1969 Torino

[FETorino]

Originally Posted by Ron Sutton

I really need to expand my terminology. I’m behind.

Does “Jiffypop” mean the wheel center is crowned out … or flush with the outer edge of the wheel … or both?

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Don't worry about learning a one off term from the Ford guy.

With the big brakes and calipers everyone want to run on these cars all (most) the centers crown out for clearance. The obvious exception is the concave spoke wheels. So that I'd say is a given.

To me JMO, Just my term, when I see that crowned center with a high offset wheel with no outer lip it reminds me of popcorn about to pop. This really becomes noticeable to me when the outer edge of the center protrudes beyond the inner step on a step lip wheel.

This obviously doesn't apply to flat lip wheels but for me, as nice as they look in some styles, those don't exist.

[Ron Sutton]

Originally Posted by carbuff

I'm loving reading this kind of detail! Thank you Rob and Ron for having most of this discussion 'publicly'.

I do have a question about the C6 spindle. I wasn't aware that the C6 ran that much caster. Can you explain a bit about the effect of running that much caster, or perhaps asked different, if you run less caster with the C6 spindle, what would the effect be?

My setup uses that spindle, but I've been told to target 6.5 - 7.0* of caster, and my recent alignment check shows 6.8* and 6.2* (I haven't had it aligned fully yet, but that is with camber dead on at the moment). While I realize my setup is different, it supposedly uses pretty close to C6 geometry points.

Keep the techie stuff coming! I'm learning from all of this too!

Okie Dokie Carbuff ... here goes ... but it is so involved it takes 3 posts.

Most everyone knows camber, caster & KPI/SAI work together, but most don’t really understand HOW they work together & how they affect each other. I’ll do my best to explain it, but we’ll need to peel the onion one layer at a time, so bear with me.

For those that don’t know what this is, KPI stands for King Pin Inclination & SAI stands for Steering Angle Inclination. They mean the same thing.

KPI was a term coined back in the day of solid front axles when spindles actually used king pins. Steering Angle Inclination is a more correct modern term & is calculated simply by running a theoretical line through the upper & lower ball joints & comparing that angle to the actual spindle pin the hub spins on (rolling axis in the photo). (I use both terms because many race car guys are used to the older term of KPI.) See photo.


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[Ron Sutton]

Guys,

I'm getting conflicting information on the C6 spindle KPI angle. So take that into account when reading #1234 & #1235.

If the KPI #'s I have been given are incorrect, I will correct the KPI # & the equation in these posts and let everyone know.

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Part 2 of 3 ... answering Carbuff's question about caster & KPI.

First … think of caster as “dynamic camber” … since caster has no affect on angle of the tires & wheels … until you turn the steering. Then caster is tipping the top of BOTH tires towards the inside of the corner you’re turning into (Good).

KPI angle is important to keep the scrub radius lower. You can look at the illustration in the previous post & imagine how big the scrub radius would be if the KPI was straight up & down through the ball joints.

Think of KPI as “dynamic camber” … since it also has no affect on the angle of the tires & wheels … until you turn the steering. But unlike caster, it is not tipping both tires towards the inside of the corner you’re turning into. KPI is tipping the top of the outside tire out towards the outside of the corner you’re turning into (BAD) and tipping the top of the inside tire in towards the inside of the corner (Good).

When the KPI/Caster Split favors the KPI … the tire & wheel, on the outside of corners, goes into a state of positive camber (BAD) … rolling over on the outside part of the tread and sidewall of the tire … with the inside part of the tread becoming unloaded. Basically, at this point, the actual tread making contact with the pavement (contact patch) gets narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had a full contact patch.

Now let’s talk about the tire on the inside of the corner. Some cars roll so much the inside suspension goes into a “droop” or state of extension … and if that car has negative camber gain built in … the droop actually helps the inside tire stand straighter. For cars don’t roll as much … and that compress the suspension on the inside tire & wheel when cornering, the negative camber gain on the tire on the inside of the corner is tilting that inside tire the wrong way. It is rolling over on the inside part of the tread and sidewall of the tire … with the outside part of the tread becoming unloaded. Also making the contact patch narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had more contact patch.

So your front tires that were already at their limit of grip … just lost a significant amount of contact patch & essentially got narrower … and lost even more front traction … creating a push or understeer condition.

The amount of dynamic camber loss is minimal with slight amounts of steering input on large sweeping corners, but grows exponentially worse with higher rates of steering input (front wheel steering angle) on tighter corners. More caster would help both situations … creating more dynamic camber the correct way for both tires … keeping the tire contact patches flatter on the track surface. But how much is enough? Read on.

Let’s start with understanding how spindle KPI/SAI works. Let’s use a C6 based spindle with 9.15 degrees of KPI/SAI. If you were to set both the caster & camber at zero … and rotated the spindle 90 degrees each direction … the difference would be 2x the KPI/SAI angle … so in this case 18.3 degrees.

We know the wheels don’t turn anywhere near 90 degrees, but this example makes everything more clear. Please humor me & follow along closely, because I’m about to share something that is one of the most overlooked keys to proper cornering set-up. We will account for the ACTUAL steering turning radius later.

If you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 9.15 degrees of camber loss (goes into positive camber). Bad … very bad for the outside tire of a corner.

If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel also experience 9.15 degrees of camber loss (goes into positive camber). But this good for the inside tire of a corner.

Reminder, we not turning the wheel 90 degrees in the real world, so don’t lock in on the numbers “too much” … just the concept.

Caster is different. If we set caster at 9.15 degrees positive (top to the rear) & leave KPI/SAI out of the equation, as if we had a spindle with zero KPI/SAI … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 9.15 degrees of camber gain (goes into negative camber). The right direction for the outside tire in a corner.

If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experience 9.15 degrees of camber loss (goes into positive camber). And this is the right direction for the inside tire of a corner.

So … caster helps both the inside & outside wheel & tire.

Here’s the most important piece of info to know at this point. It is the first & most important key to getting the front tires to use their full contract patch when cornering … increasing front end grip & turning speed. Drum roll please …

Caster offsets KPI/SAI on the wheel & tire on the outside corner … and compounds (adds to) KPI/SAI on the wheel & tire on the inside corner.
Read that again. It’s very important.

This is called KPI/Caster Split. When the Caster & KPI are equal … the caster offsets the negative effects of the spindle KPI on the outside wheel ... and compound the advantages of the KPI on the inside wheel. When the KPI is greater than the caster (unless the car has a TON of Camber) the outside wheel is going to lose camber as the steering is turned & roll over on the outside front tire. Ugly.

The greater the split favoring the KPI, the worse the problem. On the other hand if the KPI/Caster split favors the caster … meaning the caster is slightly greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires stay flatter to the road, have more grip, better turning & higher corner speeds.

Sooo … if we set the car up using spindles with 9.15 degrees of KPI/SAI and 9.15 degrees of caster … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 0 degrees of camber gain or loss.

Frankly it is zero, no matter what degree you rotate it to the front, because 9.15 degrees of caster counteracts … or neutralizes … the 9.15 degrees of KPI/SAI.

If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experience 18.3 degrees of camber loss (goes into positive camber). This is the right direction for the inside tire of a corner … way too much ... but we’re not turning 90 degrees. We’re turning somewhere from 0 to 25 degrees.

What if the wheels were turning 15 degrees? … that’s 1/6 of 90 degrees … times 18.3 … equals 3.05 degrees … the right direction.

So … at this point ... we have:
IF Tire +3.05 degrees (Good)
OF Tire +0.0 degrees (OK)

You’re probably going “Hmmmm” … but we don’t have the whole picture yet.


We have a lot of other geometry to factor in. Remember, we’re peeling this onion a layer at a time, so we’ll get to camber gain, chassis/body roll angle & static camber in steps.

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[Ron Sutton]

Guys,

I'm getting conflicting information on the C6 spindle KPI angle. So take that into account when reading #1234 & #1235.

If the KPI #'s I have been given are incorrect, I will correct the KPI # & the equation in these posts and let everyone know.

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Part 3 of 3 ... answering Carbuff's question about caster & KPI.

Camber gain & chassis roll angle are next. Chassis roll angle hurts the contact patch of both tires. Camber gain (towards negative) helps the contact patch for your outside tire & hurts the contact patch for the inside tire.

If you worked out your camber gain to be 1.5 degrees negative “in dive” on the outside tire … and add that to the combination of your caster & KPI/SAI angle of zero … and factor in the car has a chassis/body roll angle of 2.0 degrees … you would still end up with 0.5 degrees of positive dynamic camber (Bad).

Assuming you have a modern low roll angle suspension … to achieve this chassis/body roll angle of 2.0 degrees … the inside tire, of this car in the same corner, is compressed, but not as far, so it doesn’t have as much camber gain towards negative (reminder: camber gain towards negative is bad on the inside tire).

Let’s say we end up with 2/3 the travel & end up with 1.0 degrees negative camber gain (the bad direction for the inside tire) … and add in the 2.0 degrees of roll angle … makes the inside tire with 3.0 degrees of negative dynamic camber (Bad). Then we factor in the 3.05 degrees of positive dynamic camber (Good) … provided by the combination of caster & KPI/SAI … and we end up with the inside tire at 0.05 degrees of positive dynamic camber (Good).

So … at this point ... dynamically we have:
IF Tire +0.05 degrees (Good)
OF Tire +0.5 degrees (Bad)
Not optimum yet, but we’re going the right direction & we’re not done yet.

The next layer of the onion is static camber. You need SOME static camber … to help with initial steering turn-in responsiveness. Just don’t get greedy. In road racing or AutoX where you’re turning left & right, static camber is like camber gain. It helps the contact patch on the outside tire & hurts on the inside tire. For this example, let’s add 1.0 degrees of static camber.

Now with static camber added … with your car hard in the corner … suspension in dive, wheel turned 15 degrees for a tight corner … we have:
IF Tire -0.95 degrees (Bad)
OF Tire -0.5 degrees (Good)
Not optimum yet, but we’re going the right direction & we’re not done yet.

Now, here is another part I love. You simply add caster until the contact patches of both tires are flat & happy. And from this point the math is easy.

Add 1.0 degree of caster and …
IF Tire +0.05 degrees (Good)
OF Tire -1.5 degrees (Good)

Add 1.25 degree of caster and …
IF Tire +0.30 degrees (Good)
OF Tire -1.75 degrees (Good)

Add 1.5 degree of caster and …
IF Tire +0.55 degrees (Good)
OF Tire -2.0 degrees (Good)

There are many factors that will define your optimum set-up, but this creates a baseline that is darn close.

**P.S. I like to end up “around” 1-2 degrees more dynamic camber on the outside tire, since it is loaded so much more.

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There is a little more involved in this, when you involve exact steering angles for specific corners. The good news is … when you run tighter corners requiring more steering angle than 15 degrees … the caster increases the dynamic camber to help the tires maintain flat contact patches. I use a spread sheet I developed to plug in all the info & know exactly what dynamic camber I have at different steering angles & different camber gain & different suspension travels.

The best way I have found to work out a front end setting is to start with KPI/SAI & caster … then bring in camber gain … and finally static camber … to achieve the optimum dynamic camber for BOTH tires. Regardless of how you get there ... all of these geometry pieces need to work together in harmony to achieve full, optimum contact patches for both front tires in hard cornering situations … for optimum cornering grip & speed.

It probably is clearer now why getting advice on one setting that worked for a buddy’s car … without knowing the whole picture … can be misleading. As a tuner, I couldn’t imagine setting the caster without knowing the spindle KPI & the car’s camber gain … and then of course testing on track with tire crayon on the edges every run (plus taking tire temps).

It’s been said a zillion times. It’s the whole package, not one part or one setting.

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Remember the KPI/Caster Split concept ... if the caster is slightly greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires have more grip, better turning & higher corner speeds.

When I'm designing a front suspension for a specific class with rules on what spindle we can run, I pick the best spindle available under the rules and design everything else to either fix or compliment that spindle. Factory spindles usually have a lot of KPI/SAI ranging from 7-10 degrees. When I have to run a factory spindle ... I know I'm going to end up with 1-2 degrees of caster more than the KPI.

I designed & raced NASCAR Modifieds with factory GM #2 spindles with 8.75 KPI. 10-10.25 degrees of caster produced awesome results. We had a crew chief go off the range with set-ups & try 6-7 degrees of caster, but the cars always pushed in mid corner ... and snapped loose on exit. He was used to running less caster, but didn't take into account the KPI of the spindles we had to run.

The whole combination of KPI/SAI, caster, caster gain, camber, camber gain, Ackerman, toe, steering ratio, etc. ... ALL have to be designed together for optimum cornering performance. All of them are important, but the KPI/Caster Split is critical & often not fully understood.

When I'm designing a front suspension with no rules on what spindle we can run, I design the spindles & have them built. Then I'm not trying to fix anything ... and everything else in the front suspension can be designed to compliment that spindle. For a road racing car, I designed the spindle with 3 degrees of KPI/SAI ... and designed the rest of the the front suspension around what is called a "zero scrub" set-up ... & we ended up with 4.0 degrees of caster for optimum handling. This car did NOT require a high caster number, because the spindle KPI was lower. What is optimum for tight cornering is having the KPI/Caster Split slightly favoring the Caster.

Another successful car I designed with 5 degrees of KPI/SAI ... ended up with optimum handling with 6.5-7.0 degrees of caster ... depending on the track. Again, the key was the KPI/Caster Split slightly favoring the Caster.

This higher amount of caster seems odd to most mechanics & street car guys, but when you look at the newer Corvettes you'll see they run a lot more caster than what most think of as "typical" in the 1.5-3 degree range. The Factory GM specs for the C6 ZR1 is 7.7+ degrees of caster, with a 9.15 degree KPI spindle. The KPI/Caster Split is LOW, but still slightly favoring the KPI by 1.45 degrees. But owners that compete & win in the C6's increase the caster to 9.5-10.5 degrees of caster ... with the KPI/Caster Split favoring the Caster.

The common denominator is we are almost always running more caster than KPI/SAI ... if we can. We run spindles with lowerr KPI where we can too ... but it requires running wheels with DEEP back spacing to get the scrub radius low (or sometimes zero). So simply ordering a spindle with smaller KPI is NOT a bolt-on solution.


Make sense?
Got questions? ... chime in.


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[Sieg]

Thanks Ron

For detail and the sprained brain

[Ron Sutton]

Originally Posted by Sieg

Ron - Here's my track width data:

Measured outer sidewall to sidewall front and rear. Tire width is sidewall to sidewall. Nitto NT-05's IIRCC - Dale's running Nitto NT-01 255/40/17, 275/40/17.

Front: 69.0625" 245/40/18 tire on 8" rim measures 9.5"
Centerline Track = 59.5625"

Rear: 70.875" 275/40/18 tire on 9.5" rim measures 10.875"
Centerline Track = 60.00"

Hi Sieg,

Your rear TW is 7/16" wider than the front, so that will contribute a very small amount to freeing up your car in the corners.

The 12% wider tires in the rear will contribute a larger amount to tightening up your car in the corners.

Both of those items are just two pieces of the bigger puzzle.

If ... your front geometry is like most PT cars with the KPI/Caster split favoring the KPI significantly ... that would contribute the largest amount to the car's handling being unbalanced & tight/pushy on tight corners ... until the front geometry is corrected or the rear suspension tuned to reduce grip to free the car up.

On the other hand, if you're not running track days with tight corners or AutoX, it may not matter. I surely don't know your car or situation, so I don't want to make any unfounded assumptions.

...

[Sieg]

Ron,

I meant Rob - as he requested my data in a PM...........

But Ron, thank you very much

I have a spare stock subframe sitting in the garage that I plan to do on a budget somewhat "Smokey" style and this information greatly helps the quest.

My current setup is somewhat goofy as I feel it handles much better than it should, but I've yet to get it to the track and exploit the short-comings. FWIW my only road course experience is on two wheels.

Again thank you for sharing this information, it is very much appreciated.

[carbuff]

Wow... Ask what I thought was a pretty simple question, and I got an amazingly detailed answer! Thank you for taking the time to explain all of that! I've read it twice, and I'm still absorbing it, but your explanation made sense and was very well presented!

I think the next thing I need to figure out is what scrub radius I actually have now. It sounds like that would be one of the next big factors in the equation...

Thanx again!

[carbuff]

On the subject of caster, I do have another question...

I've heard that changing the amount of caster has an impact on the 'feel' of the steering. I've also heard that caster is often used in a street alignment to offset the irregularities that we may experience when driving down the road.

Can you touch on either or both of those, particular how either would relate to the KPI / SAI as you explained them above? Is there a crossover point, for example, on how increasing / decreasing the caster will affect the steering 'feel'? I ask partly because if that's true, I wonder if you can hit a point where we might 'overwork' the steering rack we are using? In other words, I would guess that if we make a change to the caster that causes us to have to use more force to turn the wheels, are we possibly putting more pressure on the rack, and risk damaging it?

I'm thinking out loud with that question, so perhaps it's irrelevant... But in my case, since I'm using a Ford rack that was likely never built to take much abuse, I wonder if that can become a problem?

[Ron Sutton]

Originally Posted by Sieg

Ron,

I meant Rob - as he requested my data in a PM...........
Oops! No worries.

But Ron, thank you very much
You're welcome.

I have a spare stock subframe sitting in the garage that I plan to do on a budget somewhat "Smokey" style and this information greatly helps the quest.
Hmmm. Where are you located? I have some ideas, but would need to show you in person on the clip.

My current setup is somewhat goofy as I feel it handles much better than it should,
Well that's cool.

but I've yet to get it to the track and exploit the short-comings. FWIW my only road course experience is on two wheels.
Cool. I love bikes but only raced motocross as a kid, never road courses. I'm sure it's a thrill.

Again thank you for sharing this information, it is very much appreciated.

Take care.

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