KPI/SAI vs Caster vs Scrub Radius/Wheel Offset vs Track Width

[DevGittinJr]

Assuming that these cars can only be driven through a corner as fast as the front end is capable of, logically one wants to maximize front grip and then tune the rear to match. That said, I'd like to hear from those of you who understand SAI/caster ratio vs scrub radius, specifically pertaining to the s197. First of all I'd like to know if it even matters. The common trend, with this chassis, seems to be to reasonably lower it and throw as much negative camber (using c/c plates), tire width, and wheel rate at it as possible. Is dialing in the front-end geometry not worth the extra effort? I understand that most likely camber bolts and c/c plates are required to adjust SAI, negative camber, and scrub radius; and, that camber bolts affect tire/strut clearance. My stuff is all pretty much adjustable, tall ball-joints, bumpsteer, 3" wheel studs, etc., so I'm not really interested in a discussion about hardware, so much as one about tuning goals. If we're satisfied with 7-8 degrees of caster on these cars, about what would our target SAI be to keep the tires doing good things (outside negative, inside positive camber) in a corner? Low SAI relative to caster keeps both tires "happy" in corners, but how much can it be lowered before adding too much scrub radius for the given wheel offset? And roughly what's too much scrub radius? At what point does track-width become a priority? If we make the tires happy in corners with SAI/caster ratio, could we run less negative camber since it does "bad" things to the inside tire, dynamically? Everything I've found on the subject is based on the wheels being turned to 90 degrees. We know that never actually happens, but about what angle is the most to which our wheels would be turned? Since it's all relative, I'd like a discussion about your experience/knowledge on how all of it relates. Statically, it's easy enough to get most of these angles, but I'd like to learn more about what our cars are doing dynamically (i.e. caster/camber gain), as well. I'm interested in all things tuning that are beneficial to front tire grip, no matter how small an improvement.

 

[Boone]

Subscribed. I like the way you think. Applied theory = reality!!!

 

[Sky Render]

It's a McPherson strut setup; there's only so much you can do. Wider tires invariably increase the scrub radius, and you can only increase the caster by a bit, so there's not much dynamic camber gain to be had. I've not seen any way to alter kingpin inclination without converting to a short/long arm suspension.

Altering the rear suspension does make a significant change in the handling characteristics. I think I've got more done to my rear suspension than front.

Sent from my Samsung Galaxy Note 4 using Tapatalk

 

[Norm Peterson]

Here's a link to Ron Sutton and a couple of ways to get to Mark Ortiz taken from the header to the latest issue of his "Chassis Newsletter".

http://www.ronsuttonracetechnology.com/

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: [email protected]. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.


Norm

 

[DevGittinJr]

Subscribed. I like the way you think. Applied theory = reality!!!

I'm way more of an "artsy" type than engineer and in way over my head, here. I had to look up what "applied theory" even means.

It's a McPherson strut setup; there's only so much you can do. Wider tires invariably increase the scrub radius, and you can only increase the caster by a bit, so there's not much dynamic camber gain to be had. I've not seen any way to alter kingpin inclination without converting to a short/long arm suspension.

Altering the rear suspension does make a significant change in the handling characteristics. I think I've got more done to my rear suspension than front.

Roughly how much is "not much" camber gain? I'd think even a degree or so would make a difference. Unless I've got things boogered up, on our cars the range is mall, but KPI/SAI is adjusted with c/c plates. Then add negative camber with bolts. Whenever camber is changed with c/c plates, so is SAI. I haven't installed the bolts but I have them, c/c plates, and plenty of tire/strut clearance. I've got as much caster as possible and was going to dial out some of the negative camber I have now, using my c/c plates, to lower SAI a bit and get it closer to stock. It'd only be a few degrees lower, as the wheels have 1" less offset than oem so SAI needs to be a bit more than stock to keep the scrub radius low. I'd then use the bolts to dial back in the negative camber.

This is how I understand it so please correct me if I'm wrong, but you can't add front grip by altering the rear suspension. Meaning, rear grip can be increased or reduced relative to the front, thus changing the overall balance, but the amount of front grip wouldn't be affected.

Here's a link to Ron Sutton and a couple of ways to get to Mark Ortiz taken from the header to the latest issue of his "Chassis Newsletter".

http://www.ronsuttonracetechnology.com/

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: [email protected]. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.


Norm

I've read through all of the free Ron Sutton stuff I could find. Very good info. I'll have to look into buying some of his books/ workshops. Thanks for the heads up about Mark Ortiz - looks like tons of good info there!


I guess more specifically for s197's though, I'm wondering roughly how much scrub radius should I be after. Front track width is about 63.4" (20x9 +24mm). I've got spacers and 3" studs so scrub can be changed a bit, separately. But I don't really want to lower the track width if possible. Assuming I can lower SAI by 3-4 degrees while keeping all other settings but scrub radius the same, about how much positive camber will that add to the inside front wheel, and how much negative to the outside, dynamically?

 

[Norm Peterson]

Roughly how much is "not much" camber gain? I'd think even a degree or so would make a difference. Unless I've got things boogered up, on our cars the range is mall, but KPI/SAI is adjusted with c/c plates. Then add negative camber with bolts. Whenever camber is changed with c/c plates, so is SAI. I haven't installed the bolts but I have them, c/c plates, and plenty of tire/strut clearance. I've got as much caster as possible and was going to dial out some of the negative camber I have now, using my c/c plates, to lower SAI a bit and get it closer to stock. It'd only be a few degrees lower, as the wheels have 1" less offset than oem so SAI needs to be a bit more than stock to keep the scrub radius low. I'd then use the bolts to dial back in the negative camber.

The Hunter site (the alignment equipment folks) used to have an SAE paper filled with geometric derivations that were basically used to describe how to determine caster from camber.

I haven't seen or looked at that paper in a while, but IIRC the relation back to camber from caster plus steering goes something like a cosine function of the angle the wheels were steered times the static caster setting. IOW, adding a degree to the 7.1 or so isn't going to change things a whole lot until you're turning circles that are pretty tight (I'm too lazy to run any numbers at the moment). I'm not saying more caster would be wasted effort here, only that unless timed competition is involved it may not be worth the trouble. You would almost certainly have to address changes in bumpsteer.

This is how I understand it so please correct me if I'm wrong, but you can't add front grip by altering the rear suspension. Meaning, rear grip can be increased or reduced relative to the front, thus changing the overall balance, but the amount of front grip wouldn't be affected.

Depends. If you have a lot of rear grip (and slip angle) that you simply can't get at because the front is limiting you a good bit sooner, it is possible to help the front without losing any ground out back by shifting the balance of LLTD rearward (springs, bars, geo-roll center heights, damping, compliances). But you might have to drive a bit differently, maybe with smoother or slower throttle application on exit.


Norm

 

[Sky Render]

Just because you add rear grip doesn't mean you somehow lose front grip or are unable to fully utilize the added rear grip.

 

[Norm Peterson]

The amount of front grip definitely is increased if you've got enough headroom on rear grip and slip angle to attract more of the LLT rearward without going loose.

Trying to use all of however much surplus rear grip exists for longitudinal acceleration (regardless of whether it was already there to begin with or if more was added in later), and counting on that to bring the rear slip angles back into balance with front tires that are still operating at their original grip level is inherently an unstable situation. It can be controlled - see drifting for this in the extreme. But that doesn't make it the fastest way around a corner or the best for driver confidence running in session traffic.


Norm

 

[DevGittinJr]

The Hunter site (the alignment equipment folks) used to have an SAE paper filled with geometric derivations that were basically used to describe how to determine caster from camber.

I haven't seen or looked at that paper in a while, but IIRC the relation back to camber from caster plus steering goes something like a cosine function of the angle the wheels were steered times the static caster setting. IOW, adding a degree to the 7.1 or so isn't going to change things a whole lot until you're turning circles that are pretty tight (I'm too lazy to run any numbers at the moment). I'm not saying more caster would be wasted effort here, only that unless timed competition is involved it may not be worth the trouble. You would almost certainly have to address changes in bumpsteer.


Depends. If you have a lot of rear grip (and slip angle) that you simply can't get at because the front is limiting you a good bit sooner, it is possible to help the front without losing any ground out back by shifting the balance of LLTD rearward (springs, bars, geo-roll center heights, damping, compliances). But you might have to drive a bit differently, maybe with smoother or slower throttle application on exit.


Norm

Rather than raising the 7.1 degrees of caster, since that can't be done, I'm looking at lowering SAI a few degrees (from 12.8 degrees) by moving the tops of the struts closer to where they where stock. Then add back in the lost negative camber at the spindle with the main goal being the added a bit of positive camber-gain to the inside tire. Of course it'd also add negative camber-gain to the outside tire. But lowering SAI also raises the scrub radius which could be solved by changing the front wheel offset (+/-spacers), but offset also affects track-width. It sounds like I may be looking at the point of diminishing returns, for my application, and I've drifted into some shit that's way above my scholastic aptitude. However, I'm leaning towards playing with it anyway, as I can at least measure static camber, before and after, on both sides with the wheels turned at a few different angles. I'll just have to wait and see if or how much scrub radius and track-width are negatively affected.

I guess you can tune the rear to add front grip. But if you shift the LLTD rearward with wheel-rate and geometry changes at the rear, wouldn't you be effectively lowering available rear grip the further away you get from the ideal rear settings, in order to accommodate that rearward load shift? I'm not understanding how you do that without reducing rear grip. Having to wait to get back on throttle on exit would seem to mean there's less available rear grip as well.

Just because you add rear grip doesn't mean you somehow lose front grip or are unable to fully utilize the added rear grip.

I agree that you can add rear grip without reducing front grip, and obviously the added rear grip will affect the overall balance of the car. If a car can only travel through a turn at the maximum speed the front end is capable of sustaining, and the car has more available rear grip relative to the front, how can one utilize all of the available rear grip?

 

[DevGittinJr]

The amount of front grip definitely is increased if you've got enough headroom on rear grip and slip angle to attract more of the LLT rearward without going loose.

Trying to use all of however much surplus rear grip exists for longitudinal acceleration (regardless of whether it was already there to begin with or if more was added in later), and counting on that to bring the rear slip angles back into balance with front tires that are still operating at their original grip level is inherently an unstable situation. It can be controlled - see drifting for this in the extreme. But that doesn't make it the fastest way around a corner or the best for driver confidence running in session traffic.


Norm

Ok, so you are taking away some rear grip to achieve this.

 

[Mark Aubele]

This is a very interesting conversation, but unless you plan on reengineering the entire front suspension, you are way overthinking things. Getting as much negative camber as you can possibly get (like -3.5 to -4*) is going to improve the grip much more than worrying about how much positive camber "gain" (it will still be negative) the inside tire will have. It is practically along for the ride with any kind of bar on these cars.

Scrub radius is important from a feel standpoint, but it is obvious from looking at fast track cars that it is all thrown out the window to get grip (much wider rubber, vastly increased track).

 

[El_Tortuga]

This is a very interesting conversation, but unless you plan on reengineering the entire front suspension, you are way overthinking things. Getting as much negative camber as you can possibly get (like -3.5 to -4*) is going to improve the grip much more than worrying about how much positive camber "gain" (it will still be negative) the inside tire will have. It is practically along for the ride with any kind of bar on these cars.

Scrub radius is important from a feel standpoint, but it is obvious from looking at fast track cars that it is all thrown out the window to get grip (much wider rubber, vastly increased track).

This. Scrub would be so much less of a consideration compared to tire size and big camber. Diminishing returns unless you have huge funds and wide open rules

 

[ddd4114]

Unfortunately, I have little to add, but I definitely appreciate the thought you're putting into this. I tend to agree that it's over-analyzing the situation a little too much, but I think it's a good way to gain understanding of how the suspension setup affects the car.

I'd be careful about increasing camber at the spindle instead of the shock tower. There is very little clearance between the shocks and the wheels, and in my experience, it's easy to make them contact by inclining the spindle. With "skinny" 275mm tires, I can only offset the top bolt by ~1/8" before having rubbing issues. With the camber plates maxed out, this gives me ~3.5 deg of camber, but of course, your results may vary.

I also agree with Norm that by adding rear grip and shifting LLTD rearward, you will increase total grip (all else being equal). Changing LLTD doesn't necessarily reduce grip at one axle; it simply shifts the distribution of load transfer (as the name implies). If you adjust LLTD to optimize total grip and then add rear grip, you're not able to take advantage of it before the front is overloaded. Therefore, shifting LLTD rearward allows you to utilize the grip of both axles better. It might feel like the car is understeering, but unless it's really bad, you can drive around it - possibly with the help of some shock tuning for corner entry.

 

[Norm Peterson]

I'd be careful about increasing camber at the spindle instead of the shock tower. There is very little clearance between the shocks and the wheels, and in my experience, it's easy to make them contact by inclining the spindle. . . . but of course, your results may vary.

May vary, indeed. I considered myself lucky with 11" wide front wheels and 285/35 tires to not have to dial OUT any negative camber at the strut/knuckle joint like at least one other person had to. You're looking at about 1.2mm clearance at both the tire and the wheel (and it took an 0.025" shim-thickness spacer to get that much on one side of the car).

Norm

 

[DevGittinJr]

This is a very interesting conversation, but unless you plan on reengineering the entire front suspension, you are way overthinking things. Getting as much negative camber as you can possibly get (like -3.5 to -4*) is going to improve the grip much more than worrying about how much positive camber "gain" (it will still be negative) the inside tire will have. It is practically along for the ride with any kind of bar on these cars.

Scrub radius is important from a feel standpoint, but it is obvious from looking at fast track cars that it is all thrown out the window to get grip (much wider rubber, vastly increased track).

This is in line with what I've seen/read for these cars. Big tires and lots of camber. However, most of the suspension books, tech articles, white papers, etc. I've read aren't vehicle specific. I was wondering how much of that stuff applies here, and it sounds like some but not all of it. If I'm understanding correctly, you're saying to get the widest wheels and tires possible with offsets that keep them under the car. Set up the front end to accommodate the tire width and as much negative camber as possible. There's nothing to be done to the geometry (outside of surgery) that will yield the same grip with less tire and/or camber.

With just lowering the car, camber was at -1.2 degrees. The c/c plates were only good for about -.8 degrees. They're at -2 degrees with no room in the strut tower to go further, as the other side actually touches the frame.

20x9, 265/35, +24, with close to 2" of strut/tire clearance. It looks like plenty of room to get more tire and camber at the spindle. Playing with 18x10.5 and 295's on a wheel/tire calculator online, it looks like I could get them under the fender and still only be 3/4" closer to the strut.

 

[Sky Render]

Dubs?

Seriously, though. I had less than a quarter inch of clearance with 18x10s. I'd love to see how you're still going to get that much clearance with 10"-wide wheels.

 

[DevGittinJr]

Dubs?

Yo soy acepto. Bad choices were made. Now that there's finally a reasonably priced step-lip wheel set available that will work (clears brakes, custom offsets, widths, etc.), 18's are on the short-list.

 

[Champale]

You don't have to apologize for dubs, yo. Ferrari 458/488, Porsche 991 GT3 (RS has 21" rears!), Nissan GT-R, etc have 20 inch wheels and no one complains about them in the slightest. Mustang world is weird (cheap?) in that regard.

I get tired of the "you have to have 25.5" Lifesaver-sized tires to be trak kool!" stuff.

 

[Mark Aubele]

People are critical because there is zero reason to run them. If you have 16" brakes, by all means run a 20".

 

[Norm Peterson]

You don't have to apologize for dubs, yo. Ferrari 458/488, Porsche 991 GT3 (RS has 21" rears!), Nissan GT-R, etc have 20 inch wheels and no one complains about them in the slightest. Mustang world is weird (cheap?) in that regard.

I get tired of the "you have to have 25.5" Lifesaver-sized tires to be trak kool!" stuff.

Even those manufacturers are catering to the "concept car look" to some extent. But just because it helps sales doesn't make it technically 'right'.

There is no need to go with a wheel size much bigger than 4" more than the rotor diameter. On any car. Wheels almost big enough to go on a Kenworth or a Freightliner should maybe be made a little bigger and go there instead.


FWIW, I don't like 27.2" tall tires for anything short of an SUV or maybe a crossover, which are vehicle categories I'm not interested in for enough other reasons already.


Norm