rough512

iTrader: (12)

Hello Setup Gurus:

What is scrub radius?

How does it affect steering?

pink pac man

i think its when when the tire flubs over ie:it rolls over itself italso might be when the tire scrapes on the bumper/chassis/bodie once again im not sure

rough512

iTrader: (12)

It has something to do with the distance from the kingpin to the hex hub... but I'm not sure...

I'm wondering about it's effect on steering. I'm using the original Xray T1 where the front pivot balls are very near the center of the contact patch of the front tires.

When the T1 first came out, people were saying that the steering is very aggressive.

Now that most people are using the C-Hubs, the general consensus is that the C-Hubs are better than the Pivot Balls.

Just wondering whether the tire scrub has anything to do with it.

pink pac man

i also use an xray but i use the c hubs sorry i couldent help

seaball

iTrader: (9)

the scrub radius is defined as the distance from the kingpin axis to the center plane of the wheel. as the steering is turned, the wheel swings about from the kingpin axis. the scrub radius quantifies how far from the pivot the arc that it makes is located. i have not played with this, but i would suspect the result or feel is very similar to castor changes. a larger scrub radius will eccentuate what castor is in the car, since the wheel will lift or sink more due to the greater arc it makes. ie. - large scrub radius will give the car better high speed steering, but might unsettle the car during quicker switchbacks? i really don't know.

talk to some karting gurus, as i understand that is a method of tuning that come into play there.

rough512

iTrader: (12)

I'm googling up the term now...

Yes I'm actuall referring to scrub radius. Thanks.

Horatio

There are other geometry considerations on an RC car that have a far greater impact on handling & performance.

On a full size 1:1 car, scrub angles affect the stationary steering effort (ie - when trying to turn the steering wheel whilst parked but off the brakes) and what happens under braking, especially if braking effort to the wheels become unequal.

Negative scrub angles cause a toe-in force and are usually considered safer, as when one of the cars wheels travels through a puddle under braking (resulting in reduced traction/aquaplaning/braking effort) the car's tendancy is to veer away from the puddle. With positive scrub, the reverse is true, as the geometry induces a toe-out force.

With RC cars, there isn't a human holding onto the steering wheel - there's a servo via a servo saver. It's ability to hold it's centre, torque and speed will determine steering accuracy.

Scrub Angles Explained

I've seen some pretty interesting videos on the subject in relation to RC cars, but I genuinely think that the effects are less relevant, due not only to the fact that our steering is electronically controlled via digital servos, but also because our car's control arms/suspension etc are much more robustly constructed (comparitively to 1:1) due to scaling.

DirkW

iTrader: (1)

Holy thread-necromancing, Batman!

Horatio

Guilty as charged!

Blame JQ - I just watched one of his videos on the subject and wanted to clarify some of the voodoo.

Airwave

iTrader: (1)

The wiki article is quite clear about Steering axis inclination and Scrub radius: https://en.wikipedia.org/wiki/Scrub_radius

Horatio

There are quite a few articles, all explaining how zero, positive & negative scrub radius affect full-size cars. Principally, the effects on steering, torque steer, stability under acceleration and braking.

However, it's important on an RC forum for people to understand why it's not a 'thing' that we generally need to tinker with.

JQ explains the difference between c-hib (generally positive scrub radius) and pivot ball setups (generally negative scrub radius). You can see this video here.:


Make of it what you will.

I hadn't seen anything explaining why on RC cars it's much less of a concern.

The forces involved are largely mitigated by modern servos with excellent centering and burly, well engineered suspension.

The only way these theoretical concepts could be put to bed, is pointing GoPro's at the wheels front and rear with the cars under race conditions to see the real time effects of these forces on front & rear toe.

I'm not convinced there's any change, or if there is, it's so small that's it's virtually insignificant compared to the effects of tyre choice, linkage adjustment etc.

Airwave

iTrader: (1)

I'm not it can be applied to buggy cars, but in conventional touring cars, the angle between the wheel and the axle of rotation is null (because of the C-hub architecture). We use "kingpin" only in specific conditions.

A friend of mine use kingpin (i'm not sure if it results in positive or negative scrub radius) on his A800 and the front was more brutal than before, but he also was destroying front tyres twice as fast as the rear ones.

Horatio

The 'scrub radius' won't be the cause of the wear rate of his front tyres. Additionally, it's not normally something that you'd seek to change or note in your setup sheets.

Whether you use a car with C hubs and kingpins, or pivot balls, it applies just as much to off-road as it does to circuit - in either case - not much at all, as it will be near the end of a list of other geometry related things that not only have more of an effect on your car's lap times, but are much more readily adjusted.




The fact remains that it's much more of a 'thing' in 1:1 cars, because the driver is directly part of the steering, Via the wheel. That and the fact that road cars often have to be steered at stationary speeds whilst parking, or made to brake in wet weather - whilst trying to stay in lane ie where braking stability is the highest priority. For that reason, road cars usually have negative scrub radius.



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