That’s basically it. The OE rubber bushings have different durometers so they deform differently under load and provide some designed-in toe change. Poly ones likely don’t.
NC Miata’s are designed to go toe out under corner enters, which is why they have such sharp steering response on turn-in. NDs go a little toe in.
Keith Tanner said:That’s basically it. The OE rubber bushings have different durometers so they deform differently under load and provide some designed-in toe change. Poly ones likely don’t.
NC Miata’s are designed to go toe out under corner enters, which is why they have such sharp steering response on turn-in. NDs go a little toe in.
That explains why my stock NC was so eager to wag the tail. 
z31maniac said:Keith Tanner said:You can have bump steer on the rear. It’s toe change with suspension travel. Not much you can do to change it without moving pickup points. I can’t say I realized it was so fatally flawed, though. All this time we’ve been dealing with junk :)
But the NA/NB Miata does have bushings that are designed to provide toe change under cornering load for stability. If you chuck in a bunch of poly without realizing, you will change the way the suspension works.
Yep. I've just never heard the term "bump steer" used in relation to the rear wheels.
I've heard that. The NC, S2000s, etc. have a toe-link in that can control or cause this. I thought the NA/NB rears were SLA setups parallel to thrust angle and didn't have any change in toe due to travel.
If you tilt the axis of the control arm mounting points up or down you'll get toe change with bump - I think. I've never bothered to measure the toe change in the rear with bump, but based on what I've seen of alignment numbers pre- and post- suspension work it's not a gross problem.
ProDarwin said:z31maniac said:Keith Tanner said:You can have bump steer on the rear. It’s toe change with suspension travel. Not much you can do to change it without moving pickup points. I can’t say I realized it was so fatally flawed, though. All this time we’ve been dealing with junk :)
But the NA/NB Miata does have bushings that are designed to provide toe change under cornering load for stability. If you chuck in a bunch of poly without realizing, you will change the way the suspension works.
Yep. I've just never heard the term "bump steer" used in relation to the rear wheels.
I've heard that. The NC, S2000s, etc. have a toe-link in that can control or cause this. I thought the NA/NB rears were SLA setups parallel to thrust angle and didn't have any change in toe due to travel.
Oh my there is a lot of toe change. Under compression, the rear tires toe in quite a bit.
This is also called "roll understeer" - body roll makes the rear suspension steer the nose to the outside, which is a nice safe negative-feedback loop. It also makes the rearend get awful squirrely under acceleration if there is uneven grip side to side. Not much of an issue with 100hp and stock tires.
Keith, I was unaware that Mazda had bushing kinematics going on in the Miata, but in hindsight, they DID pretty much pioneer it in Japan (VW had been doing it for over a decade before Mazda) so it should come as no surprise. And of course the first thing people did on FCs when they increased grip was to lock all that out.
On the FC, under light cornering, the outside wheel would toe OUT, for sharper turn-in. As cornering loads progressed, it would toe IN for stability. Worked great, if you kept the stock suspension and stock tires. And the car was new.
kazoospec said: Theres's also the cajones factor.
A buddy let me autocross his '60-something Lotus Elan with a hot twin cam on really wide hoosiers one time. Told me ahead of time "there's a line. You won't see it coming until you step over it, at which point it will be too late".
Soooooo much grip right up until there wasn't.
Also the only car I've ever been in where my knees stuck up higher than the tops of the doors.
I know all 88-00 civics and integras used similar bushings and toe links to change toe under cornering. Oddly, those are about the only thing left on my car still not swapped for an aftermarket part. The urethane trailing arm bushings are almost universally reviled on those suspensions due to not deflecting progressively, and causing binding. Most honda guys I know go OEM or full spherical on that particular bushing, while the rest of the bushings get urethane. Rule of thumb is to avoid urethane when the bushing deflects in more than one axis.
My real question about steamrollers is when does the scrub radius become a bigger problem than the increase in surface area?
Nugi said:My real question about steamrollers is when does the scrub radius become a bigger problem than the increase in surface area?
That will depend a lot on the car (as far as how much you have to change offset to fit the wider tires and what the scrub radius is before and after). Strut cars will lose positive scrub or gain negative scrub as you add negative camber up front, so that can let you push the wheels out further without ending up with tons of positive scrub.
rslifkin said:Nugi said:My real question about steamrollers is when does the scrub radius become a bigger problem than the increase in surface area?
That will depend a lot on the car (as far as how much you have to change offset to fit the wider tires and what the scrub radius is before and after). Strut cars will lose positive scrub or gain negative scrub as you add negative camber up front, so that can let you push the wheels out further without ending up with tons of positive scrub.
That all depends on how you are GETTING that negative camber, too. Using eccentric strut ear bolts will give a much different result than using an adjustable strut top.
One of my winter projects for teh RX-7 is to get tons of negative camber at the strut top, so I can take it away at the ears, because of the effect it will have on the scrub radius and the camber curve. As a bonus, if I did the geometry math right, it should also raise the roll center, so I can make the car a little lower without losing turn-in. (The main reason I want the nose lower is not for any kind of handling reasons, I'd just like it to run cooler on the highway, and making the nose lower will force more air through the radiator)
- Suspension Geometry Nerd
Today I learned that you can put a 245/50r16 on a 16x9 wheel and push a 1996 Miata tub off a Uhaul trailer with one person and if you adjust the shocks down to let the tires "just barely rub" use the wheel arches as brakes.
Knurled. said:That all depends on how you are GETTING that negative camber, too. Using eccentric strut ear bolts will give a much different result than using an adjustable strut top.
One of my winter projects for teh RX-7 is to get tons of negative camber at the strut top, so I can take it away at the ears, because of the effect it will have on the scrub radius and the camber curve. As a bonus, if I did the geometry math right, it should also raise the roll center, so I can make the car a little lower without losing turn-in. (The main reason I want the nose lower is not for any kind of handling reasons, I'd just like it to run cooler on the highway, and making the nose lower will force more air through the radiator)
- Suspension Geometry Nerd
Although the downside of getting negative camber at the strut top is that it increases SAI, and more SAI is generally a bad thing - it causes camber to become more positive as the steering moves away from center. I think that getting negative camber at the ears is generally better.
In reply to GameboyRMH :
The difference in camber loss/gain at normal steering angles is, IMO, trivial, compared to what happens to the camber curve and roll center.
Picture how often you have more than 1/2 turn of lock under heavy cornering loads, then look at how much steering angle that actually is.
I've heard of people going to negative caster geometry so that the outside tire goes negative camber. Bushing or even sheetmetal deflection will affect camber more, at the steering angles involved.
GameboyRMH said:Knurled. said:That all depends on how you are GETTING that negative camber, too. Using eccentric strut ear bolts will give a much different result than using an adjustable strut top.
One of my winter projects for teh RX-7 is to get tons of negative camber at the strut top, so I can take it away at the ears, because of the effect it will have on the scrub radius and the camber curve. As a bonus, if I did the geometry math right, it should also raise the roll center, so I can make the car a little lower without losing turn-in. (The main reason I want the nose lower is not for any kind of handling reasons, I'd just like it to run cooler on the highway, and making the nose lower will force more air through the radiator)
- Suspension Geometry Nerd
Although the downside of getting negative camber at the strut top is that it increases SAI, and more SAI is generally a bad thing - it causes camber to become more positive as the steering moves away from center. I think that getting negative camber at the ears is generally better.
Both methods increase SAI. With a wishbone system you're moving the lower BJ out relative to the upper, therefore increasing SAI. With struts you're moving the upper steering pivot in relative to the lower BJ, accomplishing the same thing. The only was to adjust front camber on a car without affecting SAI is to alter the angle of the spindle relative to the ball joints or have camber adjustment on the upright like many race cars do.
Keep in mind, having a bunch of SAI can be useful on a car with lots of caster. Caster causes you to gain negative camber on the outside tire, gain positive on the inside tire when you turn. SAI causes the opposite. But caster has that effect pretty linearly through the steering range, while the effect of SAI is very little at low steering angles and gets more significant (more camber change per degree of steering) as you get closer to full lock.
So if you balance them correctly, you can gain a bunch of camber early in the steering travel but have the gain taper off as you continue to turn, which avoids the issue I have on the Jeep with 7-ish degrees of caster and about 9 degrees of SAI where it lays the wheels way over at full lock, so if you get too close to full lock in a low speed turn, you start to lose a lot of front end grip due to excessive negative camber on the outside tire. No such issue on the BMW which runs a little less caster and just over 14 degrees of SAI, as the camber gain tapers off due to having so much SAI relative to caster (but it's not enough for the trend to reverse where you start losing camber as you keep turning).
In reply to rslifkin :
The only good reason SAI exists is for packaging reasons so that scrub radius can be acceptable. And if it weren't for SAI cars wouldn't need nearly as much caster to get good camber gain while turning. What you say about caster's effects is a good thing! When you're cornering you have both tires leaning into turn! SAI actually tries to make the outside tire lean outwards away from the turn which of course is very bad.
freetors said:In reply to rslifkin :
The only good reason SAI exists is for packaging reasons so that scrub radius can be acceptable. And if it weren't for SAI cars wouldn't need nearly as much caster to get good camber gain while turning. What you say about caster's effects is a good thing! When you're cornering you have both tires leaning into turn! SAI actually tries to make the outside tire lean outwards away from the turn which of course is very bad.
Yes, to some extent that's true. But SAI can also impact camber gain in bump. Also take a look at the graphs in this thread that a guy put together to show the combined effects of caster and SAI: https://www.trackjunkies.org/topic/4686-caster-vs-steering-axis-kpisai-wheres-my-inhaler/
Those graphs make the point of how SAI's effects on camber get greater at large steering angles (and are small at small steering angles). So the combination of the 2 can be used to get desirable camber gain early on in the steering range but without it becoming excessive at full lock.
Very interesting article, er post, thanks for sharing. I have been wanting to see what actual numbers looked like on an actual car as I have been reading more about SAI and scrub radius lately. I thought it was minimal, and that seems true until you start turning the wheel at greater steering angles.
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