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Scrub radius - how do you measure it?
Wingnut - 22/1/08 at 12:46 PM

Can anyone offer any opinions?

I'm trying to measure scrub radius on the car, I understand you take the point where he line of the king pin intersects with the ground but where do you go frm here??

Is there such a thing as a negative scrub radius?

Thanks in advance!

britishtrident - 22/1/08 at 01:08 PM

quote:
Originally posted by Wingnut
Can anyone offer any opinions?

I'm trying to measure scrub radius on the car, I understand you take the point where he line of the king pin intersects with the ground but where do you go frm here??

Is there such a thing as a negative scrub radius?

Thanks in advance!



Really just the distance from that point to the central point of the tyre contact patch.


Two schools of thought on measuring scrub, radius, the classic way is as above to measure at ground level.

However as the important effective of scrub radius is steering kick back and feel the most realistic aproach is to measure the radius at track rod end height.

Some fwd use negative scrub radius to offset torqusteer.

CaptainJosh - 22/1/08 at 01:29 PM

quote:
Originally posted by britishtrident
Some fwd use negative scrub radius to offset torqusteer.


Errr, how is that supposed to work?

Torque steer comes from having two unequal length drive shafts, a certain amount of torque ( only tiny mind you ) is lost from twist in the longer shaft, resulting in an unequal amount of force on either wheel, meaning steer.

Now the scrub radius adds to the effect, as the steering force created acts on a radius, creating more steering torque ( torque = force*distance ).

In the perfect situation, there would be no scrub at all, almost completely dialing out the effect of torque steer ( torque = force*0 ). Negative scrub sounds... silly.


Sorry to hi-jack your thread wingnut, but i just wanted to get rid of some confusion.

britishtrident - 22/1/08 at 06:45 PM

No -- basic engineering mechanics - torque can't get "lost" by transmision along a shaft every action must have an equal and oposite reaction.

[Edited on 22/1/08 by britishtrident]

britishtrident - 22/1/08 at 06:51 PM

CV joint

Note relative positions of balls transmitting the torque couple,

[Edited on 22/1/08 by britishtrident] Rescued attachment 180px-Simple_CV_Joint_animated.gif
Rescued attachment 180px-Simple_CV_Joint_animated.gif

CaptainJosh - 22/1/08 at 09:51 PM

quote:
Originally posted by britishtrident
No -- basic engineering mechanics - torque can't get "lost" by transmision along a shaft every action must have an equal and oposite reaction.

[Edited on 22/1/08 by britishtrident]


You are correct, i obviousely worded myself wrong. The torque is distributed unequally, creating torque steer, happy?

An austin mini has almost equal length driveshafts and as a result has for intensive purposes, no torque steer- just to make a point.

Wingnut - 23/1/08 at 12:09 AM

quote:

Really just the distance from that point to the central point of the tyre contact patch.



I see, thank you. So is there a difference between the centre of the tyre & the centre of the contact patch??

Centre of the tyre seems easy to find but what about centre of the contact patch??

TIA!

britishtrident - 23/1/08 at 07:36 AM

quote:
Originally posted by Wingnut
quote:

Really just the distance from that point to the central point of the tyre contact patch.



I see, thank you. So is there a difference between the centre of the tyre & the centre of the contact patch??

Centre of the tyre seems easy to find but what about centre of the contact patch??

TIA!


Centre line of tyre is fine --- unless truly crazy ammounts of camber are used.

Don't get hung up on measuring scrub radius lowering the scrub radius it is a general aim rather than something like camber or toe-in that must be very accurate.

One thing to watch out for is mismatched wheels result in different scrub radi across the vehicle, even a amall diffeence will result in the steering snatching violent to one side under braking (and in FWD undere power).

A low scrub radius is most important on off road vehicles it is not unkown for kick back to the steering wheel from ruts in the track to break a drivers thumbs.

britishtrident - 23/1/08 at 07:55 AM

quote:
Originally posted by CaptainJosh
quote:
Originally posted by britishtrident

An austin mini has almost equal length driveshafts and as a result has for intensive purposes, no torque steer- just to make a point.



On a completely flat smooth road with the steering straight ahead driven torque to each wheel will be absolutely equal but on vehicle with unequal length drive shafts such as an early base model MK1 Fiesta the driver will still experience torque steer.

Torque steer is a steering effect caused within the CV joint. The cause is the differences angle to the cv joint (when viewed in front elevation) of the two drive shafts.
It is easy enough to demonstrate in about a page of maths but life is too short.

Interestingly I didn't realise it until today that due to the nose rising under acceleration a FWD car with soft suspension will have more torque steer than a car with hard springs limited suspension tavel in droop.

Ians - 23/1/08 at 05:18 PM

Having just built a awd motorcycle engined hillclimb/sprint car where its design dictated the use of unequal length driveshafts I completely removed all the torque steer by using neg scrub offset and a nil droop front suspension . Ians