards outside front. cg height increases as wheel lift height increases.
nate
nate
09-22-2005, 07:14 AM
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Re: (descartesfool)
The roll center doens't move to the outside tire patch just because you get wheel lift. Eventually, the outside suspension will lock (either when the car is at such a high angle that there is no more axial force along the spring, or you run out of bump travel). Now the sprung and unsprung mass could be considered a rigid body, and the roll center abruptly shifts to the point of contact. It's a violent shift, but a locked suspension is a violent event.
09-22-2005, 07:27 AM
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Re: (GSpeedR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GSpeedR »</TD></TR><TR><TD CLASS="quote">The roll center doens't move to the outside tire patch just because you get wheel lift. Eventually, the outside suspension will lock (either when the car is at such a high angle that there is no more axial force along the spring, or you run out of bump travel). Now the sprung and unsprung mass could be considered a rigid body, and the roll center abruptly shifts to the point of contact. It's a violent shift, but a locked suspension is a violent event. </TD></TR></TABLE>
how can the roll center not be at the outside rear tire patch when you lift the inside tire? The car is now a 3 wheel car and how can it not pivit about the lone 3rd wheel in the rear?
how can the roll center not be at the outside rear tire patch when you lift the inside tire? The car is now a 3 wheel car and how can it not pivit about the lone 3rd wheel in the rear?
09-22-2005, 07:32 AM
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Re: (GSpeedR)
Hmmmmm...so if one could arrange a suitable roll/moment axis one could presumably transfer all of the lateral force generation into longitudinal weight transfer? Resulting in the car being supported by one axle only?
Scott, who only asks because Wiley Coyote has been bit by this kind of thing before.
Scott, who only asks because Wiley Coyote has been bit by this kind of thing before.
09-22-2005, 09:05 AM
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Re: (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR »</TD></TR><TR><TD CLASS="quote">Hmmmmm...so if one could arrange a suitable roll/moment axis one could presumably transfer all of the lateral force generation into longitudinal weight transfer? Resulting in the car being supported by one axle only?
Scott, who only asks because Wiley Coyote has been bit by this kind of thing before.
</TD></TR></TABLE>
sounds like the subject for Wile E.'s next book....
Scott, who only asks because Wiley Coyote has been bit by this kind of thing before.
</TD></TR></TABLE>
sounds like the subject for Wile E.'s next book....
09-22-2005, 09:07 AM
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Re: (solo-x)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x »</TD></TR><TR><TD CLASS="quote">
sounds like the subject for Wile E.'s next book....</TD></TR></TABLE>
As you've probably heard: all the rear wheels do is hold up the back of the kaa.
Scott, who's all excited just thinking about the potential weight savings...not to mention that Drifting will never be the same..."I'm gonna be Famous!"
sounds like the subject for Wile E.'s next book....</TD></TR></TABLE>
As you've probably heard: all the rear wheels do is hold up the back of the kaa.
Scott, who's all excited just thinking about the potential weight savings...not to mention that Drifting will never be the same..."I'm gonna be Famous!"
09-22-2005, 09:07 AM
#65
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Re: (solo-x)
i think chris and i have just agreed to partially disagree about rear roll center migration post wheel lift. some damper pots and quality data aquisition would go a long way to learning what REALLY happens on a h/a racecar post wheel lift. i'd bet it's slightly different then what you'd see on a fsae car.
nate
nate
09-22-2005, 09:15 AM
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Re: (solo-x)
And a Really Sweet skid pad.
Scott, who someday wants to have Two skid pads - one smooth, and one bumpy...or one electronically controlled variable surface skid pad...Did I mention that in my wildest fantasies I'm fantastically rich and incomparably mad?
Scott, who someday wants to have Two skid pads - one smooth, and one bumpy...or one electronically controlled variable surface skid pad...Did I mention that in my wildest fantasies I'm fantastically rich and incomparably mad?
09-22-2005, 09:17 AM
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Re: (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR »</TD></TR><TR><TD CLASS="quote">Did I mention that in my wildest fantasies I'm fantastically rich and incomparably mad?</TD></TR></TABLE>
good news! your half way there!
good news! your half way there!
09-22-2005, 09:23 AM
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Re: (solo-x)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x »</TD></TR><TR><TD CLASS="quote">
good news! your half way there!</TD></TR></TABLE>
lol!
Now thats good.
good news! your half way there!
</TD></TR></TABLE>lol!
Now thats good.
09-22-2005, 09:41 AM
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Re: (577HondaPrelude)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GSpeedR »</TD></TR><TR><TD CLASS="quote">The roll center doens't move to the outside tire patch just because you get wheel lift. </TD></TR></TABLE>
I agree with this statement.<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x »</TD></TR><TR><TD CLASS="quote">
i don't need to take a model, i have a real life car with picture evidence of what is going on. the outside rear most definitely does not continue to deflect upwards due to chassis roll after the inside rear leaves the ground. </TD></TR></TABLE>
I disagree with this statement. Are there shock pots to prove this?
If suspension is very stiff, and you are close to the g limit of the tires (since you are lifting the inside rear), then there is not much further lateral force to be had before losing it completely, so the outside rear spring will not show much extra deflection, and extra deflection would be hard to see in a picture, but shock pots would show it.
Once the inside rear (or front on a RWD) lifts, there is still a lateral force acting on the front and rear cg's, and this force can increase or decrease if you add or reduce power, and thus the outside rear can continue to change its deflection, and the contact patch is thus not under the tire. Why would it be? Why would the moment arm suddenly disappear? The lateral force has not disappeared, the cg has not moved appreciably, the spring has not locked and the outside rear is still free to deflect more or less, so what would cause it to stop moving and defy Newton's laws. This would be Voodoo physics.
Look at your right hand. Point it up with the base of your hand 1" off the table and place your thumb downwards touching the table. This is your outside rear. The car has rolled a bit (your hand). The centre of gravity of your hand is between your knuckles. The swing arm of your thumb which is pointing to the left is the inner pivot point of your thumb. Since hopefully you do not have two thumbs on this hand, consider that the thumb on the right which does not exist is the inside tire which has lifted and is out of the picture. Apply a leftward force to your palm. What happens. For my hand, the thumb stays on the table and the hand pivots about the inner joint, adding more deflection. The overall hand does not rotate about the contact point of my thumb on the table. Why would it be otherwise? If there is jacking, then you get two rotations, one about the contact point and one about the inner pivot point (swing arm). At least that is how I see it. If the hand analogy doesn't do it for anyone, try a car model with four springs and a suspension. I can see no physical reason for the rear suspension to stop deflecting once the inside rear lifts.
I agree with this statement.<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x »</TD></TR><TR><TD CLASS="quote">
i don't need to take a model, i have a real life car with picture evidence of what is going on. the outside rear most definitely does not continue to deflect upwards due to chassis roll after the inside rear leaves the ground. </TD></TR></TABLE>
I disagree with this statement. Are there shock pots to prove this?
If suspension is very stiff, and you are close to the g limit of the tires (since you are lifting the inside rear), then there is not much further lateral force to be had before losing it completely, so the outside rear spring will not show much extra deflection, and extra deflection would be hard to see in a picture, but shock pots would show it.
Once the inside rear (or front on a RWD) lifts, there is still a lateral force acting on the front and rear cg's, and this force can increase or decrease if you add or reduce power, and thus the outside rear can continue to change its deflection, and the contact patch is thus not under the tire. Why would it be? Why would the moment arm suddenly disappear? The lateral force has not disappeared, the cg has not moved appreciably, the spring has not locked and the outside rear is still free to deflect more or less, so what would cause it to stop moving and defy Newton's laws. This would be Voodoo physics.
Look at your right hand. Point it up with the base of your hand 1" off the table and place your thumb downwards touching the table. This is your outside rear. The car has rolled a bit (your hand). The centre of gravity of your hand is between your knuckles. The swing arm of your thumb which is pointing to the left is the inner pivot point of your thumb. Since hopefully you do not have two thumbs on this hand, consider that the thumb on the right which does not exist is the inside tire which has lifted and is out of the picture. Apply a leftward force to your palm. What happens. For my hand, the thumb stays on the table and the hand pivots about the inner joint, adding more deflection. The overall hand does not rotate about the contact point of my thumb on the table. Why would it be otherwise? If there is jacking, then you get two rotations, one about the contact point and one about the inner pivot point (swing arm). At least that is how I see it. If the hand analogy doesn't do it for anyone, try a car model with four springs and a suspension. I can see no physical reason for the rear suspension to stop deflecting once the inside rear lifts.
09-22-2005, 10:29 AM
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Re: (descartesfool)
claude, damper pots would be the ultimate in knowing what happens to that outside rear. analyzing photographs is a less ideal way of doing it, but is still valid.
like i said before, lapsim agrees with my conclusions on roll center displacement/migration. iirc you've used the program extensively yourself. so for the moment, lets assume that both lapsim and myself are correct on rear rc location post wheel lift.
going back to our buddy newton, we know that for every force there is an equal and opposite force. post wheel lift we know that the outside rear is carrying 100% of the load (or some other very high percentage if we assume there might also be some longitudinal load transfer which i won't get into since i can't make it make sense in my head). for it to compress further you would need some force to act upon it. in pure roll we assume no longitudinal acceleration, so the force can't come from there. there is no more load to transfer either, so the force can't come from there. barring a bump i don't see where any other external force can act to compress the outside rear.
if we assume the rear rc is not actually at the outside rear contact patch or a point very near it, then your argument is valid. it all comes down to figuring out where that rc migrates to. does anyone know if lapsim uses a force based analysis for determining rc migration? or is it geometric?
nate - not sure if that helped the discussion or not
like i said before, lapsim agrees with my conclusions on roll center displacement/migration. iirc you've used the program extensively yourself. so for the moment, lets assume that both lapsim and myself are correct on rear rc location post wheel lift.
going back to our buddy newton, we know that for every force there is an equal and opposite force. post wheel lift we know that the outside rear is carrying 100% of the load (or some other very high percentage if we assume there might also be some longitudinal load transfer which i won't get into since i can't make it make sense in my head). for it to compress further you would need some force to act upon it. in pure roll we assume no longitudinal acceleration, so the force can't come from there. there is no more load to transfer either, so the force can't come from there. barring a bump i don't see where any other external force can act to compress the outside rear.
if we assume the rear rc is not actually at the outside rear contact patch or a point very near it, then your argument is valid. it all comes down to figuring out where that rc migrates to. does anyone know if lapsim uses a force based analysis for determining rc migration? or is it geometric?
nate - not sure if that helped the discussion or not
09-22-2005, 10:45 AM
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Re: (solo-x)
Claude - I tried to do with my hand what you said to...talk about Voodoo Physics!
Nate - it's possible that Nothing will help the discussion.
Scott, who's glad to see some views on Chris' thread - how many of them came from here though?
Nate - it's possible that Nothing will help the discussion.
Scott, who's glad to see some views on Chris' thread - how many of them came from here though?
09-22-2005, 11:06 AM
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Re: (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR »</TD></TR><TR><TD CLASS="quote">As you've probably heard: all the rear wheels do is hold up the back of the kaa.
Scott, who's all excited just thinking about the potential weight savings...</TD></TR></TABLE>
I am reminded of a photo.
Scott, who's all excited just thinking about the potential weight savings...</TD></TR></TABLE>
I am reminded of a photo.
09-22-2005, 11:29 AM
#74
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Re: (solo-x)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote »</TD></TR><TR><TD CLASS="quote">Why would the moment arm suddenly disappear? The lateral force has not disappeared, the cg has not moved appreciably, the spring has not locked and the outside rear is still free to deflect more or less, so what would cause it to stop moving and defy Newton's laws. This would be Voodoo physics.</TD></TR></TABLE>
It does not suddenly disappear but it gets smaller and smaller as the roll axis get closer and closer to the outside wheel. Think of 2 wheels (a bike), the roll axis is going to be right through the 2 tire patches. What is the roll axis of a 3-wheeler? it is going to to through the lone wheel on the front, and through some point between the rear 2 tires. How can your have any weight transfer to the lone front wheel from a side force?
It does not suddenly disappear but it gets smaller and smaller as the roll axis get closer and closer to the outside wheel. Think of 2 wheels (a bike), the roll axis is going to be right through the 2 tire patches. What is the roll axis of a 3-wheeler? it is going to to through the lone wheel on the front, and through some point between the rear 2 tires. How can your have any weight transfer to the lone front wheel from a side force?
09-22-2005, 12:16 PM
#75
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Re: (577HondaPrelude)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by superpilun »</TD></TR><TR><TD CLASS="quote">577, I sketched some 3d force body diagrams and all my results are in agreement with your analysis. In the steady state mode, all forces must balance out eachother in the x, y, and z direction. In addition, all moments about the x, y, and z axis must balance. Otherwise, when you went in circles on a skidpad the car would start doing rolls, somersaults, and cartwheels (which it doesn't). If we consider only a pure lateral force (along x-dir) then the only forces the tires are balancing are x-dir to balance the centrifugal force, load transfer in z-dir to balance moment from the lateral forces which sum to weight. When the outside rear load equals rear weight (inside rear lifted). There cannot be any more load shifted from the front tires to the rear because it would result in a pitch moment when there are no longitudinal forces to balance. These would have to come from accel or deccel to allow this to happen.</TD></TR></TABLE>
Or...the pitch moment is balanced by the vertical forces from the suspension = front/rear load transfer.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 577HondaPrelude »</TD></TR><TR><TD CLASS="quote">It does not suddenly disappear but it gets smaller and smaller as the roll axis get closer and closer to the outside wheel. Think of 2 wheels (a bike), the roll axis is going to be right through the 2 tire patches. What is the roll axis of a 3-wheeler? it is going to to through the lone wheel on the front, and through some point between the rear 2 tires. How can your have any weight transfer to the lone front wheel from a side force?</TD></TR></TABLE>
A bike is a completely different machine, which different degrees of freedom. The front wheel of a three wheeler does not follow an arc in the lateral plane (it only moves vertically), so that too has different DOFs. The vehicle will roll along an axis that the suspension allows.
The entire vehicle is not a rigid body, so even if there is one contact point, the sprung mass only "knows" that there are 4 forces where springs are connected and now one of them is zero. It moves according to how these forces "tell" it to. You need suspension lock for a full rigid body analysis at least in the rear.
I have pot data for a FSAE car, and there is outside wheel compression even during wheel lift, which indicates that the roll center is not at the outside tire patch.
Modified by GSpeedR at 4:41 PM 9/22/2005
Or...the pitch moment is balanced by the vertical forces from the suspension = front/rear load transfer.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 577HondaPrelude »</TD></TR><TR><TD CLASS="quote">It does not suddenly disappear but it gets smaller and smaller as the roll axis get closer and closer to the outside wheel. Think of 2 wheels (a bike), the roll axis is going to be right through the 2 tire patches. What is the roll axis of a 3-wheeler? it is going to to through the lone wheel on the front, and through some point between the rear 2 tires. How can your have any weight transfer to the lone front wheel from a side force?</TD></TR></TABLE>
A bike is a completely different machine, which different degrees of freedom. The front wheel of a three wheeler does not follow an arc in the lateral plane (it only moves vertically), so that too has different DOFs. The vehicle will roll along an axis that the suspension allows.
The entire vehicle is not a rigid body, so even if there is one contact point, the sprung mass only "knows" that there are 4 forces where springs are connected and now one of them is zero. It moves according to how these forces "tell" it to. You need suspension lock for a full rigid body analysis at least in the rear.
I have pot data for a FSAE car, and there is outside wheel compression even during wheel lift, which indicates that the roll center is not at the outside tire patch.
Modified by GSpeedR at 4:41 PM 9/22/2005