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Scrub and KPI

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Vladmir123 View Drop Down
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    Posted: Sep/24/2012 at 1:41am
Just curious .. what kinda scrub radius and Kingpin do teams use here ? We use ATP Mudlite tires and honestly its a bit hard to package the whole knuckle assembly into it without compromising on either one .. we wanted to go for a fairly moderate 8 degrees of KPI with around -5 mm scrub. But from the looks of it, thats fairly hard to achieve .. Last year we had too much of both and gave us way too much camber gain and torque steer. How do all you guys get around this ?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cujdubs Quote  Post ReplyReply Direct Link To This Post Posted: Sep/25/2012 at 8:35pm
Well the positive camber effects of KPI can be compensated for  by increasing castor.  As for your scrub that seems perfectly reasonable and is probably not where your issue is. I am not really sure under which conditions you encounter torque steer but it is possible that that is due to less than ideal steering rack placement or too high of a ratio if you are using a fast rack with short steering arms. hope this helps
John Hannum
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Vladmir123 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vladmir123 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2012 at 8:34am
is having a poitive scrub a bad thing ? i kinda know the downsides of it but am not sure if v should go for it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2012 at 11:24am
(I can't remember off hand which side is positive) However, when you go hit a water hole or any other obstruction, do you want the wheel to turn into the obstruction or out of it? Think about it for a moment.

 And if its near zero, it is not going to matter much anyways due to how much the tire rolls over anyways.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote cujdubs Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2012 at 2:34pm
Positive is to the inside. To have it outside of the contact patch(or negative) is not only not a good thing but you would most likely need a ton of KPI to get it (a bad thing). 

So many properties effect handling that its hard to really pin down one thing as the source of your woes... especially given the extemely varied terrain that our cars travel over.

The main thing that i have been looking at is maximizing front traction in the most situations as possible because we had problems with low speed maneuverability last year (obviously affected by front suspension as well)

So see if you can make some compromises and find a set of geometries (mainly caster and KPI) that keep your wheels from gaining positive camber when you turn
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Vladmir123 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vladmir123 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2012 at 9:20am
For now we've set up our geometry with +6 mm with around 9 degree KPI and 6 degree caster. i think thats good enough.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2012 at 12:37pm
Scrub is nil. Good job
Mooooore Castor! At least double digits.
Kpi isn't bad.
Just my opinion, and I'm an idiot.
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Vladmir123 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vladmir123 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2012 at 3:10pm
Originally posted by ErikHardy ErikHardy wrote:

Scrub is nil. Good job
Mooooore Castor! At least double digits.
Kpi isn't bad.
Just my opinion, and I'm an idiot.


Hahaha, highly doubt the idiot part.
But why run a higher caster ? our camber gain in roll and bump is decent enough with the current combined effects of caster and KPI. Will the KPI need to increase too to compensate for the caster camber gain ? That comes with its own disadvantages doesnt it ?

Edited by Vladmir123 - Oct/03/2012 at 3:14pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richie_Dagger Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2012 at 11:50pm
Originally posted by ErikHardy ErikHardy wrote:

Scrub is nil. Good job
Mooooore Castor! At least double digits.
Kpi isn't bad.
Just my opinion, and I'm an idiot.

Double digit caster seems a bit extreme, what's your reasoning behind that?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2012 at 8:13am
10-15 degrees of castor is certainty not unheard of in the racing world. There are too many parameters to simply just point at and say ahh, there is your problem when it comes to steering geometry so I probably should not have commented before. However, when it comes to KPI, I've come to figure out that its a good idea to keep the KPI as minimal as possible, limited usually by packaging. In simple terms, KPI fights against your castor upon turning taking away from camber gain induced by the castor, therefore get rid of it. Keep in mind your bump and roll conditions might be negligible when it comes to a tight maneuverability course when you are slowly weaving through trees.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2012 at 10:07am
10-15 is high but certainly reasonable. We have been even higher than that in the past. It all depends on what you want out of the car and how you balance it with all of your other parameters.
Danny

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/22/2013 at 10:00am
hello guys,
I'm using lotus suspension software. I'm designing the front suspension and in the results i get "half track change" of about 40 mm along the wheel travel.
does the half track change means the scrub radius? and if not what does it mean?i want to judge this value (40 mm)
thank you
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richie_Dagger Quote  Post ReplyReply Direct Link To This Post Posted: Aug/22/2013 at 2:12pm
Scrub radius shouldn't change through travel, my guess is it's one half the track width of your car, i.e. when you move one spindle up and down through it's travel, how much car's width changes.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/22/2013 at 8:47pm
so how are scrub radius and track width related? and how cant scrub radius change with travel?!
i know you u have a static scrub radius but kingpin angle and tire vertical axis changes with travel , so accordingly the scrub radius will change?
um still confused if a 40 mm change in half track is a big deal :(
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zmpeck Quote  Post ReplyReply Direct Link To This Post Posted: Aug/23/2013 at 7:30am
I believe you're referring to scrub, which is the chance in track width over the suspension cycle.  Scrub & scrub radius have no relationship to each other.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/23/2013 at 11:28am
yes , i've realized so recently.
so how much scrub usually you can expect to have in an off-road car ?
thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Aug/23/2013 at 1:55pm
Originally posted by ahmed ahmed wrote:


so how much scrub usually you can expect to have in an off-road car ?
thanks

That depends a lot on the suspension type and how important a parameter that is to you.  Trailing arms and solid axles typically have no scrub whereas semi trailing arms or 3 links tend to have a good amount. Dual a arms can vary a lot depending on your other design constraints.

The more scrub, the more speed lost driving over uneven ground and when landing jumps. Front scrub can cause loss of traction while cornering which causes understeer. Rear scrub can cause loss of traction in a corner causing oversteer, or just loss of traction and speed on bumpy courses.

Our 2011 car that took 1st and 3rd had more rear scrub than that and still won. However, that was one of the things we disliked most about that car and it was cut down a ton for 2012.

So in the end it is up to you. Nobody can eyeball that number and say whether that is good or bad conclusively. Less scrub is better, but at what tradeoffs do you draw the line?
Danny

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/23/2013 at 4:41pm
Thank you Soccerdan   
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