69427

Like the title says, I'm curious what the KPI angle is. I'm just considering doing some suspension geometry tweaks and am curious what the angle is.

Thanks.

Don Keefhardt

It's a long way of saying "caster".

Last car I had with an actual kingpin was an Austin Healey.

Solofast

No, the kingpin inclination angle is defined by a line between the upper and lower ball joints when VIEWED FROM THE FRONT OF THE CAR

Caster is the angle when viewed from the side.

The kingpin inclination angle effects camber as you turn the wheel and bigger kingpin inclination angles result is loss of camber when the wheels are turned to higher steering angles.

Hope that all makes sense...

Hat_Trick_Hokie

For the visually-challanged among us, it's the angle between the yellow (kingpin) and green (vertical reference) lines:

steve J06

Might try calling an alignment shop and see what they have for SAI in the machine default settings. The Hunters I've used have those values pre-programmed, its in the secondary menu. Where SAI - Camber = KI.

69427

I'm doing some additional geometry changes (trying to get rid of a medium speed corner understeer issue) and I need to figure out my (static and dynamic) caster needs, and the KPI angle for some pre-welding calculations (I'm still striking out finding the actual number).

If I can't find it here or on some site I may take Steve's advice and just bother somebody at an alignment shop. If nothing else, it might be the extra motivation I need to get an appointment for my Tahoe to get the front alignment checked.

Thanks.

BrianCunningham

Here's everything I have
http://s242.photobucket.com/user/swi...?sort=3&page=1

I also have a SolidWorks model

theseal

Pretty sure the 88-96 has 13 degrees Kai. 84-87 was about 7.5. If I recall correctly for comparison, c5-6 is about 7.5 also.

69427

I appreciate it.

I've looked all over the 'net for this bit of information, but it almost seems to be a state secret for some reason.

MatthewMiller

I'm raising this topic up from the dead to report new empirical findings. I sold my 96 in February, but I am friends with the new owner. He took it to a local-but-nationally-known shop in town today for an alignment and corner weighting. I suggested that while the alignment tools were on the car, he should have the owner/tech get a number for KPI(SAI). With -3deg camber (which adds a like amount to KPI), they measure 19deg. That's not a typo: I said nineteen. So with a stock setting of 0deg camber (the book calls for plus or minus 0.5deg), that car would have had 16deg KPI. Mother of God, that's a **** ton of inclination! This really drives home why people feel the early C4s had better front ends.

Stock caster calls for 6deg, and I did some things to get a touch over 8. It's not nearly enough, but I have no idea what 19deg caster would do to other things, but I know you can't do it with differential shimming. And even if you could, you wouldn't want to because splaying out the pivot axis for the upper control arms in plan view introduces unfixable bump steer. Ideally, you'd shim them equally and make a whole new upper control arm that sets the ball joint back a lot. My rough calculation was that 1/2" of upper ball joint setback adds 2deg caster. But in the quantities required to neutralize 19deg KPI, you'd probably have the tire too far back in the wheel well. It might require a custom lower arm to move the lower ball joint forward an equal amount as the top moves backward. By the time you do all that, it may make more sense to just swap in an old subframe.

69427

I appreciate the information/update.

I made a few tweaks along the way to improve the front grip, and like you, I wasn't a fan of the differential shimming. I modified the UCA mounts (the C4 suspension UCA and LCA mounts are bolt-ons on my '69, so this Carroll Smith wannabee can make tweaks or correct any design mistakes). I moved the UCA mounts rearward about 5/8" (going by memory), which improved the camber gain during cornering. I'm 75% done with a new lighter (aluminum) LCA crossmember, and just waiting for some track time to decide whether to tweak the LCA location.

BrianCunningham

I have some CAD files I can upload later

Supercharged111

Doesn't caster affect camber linearly? Meaning it has a more pronounced effect at smaller angles. Since the KPI makes a frownie face as it swings through it's arc, it has a smaller effect with smaller angles, but bigger effect when the wheel gets cranked farther. That said, I thought they were only supposed to be a couple degrees apart.

MatthewMiller

Yes! Which is something I hadn't thought about until it was brought to my attention by an acquaintance in the industry. So you can have too much caster. He suggested shooting for a caster value of 2/3 your KPI angle. This is also why KPI/SAI probably doesn't matter much for road racing, where steering angles are small.

Solofast

Exactly!! At lower steering angles it doesn't matter so road racers never saw it as a problem. With later C4's you could feel the front end wash out once you went past a certain steering angle and if you backed off just a bit with steering angle the grip would come back. What was also interesting was that even a bit more caster would make a big difference in where you hit that point. Jeff Glorioso made a set of custom "bushings" for his nationals winning BSP car that moved the upper control arm back about at least a half an inch or maybe a bit more. Our BSP car used the early front suspension, so we didn't bother, but when I asked Jeff if that helped front end bite he got all evasive and stuttered that he didn't think it helped "all that much".. But if you're doing autocross on a late C4 it's a huge help. On stock class cars we used to push the arm as far aft as we could with the stock parts. Even if you got 1/4 of an inch it made a difference that you could feel.

69427

I had that exact same wash out experience big time in the slow tight corners, while the fast sweeping corners were no problem in my'69 (with a '96 suspension). I've since redone the UCA mounts to move the arm mounts back a bit, and am anxious to get back out to a track day to see what level of change or improvement there is.

Thanks for your post. I wasn't aware that this issue was "common" on later C4s, and I just assumed my car's tight-corner understeer was a result of a mistake/error in my fabrication and installation of this suspension into my'69. If nothing else, it did force me to study up a bit more on suspension dynamics to learn how to minimize/rectify this issue.

MatthewMiller

Heh heh. After PMs with you, it's possible I might have some experience in this area. It definitely helps. I was able to get 8.5-deg caster that way. If one didn't care about staying legal for NCCC Group 3, one could use a custom offset upper control arm to move the upper ball joint backward further than that. CAM allows for that. OTOH, swapping to an early C4 subframe and suspension is a definite possibility - it's just a lot of work. It may be that the best combo is an early front end and a later rear end.

CraigStu

Would it be legal to just use some parts from the early C4 especially the spindle? KPI causes a loss of negative camber on the outside wheel as it is steered. Making up for that via caster I think is a real losing proposition. One negative result of increasing caster is that you put negative wedge into the car as the steering angle increases. Think of the LF wheel in a right hand corner. Caster causes that wheel to rise and causes the right wheel to drop. So it takes weight off the LF and adds weight to the RF. If one had a car that understeered badly that could help but my feeling is that >10 deg caster is not a good idea. I would rather spend time on fixing the kpi than trying to compensate for it via caster.

MatthewMiller

It would be legal in any class that also allows custom upper control arms, yeah. The question is can the early spindle and upper and lower control arms just be bolted on to the later subframe and work properly? I really don't know. One would think not, or else lots of people would have done it by now. Maybe the problem is that the mounting locations on the subframe are way different. In that case, maybe custom control arms would allow proper alignment with the old spindle, with a favorable KPI? I don't know that, either. If I still had the car, I'd be looking into it!

The caster jacking effect is certainly a thing. However, given that we're saying later C4s understeer due to KPI-induced camber loss at high steering angles, it's hard to view that as a negative. However, there are other reasons why huge caster is not a great idea. Keep in mind, though, that moving my upper ball joints back 1/2" still only got me 8.4-deg caster, so even if we say 10 is a good limit, there was room for another 1/2" of rearward movement before hitting that limit. There's just not room with stock A-arms and shafts: you run out of room on the shaft and the A-arm also hits the shock tower. An offset arm with the ball joint set back is a better way to do it.

All that said, fixing KPI would be a better option.

BrianCunningham

I put these on mine