King Pin angle for desert racing???

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atomicjoe23

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What's the normal range of the king pin angle for desert racing trucks/buggies?

I read Herb Adams Chassis Engineering and he suggests running a smaller KPA because the KPA affects the scrub radius which he states should be as close to zero as possible. . .I know the book is written with asphalt going race cars in mind but it's about the only decent suspension design book out there (at least that I'm aware of. . .if there's another one someone please let me in on the secret). . .I'm sure that off-road vehicles have different needs than road going ones, but today is the first time that I actually saw a KPA listed for a desert buggy and it was 14*. . .which to me seems like a lot. . .

So back to my question. . .what's the normal range of the KPA and why?

Thanks!
 
tim_krueger

tim_krueger

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Wheel, Hub and Tire all play a big role in determining the ideal KPI. I'd say 8* (factory king pin i-beam) is at the low end and 14* would be the higher end. Average hub with a standard 17x8 bead-lock and a 37"tire is right around 12*
 
zjohnson

zjohnson

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Like tim said, hub wheel and tire play a big part in the decision process for KPI. The KPI for onroad could be considered for off road, but conditions are completely diff't. For 1, road car operate on a surface where traction is avaliable in ideal surfaces, body roll, along with wheel travel are extremely low in comparison to an off road racecars. Off road racecars ussually operate in low traction surfaces, and sometimes operate in the surface (sand). Because of wheel travel and suspenion setup, body roll is high. One place to look at relative KPI is on pro-am's website. For class 1, they offer 12-16 degrees, many is dependent upon designer preference and hub.
As for my choices . . . KPI is a tool I use to achieve desired suspension geometry. I find other aspects more important in the world of off road racing, such as scrub radius, wheel travel, roll center, trackwidth change, etc . . . Its all a big compromise to achieve desired dynamic characteristics.

Hope this helps
 
motorhead

motorhead

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atomicjoe23 said:
What's the normal range of the king pin angle for desert racing trucks/buggies?

I read Herb Adams Chassis Engineering and he suggests running a smaller KPA because the KPA affects the scrub radius which he states should be as close to zero as possible.
Click to expand...

The steeper the KPA the more lift you will have at the steering arm causing bumpsteer when the tires are turned.

In off road cars, 12-16 degrees seems to be the norm considering ground clearance, wheel back spacing, hub dimensions and upright height.
 
Mark Newhan

Mark Newhan

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I believe that it is important to use all of the enginneering pricipals available when designing a front end. The purpose of KPI is to have the center of the scrub radius in the center of the tire. If you stray to far from this you load the steering components and ultimately the steering wheel when you get feedback. The further negitive offset (or outward spacing) the more leverage on the steering components. So the conventional wisdom has been "make the steering more powerful", which does not cure the issue. The enginneered parts that you purchase from Pro-Am are designed with the wheel and tire application used in off road racing.

A four wheel drive vehicle is very pron to showing an operator of a vehicle the illeffect of improper KPI. Anyone that has driven a Rhino with either wheel spacers or negitive offset wheels can attest to the dramatic effect of "torque steer". It is the same feeling you get with a front wheel drive car when you turn the wheels and stand on the gas. This is the exact opposite of what a two wheel drive feels with the improper wheel offset, which destroys the effects of proper KPI. It is a slow hammering of the steering, which will put an off road race vehicle out of a race.
 
Bajated

Bajated

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Mark Newhan said:
It is a slow hammering of the steering, which will put an off road race vehicle out of a race.
Click to expand...


Amen, we have too much scrub radius on our 7S truck (no choice due to OE pivot points and stock parts required) and it causes damage to our steering racks, so we replace them every few races.

In designing a new front susppension, what's ideal for scrub radius? I've heard that you need a little to be able to feel the front end getting loose. Is 1/2" enough?
 
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atomicjoe23

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I don't know for sure. . .but 1/2" would sound about right. . .like I said before. . .in Herb Adams' (who is a suspension engineer) book "Chassis Engineering" he said you should shoot for as little scrub radius as possible. . .
 
Bajated

Bajated

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Well I can tell you that 2.5" breaks steering racks if you hit a big rock at the wrong time : (

Herb adams used to build camaros right? I think he worked for GM and then started a company called VSE?
 
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