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PostPosted: Mon Mar 17, 2008 3:53 pm 
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Miatav8,MstrASE,A&P,F wrote:
Why so much kpi? Do you need that much for the wheel offset and tires your using to eliminate the scrub radius? I was suggesting how you could reduce the kpi in a previous post.

The more kpi you have, the more the camber will change when turning. It may be fine for a vehicle like a suv with a high cg, a lot of body roll, and power steering, but less is more for a locost.


If you go too low on KingPin Inclination angle you won't have any steering feel or communication with the road, especially on a light car... and you'll end up with too much scrub radius (disturbances will kick the steering)... and the car will lack self centering (primarily provided by KPI, NOT caster angle).


================... KPI ......... caster

Lotus Elise........................ 12° ......... 3.8° caster
Corvette C5, C6................. 8.8° ...... 6.5° caster
Pinto/MustangII/Wilwood... 11°
Ferrari 512BB............... 9 or 13°
ATS AFX Tall..................... 8°
Triumph Spitfire................ 7° ..........4.7° caster
Lotus Seven..................... 9° .......... 5° caster
Mazda Miata................... 11.3° ........ 5° caster
Ferrari F355................... 13.16°
McLaren F1...................... 9° ........... 6° caster ...... 16.25mm scrub

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PostPosted: Mon Mar 17, 2008 4:58 pm 
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Where most of the stability comes from on production cars is not the issue. There are several ways to make the steering self-center, and
increasing the scrub radius by using more kpi is not one of the better ones.

Having kpi is a result of packaging, including a specific wheel offset and tire diameter.

Miata's are designed to have zero scrub radius. The miata kpi, in combination with it's odd-ball 45mm wheel offset and tire diameter accomplishes this.

Zero kpi with a 8 inch wheel could be 7 inches of back spacing, which isn't practical. If wheels that are wider than what came on the upright are used and those wheels place the wheel centerline further inboard than the original wheels, then the location of the upper joint can be moved to keep the scrub radius zero. It makes no sense to use a kpi greater than necessary for zero scrub radius for stability, when that stability could be gained from more caster.

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PostPosted: Tue Mar 18, 2008 1:23 am 
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Alright. You guys have scared me talking about scrub radius. So, 1 hour of google and wikipedia later, I think I know what's going on. First: zero scrub radius, under hard braking, causes the suspension to be skittish because varying road conditions created varying amounts of torque (both positive and negative) around the steering axis. Therefore, some amount of scrub radius, positive or negative, is preferred. Having a small amount of scrub radius, under hard braking, generates a small bit of torque to a predictable side. While this torque isn't desirable, it's predictable and relatively not-affected by changes in road conditions, which makes the steering smooth.

Now, positive or negative. From what I understand, negative scrub radius is used on front wheel drive cars. This is because in the case that one tire blows out, or the brake fails, or one wheel is on ice, a positive scrub radius will create a very significant torque around the steering axis, which will jerk the wheel out of the driver's hands. This is the principle behind torque steer, it seems. Positive scrub radius is, in theory, used in RWD cars. However, it seems as if negative scrub radii are being used in many RWD and even some rear-engine cars. I can't seem to figure out why though. Any ideas?

I played with the geometry in solidworks, and re-designed by knuckle attachment. Since i'm limited by the weld-in nut, my KPI angle is around 16 degrees. With the RSX stock tires, this gives me a negative scrub radius of 15.6 mm. I don't know whether this is good or bad. I doubt that I can decrease the KPI angle any more. I could add more offset to the rsx's 45 mm offset to decrease the scrub radius, but I don't think I can get it positive.

Any comments?

Sports-car, thanks for the list of KPI angles. You wouldn't happen to have the scrub radii for any other cars, would you?


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File comment: The KPI angle
KPI angle.JPG
KPI angle.JPG [ 75.39 KiB | Viewed 9958 times ]

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 Post subject: KPI
PostPosted: Tue Mar 18, 2008 7:46 am 
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The 16* KPI will make for heavy steering and will compromise your camber gain while cornering. The increase camber could be a problem with low profile tires. Most builders should have a KPI between 8* and 12* depending on the choosen caster. Typical the more caster you use the less KPI is needed.
Your scrub is more influenced by the wheel offset that you select. Work out the design end for the KPI and caster first. If the scrub intersection falls within all but the outer 10% of the contact patch, from the center line of the tire to the edge of the tire, you should be OK.
When designing a suspension system, you are always stealing from Peter to pay Paul. No body has a perfect system, but most seem to work.

Good luck on the design Dave W
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PostPosted: Tue Mar 18, 2008 8:43 am 
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proximacentuari wrote:
Sports-car, thanks for the list of KPI angles. You wouldn't happen to have the scrub radii for any other cars, would you?


RE: 16° KPI... As long as you're fabbing the upper half of the upright, maybe you should do the whole thing with 10ish degrees. :P Does the Civic by chance have a bolt-on axle stub at the rear? (that you could use along with its hub)

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scrub.jpg
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Last edited by SportsCarDesigner on Thu Nov 19, 2009 10:49 pm, edited 2 times in total.

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PostPosted: Tue Mar 18, 2008 9:09 am 
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The kpi used on a fwd is a way of compensating for the torque steer, which is a result of differences between axle lengths and angles of the joints as a result of the different axle lengths, as well as the transaxles movement on its mounts.

Use the kpi required for zero scrub based on your wheels and tires. It will not be zero. Allan Staniforth specifically states that zero kpi is a desireable thing, though impractical. IMHO, spliting the difference between the caster and kpi is the best solution.

FYI, I put aftermarket wheels and tires on my wifes miata (with powersteering) which had a 35mm offset, creating a 10mm positive scrub. I took them back off because the steering was quite annoying, pulling left and right over uneven surfaces. A increase in kpi could have zeroed the scrub.

(Edited to indicate positive scrub instead of negative offset.)

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PostPosted: Tue Mar 18, 2008 1:10 pm 
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A mazda engineer who worked on the miata project, Norman Garett describes in his miata performance book why the miata has zero scrub, and how changing the scrub makes the car less pleasant to drive.

My own locost is using oem wheels and spindles with much lower profile tires that increase the scrub radius. My kpi is 15.19 degrees. unlike your spindles, it would be difficult for me to correct it.


http://www.caesionline.com/wheelalign/s ... tions.html

Edited to remove my included angle math, which was wrong.

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Last edited by Miatav8,MstrASE,A&P,F on Wed Mar 19, 2008 7:35 am, edited 2 times in total.

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PostPosted: Tue Mar 18, 2008 1:30 pm 
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The difference between positive and negative scrub radius is a stability thing. If you hit a puddle at speed on one side of the car, with negative scrub radius the torque on the steering will steer you away from the puddle. Which balances the effect of drag on one side of the car. With positive scrub radius the effect is to tug the car into the puddle, requiring much higher driver involvement and also competence to maintain control.

So, my understanding is that Porsche used positive scrub on the 911 cars, but for the 928 they used negative. Reflecting their idea of the drivers likely to buy the cars.

It seems to me that with zero scrub, you would have less feel to the steering? So you should have some of one or the other so you can sense some forces on the front wheels.

So MV8, do you think it's best to have zero or is that just a good choice for Miata along with other tradeoffs?

It's not something you want a lot of, I have seen rigs with too much go into the woods on hillclimbs. So slapping any width/offset wheel on a car can hurt you...

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PostPosted: Tue Mar 18, 2008 8:36 pm 
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I have a stack of Carroll Smith's books. He doesn't dwell on kpi and scrub radius, but he does state that kpi is normally 6 to 8 degrees. He goes on to say that increasing scrub radius is unlikely to have any beneficial effects on the steering.

Scrub radius is analogous to bumpsteer. Like bumpsteer, scrub is feedback through the steering wheel. Less feel from scrub is a good thing. If you want more feel, use a numerically lower ratio rack.

Herb Adams states that scrub radius should be reduced to the minimum and by minimizing it, there will be an improvement in handling, control, and steering effort.

Norman Garett states that scrub radius is not dangerous, but it is "tiring and unnerving". "Mazda spent millions of dollars developing the Miata suspension to avoid exactly this kind of bad behavior."

I am inclined to agree with these fellows. I don't feel there is any worthwhile reason to add to the scrub radius on the front of a rwd car. On my car, I've decided not to modify my spindles and see how it goes. I have not determined what the tbird's scrub radius is, so at this point, I don't know if I am making it worse or better. I will be checking it.

The 928 has a Weissach-axle, with links that flex intentionally, causing toe-in during braking.

BMW has a complex dual lower balljoint suspension which moves laterally and longitudinally when steered. They call it a "double linked strut".

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PostPosted: Tue Mar 18, 2008 9:57 pm 
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Thanks MV8, that's all good info. I don't think adding to the scrub radius is all that great an idea. When you figure out what a T'bird had it would be nice if you let us know.

I can still remember the feel ( dating myself here ) on my '78 Ford Fiesta. It had a somewhat auto-magical way of recovering or adapting to hitting puddles or slush/snow at speed. The scrub on the two sides balance against each other, but when you hit a resistance on one side - negative scrub will adapt, but positive scrub needs to be corrected by the driver. Having to correct would be tiring.

So it seems the feel provided by scrub compared to caster would be different. With scrub you would feel the drag on a wheel, but not with castor.

You would get feel of the road surface going in a straight line. In a turn you would get some easing of steering effort with positive scrub?

I didn't know you should design for zero, but figured it should be a small number. Is this the biggest issue with say running smaller wheel/tire diameters at the tack vs. street?

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PostPosted: Wed Mar 19, 2008 2:27 am 
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Apologies for extending this thread hijack Proximacentuari! But this is a subject that I'm currently considering. Having seen that the Elise runs 12deg KPI was a bit of a surprise. I had assumed that it would be lower as -I had read in Milliken:

"with no KPI (and no caster) there is no camber change with steering lock. As KPI is increased (but still no caster) the wheel will "lose" camber with steer lock, or in other words it will change in a direction giving positive camber on the outside wheel. As caster is added, this modifies the affect of KPI. With positive caster and no KPI, the wheel gains negative camber on the outside wheel and positive camber on the inside wheel. Thus caster can add favorable camber angle to the affect of KPI. In other words, the reason that low KPI is desirable is that KPI subtracts from the negative camber gain due to caster on the outside wheel." (quote from section 17.5)

If you consider a design with 10deg KPI and 10deg caster, when the wheel is turned through 90 degrees you'd have the sum of:

10deg positive camber as a result of the KPI
10deg negative camber as a result of the caster

The net camber would be zero. Of course we'd never apply 90 degrees lock, I've read that the maximum steering angle see on a track is around 8 - 9 degrees. At 9 degrees you'd get about 10% of the camber change produced by the combination of KPI and caster.

So if we had 0 KPI and 10 deg caster, and applied 9deg of lock (instead of 90) we'd see about 1 degree negative camber.

Based on this, and other readings, I was intending to design a front upright (knuckle) and wishbones that:

minimises KPI
allow caster to be altered
allow camber to be altered
sacrifices zero scrub to achieve minimal KPI

My target is to try and achieve -2.5 deg negative camber on the outside wheel in a 1G turn and compromise the amount of negative camber present when running straight (i.e. ideally there would be 0deg but -1deg would be acceptable).

I'd be interested to read other peoples thoughts on this approach.

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PostPosted: Wed Mar 19, 2008 6:57 am 
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Miatav8,MstrASE,A&P,F wrote:
The kpi list above appears to be the included angle. The kpi plus the caster equals the included angle. For example, a older miata has a included angle of 11.75. Subtracting the recommended caster setting of 4.44 will give you the kpi, which is 7.31 degrees. I haven’t looked up any others.


The angles I've quoted are KPI, plain and simple... the angle a line through the ball joints makes relative to vertical viewed in the YZ plane (i.e. viewed from the front of the car).

Caster is the angle a line through the ball joints makes relaive to vertical viewed in the ZX plane (i.e. viewed from the side of them car).

Subtracting the caster from the KPI makes no sense.

You seem to be trying hard to fit theory to an anecdotal experience. Fitting 35mm offset wheels to your Miata should give you the same general situation as the Elise, F1 and C5 shown. If you have problems with it, look elsewhere for answers; it's not likely the +10mm scrub radius at fault. Gordon Murray ain't stupid.

A former Lotus chassis engineer explained that keeping the (static design) scrub radius a little bit positive is better than zero because it keeps the state the same as the tire squirms from normal road irregularities and minor steering corrections... you don't want the scrub radius constantly flopping from pos to neg as you drive, alternately loading the joints in different directions when you just want to go straight. Makes sense to me.


Miatav8,MstrASE,A&P,F wrote:
I have a stack of Carroll Smith's books. He doesn't dwell on kpi and scrub radius, but he does state that kpi is normally 6 to 8 degrees. He goes on to say that increasing scrub radius is unlikely to have any beneficial effects on the steering.

Scrub radius is analogous to bumpsteer. Like bumpsteer, scrub is feedback through the steering wheel. Less feel from scrub is a good thing. If you want more feel, use a numerically lower ratio rack.

Herb Adams states that scrub radius should be reduced to the minimum and by minimizing it, there will be an improvement in handling, control, and steering effort.

Norman Garett states that scrub radius is not dangerous, but it is "tiring and unnerving". "Mazda spent millions of dollars developing the Miata suspension to avoid exactly this kind of bad behavior."

I am inclined to agree with these fellows. I don't feel there is any worthwhile reason to add to the scrub radius on the front of a rwd car. On my car, I've decided not to modify my spindles and see how it goes. I have not determined what the tbird's scrub radius is, so at this point, I don't know if I am making it worse or better. I will be checking it.


Changing rack ratio can't magnify a signal that you've eliminated. In any case, scrub radius is dynamic. The Miata only has zero INITIAL scrub radius .

King pin inclination causes the contact patch to move outward when you steer off-center; to a positive scrub radius situation. So it's not zero scrub radius vs. non-zero... it's zero migrating to positive, positive migrating to more positive, or negative migrating to less negative and possibly crossing over to positive. Which of these three situations is constant in sign?

Always keep in mind, when reading textbooks, not just the authors' credibility, but also the era, environment, and practices at the time of writing. I've had, and frequently re-read, a (first draft, hand corrected) copy of the works of Maurice Olley ("father of vehicle dynamics"... brilliant guy) since 1974, twenty-five years before the Millikens were given permission to publish them. I've worn out Carroll Smith's books and gone back for seconds. I worship these guys, but temper their words with the realization that they were only the best understanding at the time. For example when someone rails against scrub radius it's helpful to know that it's in the context of many production cars with 4" (made parking easy with manual steering... roll the tire around the steering axis rather than twisting a patch) and race cars with 6" at the time. That 6" is very bad doesn't necessarily make 0.6" worse than zero. Be aware that what applies to a race car with massive downforce and front tires a foot wide may not make sense for a car with neither... different situations call for different tradeoffs.

I'd point out that none of the engineers involved with the four examples I gave were hindered by using donor car parts, nor were they limited to existing wheels available from Tire Rack; they could manufacture any geometry, KPI, scrub radius, caster they want... they don't lack performance knowledge; they are closely linked to the (factory effort) experience from multiple F1 WCs, multiple LeMans overall and class victories, etc. They have test tracks, instrumentation, contracted test drivers, and the means to test anything they think up. When they build their highly visible "halo" high-performance road cars they all settled on KPI around 10°, caster around 5°, and three out of four have significant positive scrub radius. I hate to discourage thinking for oneself, but I can't ignore the conclusions of the aforementioned either.

Don't assume everything you read is right... I've seen chassis engineers and tire engineers ridicule Herb Adams' book (quote: "I view it as the absolute worst book on suspension design ever written. I figure the fewer people read it, the fewer will get hurt.). :shock: My copy is hidden away somewhere in storage, so I haven't looked at it for years (not sure what it is that upsets them), but I'd be a little more kind... he was racing at a time of rapid change 40 years ago when no one really quite understood what they were doing (compared to now, when the best halfway understand almost half of what they're doing)... and trying to boil down all the complex interactions to simple explanations does tend to produce rubbish.

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 Post subject: Caster and KPI
PostPosted: Wed Mar 19, 2008 7:23 am 
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I'm NOT trying to flame anyone, But the KPI adds camber to the outer wheel in a turn and subtracts it from the inner wheel. The caster angle has a direct affect on how much increase the KPI influences the camber change. There are several reason to alter these angles depending on the suspension systems total camber gain in jounce and rebound. If you have to little gain or not enough, you can increase / decrease the system by adjust the KPI and Caster. In Racing systems a lot of times these set-up adjust can be made relative easily, but for most Locost builders we will have a fitted KPI so if you have a choice, look for a spindle in the middle range of the KPI's.

Good results from 5.5* caster & 11.3* KPI Dave W


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PostPosted: Wed Mar 19, 2008 8:22 am 
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We are talking about the static scrub radius. Yes the scrub changes as the wheels turn, so there is still feedback with zero static scrub. Do you really want the steering to turn on it's own if one wheel goes over a bump?

We are talking about vehicles that are more closely related to a miata than a F1.

I mistakenly subtracted caster instead of camber from the included angle, and I have corrected that.

I don't have time to get into this right now, more later.

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PostPosted: Wed Mar 19, 2008 10:17 am 
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Gee, I hope nothing I say here is considered flaming... just stating my opinion and understanding. Vehicle dynamics and what goes on in those floppy black rubber things is horribly complex. That's why the books are thick and have disagreements. It's also why I would lean toward proven geometries. Concentrating on one aspect is probably going to have a downside overall. Such is the fun and the curse of engineering.

Just to be clear, the example is the McLaren F1, not an F1 McLaren. Although expressly and purposely NOT designed to be a race car (different tradeoffs), it did win LeMans overall in 1995 in nearly stock form, including, presumably, :wink: KPI.
Image
After 1995, rules creep, and Porsche and Mercedes buildng cars designed to be pure race cars, forced McLaren to build more-modified versions, but they were still very much based on the road-going chassis/suspension.

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Last edited by SportsCarDesigner on Thu Nov 19, 2009 10:55 pm, edited 1 time in total.

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