Please Help Me Understand "Instant Center"

[BOSSSTANG]

Does anyone know how adjusting the instant center on our Mustang's affects cornering performance? All the information I can find online is for drag racing, I could not find much about road racing.

Here is a stolen diagram to easily describe what I'm talking about:

 

[Grant 302]

For handling, I think of instant center as being a good indicator of corner exit performance.

Much more complex modeling needs to be done to evaluate handling properties like 'roll steer'. That's how rear geometry changes as the car body rolls. Instant centers can be evaluated separately for each side of the car to evaluate power and braking effects.

 

[BMRTech]

Excellent post above ^

There is a lot more to dropping the LCA down, than most know. For handling use, users can benefit from the extended (dropped down) LCA, in several ways. One of them, as mentioned above, is improved traction on corner exit. One of the best ways for this chassis to pick up/decrease lap times is by being able to apply more power, earlier in the corner exit stage.

On the topic of corner exit, you can get to a point where you will actually reduce traction on corner exit, and reduce axle stability. People who run into this will likely tell you "it ain't fun"

There is a balance. Finding the balance is done with adjusting the height of the forward RUCA Mounting point, along with the RLCA Mounting Point. We use modeling software to engineer our mounting points into our parts, and of course, test real world as much as possible. What we have found is, in a handling application....there really is no perfect AS setting. Some like aggressive, some like neutral, and some like hardly any AS at all. My point is, it is usually best to test multiple settings, and figure out what you like best. What one person may like, and what one persons effective wheel rates are....may not match the next. Roll stiffness and dampers will play a large factor in what AS will work, and what will not.

Then you have roll steer. Some like increased roll steer, some do not. Again, it boils down to driver preference based on how they drive, their comfort, and their supporting suspension modifications. Excessive roll steer can cause serious discomfort, and also cause a very abrupt loss of traction. It can also work well for someone. For example, increased roll steer from a steep LCA Angle, can be beneficial on select applications, with select drivers in an AutoX environment. On a road course, that increased roll steer may A: Never even be noticed / come into play, or B: Take away corner stability. It can be compared to wheelbase, kinda.... A long wheelbase improves cornering stability on most road courses, and can lead to instability issues during AutoX driving. You can throw a shorter WB around, easier, during autoX.

Then there is "brake hop". Too much AS, and you can experience erratic axle movements under heavy braking. No fun! Too little AS, and you run into unloading rubber. I cannot stress enough though, supporting suspension modifications and settings can and will affect these characteristics greatly.

Here are some drawings I made many years ago, to help people get a good idea of the terms Instanct Center, Anti-Squat, PoR, etc.






"Launching' on my drawing, would be no different than say, simply going WOT on any car. The force will still cause the body of the car to act a certain way, in relation to the rear axle assembly.

It is also important for people to remember, when you do change your RCA angles, you are changing their effective lengths....and the rear axle assembly pivots around these mounts. So, in other words...when you change the effective lengths of the mounting points...you change the "arc" during suspension travel. Hence the reason people run into roll steer conditions. So for example, if you mounted the rear of the LCA 8" lower than the front mount, 2" of suspension travel would pull the rear axle closer to the car, than OEM positions during 2" of travel. This movement is more erratic too, because you have increased leverage, but shortened the effective length.

Maximum Motorsports recently release some very nice RLCA Brackets, and they provide some very nice instructions, that I feel anyone could benefit from reading.

http://www.maximummotorsports.com/assets/install/pdf/rear_susp/Mm5RLCA-57.pdf

Hope that helps! I tried explaining it in a fashion that can be easily understood.

 

[BOSSSTANG]

Hope that helps! I tried explaining it in a fashion that can be easily understood.

This is great! It will take some time for me to fully understand and digest. The reason I ask is because I am currently running a Roush UCA with aftermarket LCA's (non adjustable) and relo brackets. I have read (might have even been from you on another forum!) that the Roush UCA moves the IC point to a unfavorable position on lowered cars, and I've been debating going a different route on the UCA. I tried a Steeda UCA for a while but it just made too much noise, I was thinking that possibly the BMR with the grease-able joint would help this noise issue and get my IC back to a decent place.

 

[BMRTech]

This is great! It will take some time for me to fully understand and digest. The reason I ask is because I am currently running a Roush UCA with aftermarket LCA's (non adjustable) and relo brackets. I have read (might have even been from you on another forum!) that the Roush UCA moves the IC point to a unfavorable position on lowered cars, and I've been debating going a different route on the UCA. I tried a Steeda UCA for a while but it just made too much noise, I was thinking that possibly the BMR with the grease-able joint would help this noise issue and get my IC back to a decent place.

It was likely me.

I like the Roush UCA. The pros are, it does an excellent job at what it was designed to do.

The cons are, they do drop the front mount down, which increases AS / shortens the IC. What they also do is, unless it has been changed, the UCA in their kit is the traditional 05-10 S197 Length of 8.5".

The longer the UCA, the better, for everyone. 11+ cars came with a 1" longer UCA, and that 1" helps stabilize the axle, and helps with excessive axle movement during suspension travel.

Anyhow, when you lower your car, you are lowering the front of the UCA Mount....but the axle side, stays the same. When you additionally lower that front UCA Mount with the Roush, or our lower hole in the BMR Mount, etc....you are typically increasing that downward angle too much....because, you will likely use a RLCA Relocation mount to "fix" your angle of the LCA.

Combining a Roush UCA, with a set of RLCA Brackets, on a lowered car tends to work against the driver. Yes, you rid of the OEM deflection, but you are also shortening the IC too much, and ALSO reducing the UCA length, which is opposite of what you truly want with the 3-Link Suspension.

 

[BOSSSTANG]

That makes sense, when I had the OE arm next to the Roush I could have sworn it was a good bit shorter...

So my next and last question is how does a car handle with a shorter IC?

It was likely me.

I like the Roush UCA. The pros are, it does an excellent job at what it was designed to do.

The cons are, they do drop the front mount down, which increases AS / shortens the IC. What they also do is, unless it has been changed, the UCA in their kit is the traditional 05-10 S197 Length of 8.5".

The longer the UCA, the better, for everyone. 11+ cars came with a 1" longer UCA, and that 1" helps stabilize the axle, and helps with excessive axle movement during suspension travel.

Anyhow, when you lower your car, you are lowering the front of the UCA Mount....but the axle side, stays the same. When you additionally lower that front UCA Mount with the Roush, or our lower hole in the BMR Mount, etc....you are typically increasing that downward angle too much....because, you will likely use a RLCA Relocation mount to "fix" your angle of the LCA.

Combining a Roush UCA, with a set of RLCA Brackets, on a lowered car tends to work against the driver. Yes, you rid of the OEM deflection, but you are also shortening the IC too much, and ALSO reducing the UCA length, which is opposite of what you truly want with the 3-Link Suspension.

 

[BMRTech]

That question is too broad.

It depends. You can shorten the IC via the UCA, or the LCA, or both.

I tried to explain some of the characteristics above, but it really just depends on each person and their set-up.

Shortening the IC can have various impacts on Anti-Squat, based on how you shorten it.

VERY generic example - it should help you when you look at the pics above.

 

[Grant 302]

Here's a simple calculator that I like to use for estimating IC:

http://www.baselinesuspensions.com/instant-center_general.php

Play with it a little and see how AS values change.