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Zero suspension carts - Rage Carts


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#1 meb58

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Posted 03 July 2013 - 20:05

I hesitated before writing this question because the answer at first blush seamed clear...but then, there are so many aspects to suspension design.

I am working with a number of parents with slightly Autistic children who have begun to race Rage Carts. I am unfamiliar with these types of cars but physics is physics...geometry is geometry.

I ask about SAI (and scrub radius) because as I understand it, SAI can help to generate self aligning torque at the contact patch even in straight ahead if not set up for center point steering. I have always assumed that self aligning torque can be a good thing where steering feel is concerned, but, that self aligning torques create friction and thus sap speed. The motor in this type of cart in restricted form produces 6hp, so I believe that tiny differences at 50 mph might be meaningful...these are truly momentum cars!

I assume that if the center line of the bearing is the center line of the tire then we have what I might call center point steering? If my assumption is correct, there is no self aligning torque at the contact patch as long as the tire is pointing straight ahead? I imagine that the center line of the bearing and tire do not line up during a turn due to tire deformation at the contact patch?

The spindles and chassis are designed to flex during a turn and probably add some degree of camber while aiding weight transfer.

As a side note, it's quite incredible to experience 7-10 year old children driving at 50mph in a competitive environment! ...then there are those parents who expect the feed back of an F1 driver from their 7 year old when they don't win...one wonders! ...there's a correlation too...best gear, best car, best data acquisition equipment, most screaming.

Edited by meb58, 03 July 2013 - 20:25.


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#2 rgsuspsa

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Posted 03 July 2013 - 22:30

I hesitated before writing this question because the answer at first blush seamed clear...but then, there are so many aspects to suspension design.

I am working with a number of parents with slightly Autistic children who have begun to race Rage Carts. I am unfamiliar with these types of cars but physics is physics...geometry is geometry.

I ask about SAI (and scrub radius) because as I understand it, SAI can help to generate self aligning torque at the contact patch even in straight ahead if not set up for center point steering. I have always assumed that self aligning torque can be a good thing where steering feel is concerned, but, that self aligning torques create friction and thus sap speed. The motor in this type of cart in restricted form produces 6hp, so I believe that tiny differences at 50 mph might be meaningful...these are truly momentum cars!

I assume that if the center line of the bearing is the center line of the tire then we have what I might call center point steering? If my assumption is correct, there is no self aligning torque at the contact patch as long as the tire is pointing straight ahead? I imagine that the center line of the bearing and tire do not line up during a turn due to tire deformation at the contact patch?

The spindles and chassis are designed to flex during a turn and probably add some degree of camber while aiding weight transfer.

As a side note, it's quite incredible to experience 7-10 year old children driving at 50mph in a competitive environment! ...then there are those parents who expect the feed back of an F1 driver from their 7 year old when they don't win...one wonders! ...there's a correlation too...best gear, best car, best data acquisition equipment, most screaming.


The racing is for the kids' enjoyment. They will not understand nor care a whit about all the technical jargon and tinkering. As for the parents, remind them of same and encourage them not to detract from or spoil the kids' fun.

Ron Sparks

Edited by rgsuspsa, 03 July 2013 - 23:04.


#3 Greg Locock

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Posted 03 July 2013 - 22:34

What do you mean by SAI? Steering axis incliniation, which we'd normally call KPI?

A tire that is upright and aligned with the direction of travel will still generate all sorts of funny forces, ply steer , coning steer, and overturning moment and presumably ply moment and coning moment. These are small in the scheme of things but not zero. So don't be surpised if a slight tweak in toe away from 0 is faster.

The signs and magnitudes of all these things is entirely under the tire makers control.



#4 JimboJones

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Posted 03 July 2013 - 22:45

The signs and magnitudes of all these things is entirely under the tire makers control.


Unless they're Pirellis...

#5 meb58

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Posted 04 July 2013 - 00:20

Yes King pin axis...I sometimes forget that SAI is an American thing.

Well, I've just learned about a few more terms I've not known about...we are going to do a little testing this weekend with toe, oddly enough. Is the idea that a tire that drags a bit over-rides the forces you wrote about thereby potentially organizing forces a bit better - better meaning less friction?


What do you mean by SAI? Steering axis incliniation, which we'd normally call KPI?

A tire that is upright and aligned with the direction of travel will still generate all sorts of funny forces, ply steer , coning steer, and overturning moment and presumably ply moment and coning moment. These are small in the scheme of things but not zero. So don't be surpised if a slight tweak in toe away from 0 is faster.

The signs and magnitudes of all these things is entirely under the tire makers control.



#6 meb58

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Posted 04 July 2013 - 00:22

Poor Pirelli!


Unless they're Pirellis...



#7 Greg Locock

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Posted 04 July 2013 - 04:23

Yes King pin axis...I sometimes forget that SAI is an American thing.

Well, I've just learned about a few more terms I've not known about...we are going to do a little testing this weekend with toe, oddly enough. Is the idea that a tire that drags a bit over-rides the forces you wrote about thereby potentially organizing forces a bit better - better meaning less friction?


Toe more or less counterbalances all the others, that is, by using toe you can reduce the net sidethrust of the wheel to zero, so all the force*v components are small.

That's a sort of practical interpretation, obviously toe can't neutralise all of those forces and moments completely.

Edited by Greg Locock, 05 July 2013 - 02:23.


#8 Greg Locock

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Posted 04 July 2013 - 04:24

I suspect you'd get more lap time out of tire pressures than toe to be honest.

#9 fredeuce

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Posted 04 July 2013 - 22:13

Yes King pin axis...I sometimes forget that SAI is an American thing.

Well, I've just learned about a few more terms I've not known about...we are going to do a little testing this weekend with toe, oddly enough. Is the idea that a tire that drags a bit over-rides the forces you wrote about thereby potentially organizing forces a bit better - better meaning less friction?


I'm not so sure that SAI is just an american thing. I encountered that term here in Oz as an apprentice mechanic back in the 70's . By then front suspensions on cars didn't have king pins any longer so the SAI terminology was being used at trade school at that time. Trucks of course still had beam axles and therefore kingpins so KPI was still used. Now that acronym has been pilfered by those human resource managment boffins to describe "key performance indicators" . Not to be confused.;) ;)

Edited by fredeuce, 04 July 2013 - 22:16.


#10 munks

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Posted 05 July 2013 - 15:23

They will not understand nor care a whit about all the technical jargon and tinkering.


If they're slightly autistic, some of them may actually be interested in certain aspects.

#11 meb58

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Posted 05 July 2013 - 21:43

I agree. We are trying to improve time by working from simple to complex...air pressure is the first bit of data we will begin to experiment with.

We're also using a Go-Pro camera pointed as straight ahead as possible while also capturing the driver's hands and feet...since we don't get good feedback from the kids...and even if they weren't slightly handicapped, 7-10year old children simply have no prior driving experience. We're hoping that the camera helps us to recognize balance, driver's hand movements during turns etc.


I suspect you'd get more lap time out of tire pressures than toe to be honest.


Edited by meb58, 05 July 2013 - 21:43.