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PostPosted: March 19, 2008, 11:46 am 
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SportsCarDesigner wrote:
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.

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MV8
I am suprised you would compare a F1 to a miata. Gordo did not design my car. The fellow that was part of the team that did has provided an explanation. Do you have a better one? Many may say something is bad, but few provide a solution.
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SportsCarDesigner wrote:
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.



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?

--------------------------
MV8
Thank you for making my point. Less kpi equals less migration of the scrub radius, positive, negative, or any combination. This means less static scrub radius is required to prevent “oscillation” between positive and negative, if any static scrub radius is required at all.

The kpi is a compromise. Adding kpi for parallel parking a vehicle with 7 inch wide rims that weighs less than 1,500 lbs should be very low on the list of objectives.
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SportsCarDesigner wrote:
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

-----------------------------
MV8
Those unhindered engineers are still compromising in many ways. A certain size car needs a specific amount of tire for the intended use. That tire needs a specific rim width. There are advantages to maximizing the distance between the upper and lower balljoints, so now we have a spindle buried in a high offset wheel with the joints close to the rim’s inside diameter. The wheel must turn so many degrees. Since we don’t want the control arm links to contact the rim, the distance between the control arm to chassis mounting points is limited, which affects the loads transferred into the frame at those points, affecting how strong the frame must be. Increasing kpi out of necessity due to the wheels and tires makes the job easier. If they have a lot of positive scrub, it is probably because they didn’t want to increase the kpi any more than they already had. The F1 is a perfect example, with its 9 inch wide, high offset front wheels.

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SportsCarDesigner wrote:
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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MV8
Regarding Herb Adams, I am curious as to who you are quoting? Herb does an excellent job of explaining what is going on in layman terms. While his conclusions may not be correct for every possible scenario that a professional engineer could dream up, they don’t need to be to fit the scope of the book.

Our conversation seems to come down to this. You feel there should lots of kpi and scrub radius for their own sakes and not as a result of physical limitations, while I think kpi and scrub should be minimized. I don’t think this is something we can agree on.
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PostPosted: March 19, 2008, 1:55 pm 
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I did some more cad work, And I managed to decrease the KPI by 1 degree to 14.94 degrees. This gives me a negative scrub radius of 11 mm. I don't think I'll be able to do much better than this, although I don't like how close the tire is to the ball joint. Having the ball joint close to the rim is ok, but the tire flexes, as we know all too well.

Sportscar and MV8 - Since I'm going after the At-om design, I'm going to assume that dimensions from two cars, the Mclaren F1 and the Lotus Elise, both 40/60 cars, will work for me. This means a small, but not zero scrub radius (<15 mm, and a relatively large KPI and 4-5 degree caster.

I'll post other other suspension info in my build log


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PostPosted: March 19, 2008, 4:05 pm 
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The less weight on the wheels, the more caster you'll probably want. My mid-engine Mini has about 300lbs on each front tire and I have about 5 deg caster. Doing it over I'd go to more like 6-7 degrees, but it's all personal preference anyway.

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PostPosted: March 19, 2008, 6:34 pm 
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Keep in mind what that negative scrub radius you have is going to do to steering feedback. The scrub radius acts as a torque arm from the center of pressure of the tire (COP) to the steering axis (the line between the UBJ and LBJ).

Your steering "feel" is going to depend on the size of this torque arm and which direction it goes (in or out). If this radius as well as the mechanical trail is right, the car will have a good feel; if either/both are too great, the car is going to be extremely heavy to drive (on a positive note, you'll get a good upper body work-out every time you drive).

Personally, I can see the negative scrub radius causing a light RWD car to want to jerk to full lock under hard braking and cornering. Rather than the driver exerting a force on the wheel to keep the car turned, you will be exerting a force to keep the car from turning; it seems backwards from what you normally do. I guess it will go back to personal preference, but that seems like it could make the car very difficult to drive.

I've always gone for relatively small positive scrub and low KPI. The only thing KPI does is dial in positive camber to the heavily loaded wheel in the turn, which I would consider bad and a reason to minimize it.

On FSAE cars, I've always aimed for 1 degree of KPI just to make sure it never went negative due to build tolerances, camber, etc; 3-4 degrees always ended up being more the reality once brakes, arms, the upright, the hub, and everything else was stuffed into the wheel.

Seeing as these cars have more travel, more body roll, more compliant tires, and need to be safe in a wide variety of conditions (well track and road) unlike the FSAE cars I've done; something different may be better.

For comparison's sake, currently my 7 design has 6.7 deg of KPI, a +1" scrub radius, 3 deg of caster and +1" mechanical trail.

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PostPosted: March 20, 2008, 4:45 am 
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a.moore wrote:
For comparison's sake, currently my 7 design has 6.7 deg of KPI, a +1" scrub radius, 3 deg of caster and +1" mechanical trail.


Andrew in comparison with the values that SportsCarDesigner was good enough to compile and share, your KPI and caster values are at the lower end of the range. Could I ask what is driving this? Are the values being forced on your design by having to use an existing upright? Are you minimising KPI in order to minimise the detrimental effect on camber when the wheel is turned from the straight ahead? None of the above!

Thanks,
Puk

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PostPosted: March 20, 2008, 8:52 pm 
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In my book, if you have low KPI, you need low caster and if you have higher KPI values, it would be appropriate to have higher caster values. I would be concerend if one was large and the other was small since KPI adds positive camber to the heavily loaded wheel and caster subtracts camber (adds negative camber) to the heavily loaded wheel in a turn.

If they're somewhat equal, they somewhat cancel each other out in the camber department. Since I'm doing custom uprights and this theory has seemed to work so far, I'm going to keep rolling with it. The nice thing is my design allows for some relatively easy changes of caster, KPI, scrub, and mechanical trail. I need to get a build log up one of these days.

As far as use, it will be street and track. Since tires in general don't like large camber values, especially positive ones, I'd prefer to minimize whatever would cause that. Eventually, the car will have some sort of r-comp like tire on it.

Puk wrote:
a.moore wrote:
Are you minimising KPI in order to minimise the detrimental effect on camber when the wheel is turned from the straight ahead?


So to sum it up, yes.

On a side note, springs and arb's play a big role in camber as well since they're responsible for controlling chassis movement so there are several factors.

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PostPosted: March 20, 2008, 9:27 pm 
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For track or fast road use, I don't understand your concern about camber change from steering input. In a parking lot with very tight radius turns, sure.


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The only thing KPI does is dial in positive camber


You don't consider lifting the weight of the front of the car a factor? Considering light car, with relatively large and heavy tires/wheels. A good amount of KPI seems sensible...

I see descriptions of people getting tired in their locosts on the highway, from constantly having to be on top of them. So at least some of these seem to be lacking in stability.

Adjustability is good. Best case you try these things out and give us real world data...

Heavy steering is not much fun on the track. I have owned two Formula Fords, the first had much lighter steering and was more fun. I *really* wish I had the numbers on their suspensions now for comparision.

Andrew, how are you going to arrange to adjust your KPI? Seems hard to do...

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PostPosted: March 20, 2008, 10:00 pm 
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You got me; 2 things :) - I forgot about jacking from KPI. I wouldn't consider it particularily good though. The lifting you're referring to isn't going to do much in terms of making the car easier to drive or steer in my book since its going to cause the contact patch to move to the edge of the tire (increase in scrub radius) along with some lateral weight transfer. If it were longitudinal WT, I'd buy into it being able to make the car easier to steer.

Things like steering arm length, steering rack ratios, steering wheel size, mechanical trail (infact I'd say this is the most important), roll center migration, and suspension symmetry are also going to determine how easy/hard a car is to steer and how well it tracks. Its really hard to nail it down to just one item like KPI.

My concern is along the same lines as what you've said. In a parking lot and under high steering angles, you see the greatest change in camber from KPI and caster but under hard cornering, you're also seeing roll, causing more positive camber on the heavily loaded wheel. This cornering G could be from high corner speed or high steering angle; as you turn more though, its only going to make the camber worse. If KPI and caster are low, you will see less of a change in camber.

You could have ideal camber in a parking lot and under light cornering but this roll factored in with a large KPI could cause the outside front wheel to go positive.

Once I get running, I'd defiently be interested in doing some testing to see how stuff actually effects how the car handles.

I'll post some pics of my uprights later.

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PostPosted: March 20, 2008, 11:00 pm 
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above statement
"You could have ideal camber in a parking lot and under light cornering but this roll factored in with a large KPI could cause the outside front wheel to go positive." WRONG

The KPI will increase the neg camber on the outer wheel and decrease it on the inboard wheel in a turn. You would have to have zero caster to go positive camber relative to the KPI influence. With the higher KPI and the typical 5* of caster you can see the spindle actually raise the front of the vehicle while increasing the negative camber of the outer wheel.

That why you want the larger KPI so you do not have to have as high of a camber gain in the suspension system, that starts to effect things like braking and camber thrust, but getting the increase in negative camber on the outer wheel while cornering.

We are all still looking for the perfect suspension Dave W


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PostPosted: March 21, 2008, 1:18 am 
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davew: I think you have the camber effect of KPI backwards. That's what this chop-stick and index card are telling me. Also, I think that was mentioned above. In addition castor subtracts from lifting the car on the outside wheel and adds to it on the inside wheel.

I think the base reason for using KPI instead of zero angle is that it causes the wheel to self center, due to the jacking effect. If a car is parked and you turn the wheel, you can see that your lifting the front of the car up.

I mentioned parking lots because that's where you find turns with a radius closer to the length of your car causing you to need a lot of steering angle. On a track or at speed you shouldn't need more than a few degrees of steering angle. You don't want to have to push the front tires around the turn.

I wonder if Miata's have zero scrub because they used wheels with offset similar to a front wheel drive car.

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PostPosted: March 21, 2008, 7:21 am 
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The miata wheels were not used on anything else in the mazda fleet. I think a standard (if there is such a thing) fwd offset is 35mm. The early miata is 45mm.

A motorcycle with a front fork has no kpi, yet it is stable and has feedback, with it's 1:1 "rack". It is all caster.

Everything I have read says to minimize kpi and that it isn't practical to make it zero, but SCD makes good points too. I need time to do more study and modeling in order to support a conclusion.

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PostPosted: March 21, 2008, 10:50 am 
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I agree if you are just looking at KPI, But as you increase the caster the tilt of the uprights KPI actually increases the lifting affect of the spindle. This means that you increase the self centering effect of the geometry. Only with low caster settings "possible racing set up" would you go over center and then see a decrease in camber on the outer wheel. Most street cars have a much higher caster setting and while cornering you would not be at such a turn angle that you would go over center and see a decrease in camber. From a practical stand point most locost builders would be better off with greater caster and KPI suspension set ups, assuming that they do have big flat F1 tires and want reasonably stable road vehicle.
Sorry no wide profile racing rubber here Dave W


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PostPosted: March 21, 2008, 11:41 am 
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MV*, yes. I really have no idea if KPI is useful. Was just mentioning what it does, a bit. Honestly before this thread I'd never really thought about it, just noted to read up on it. You see castor mentioned much more often.

Motorcycles use a LOT of castor. Numbers more then 20 degrees are coming to mind, but not sure. Yet another vehicle from the 70's buried in the garage.

The numbers used by motorcycles are very specific. I've always wondered about that. For instance, I had a 350 Yamaha and borrowed my friend 400 of the same model. It just about threw me the first turn it came to the feel was so weird. I think it was a only a degree or two different on castor, I don't recollect being able to see the difference.

I am curious though, is there a downside to having some KPI?

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PostPosted: March 21, 2008, 1:33 pm 
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Some KPI gives you more freedom in what you want your scrub radius to be. I personally think this is the main reason we see values like we do on production cars since manufactuers want to make cars people want to and can drive. If they went with small KPI angles, they may have to go with huge scrub radii to allow for everything to be packaged; how else would you get a McPhereson strut to not hit the tire?

MV, I've also read the same thing about KPI.

Davew, when you increase caster, if all other things are kept the same (IE you just lay the upright back), you're also going to see an increase in mechanical trail. Since this is essentially the same thing as scrub radius just fore/aft instead of left/right, this is what is causing the centering more so than the caster.

Try downloading one of these suspension simulation programs listed somewhere on this board. Put in a geometry, set the steering to some angle, and see whats your camber on your outside wheel. Next, keep everything the same and make the KPI 0; note the camber. Now make the KPI greater than the value you originally started with (maybe 5 degrees more); note the value.

I did this experiment last night in Racing By The Numbers. The first tests were done with 3 degrees of caster, 1.5" of rack travel, and 1 degree of chassis roll. The first run was done with 6.7 degrees of KPI and the outside wheel had -0.805 degrees of camber. The second run was with 0 degrees of KPI and had -1.20 degrees of camber. The last run was done with 15.15 degrees of KPI and has -0.36 degrees of camber (the lowest).

I ran the same procedure again but with 15 degrees of caster. I ended up with -4.25 degrees of camber on the test with 6.7 degree KPI, -4.58 degrees of camber with 0 degrees of KPI, and -3.865 degrees of camber with 15.15 degrees of KPI.

In both tests, the lower camber values occurred with the higher KPI value and the highest camber values occurred with the lower KPI value.

As promised, heres my upright design. The aluminum blocks that bolt in to the end of the upright can be swapped out for ones with the balljoint hole in different positions to allow for different KPI angles. The caster is adjusted inboard on the upper arm by having the bolts oriented fore/aft; the rod-ends can be shimmed along the bolts. Its not ideal but its something to start with.


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 Post subject: KPI and caster
PostPosted: March 21, 2008, 2:33 pm 
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Andy

I stand corrected, you test indicates that for every degree of KPI you added to the design of your up right, you loss .048* in camber gain.

Dave W


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