Trying to cure understeer

[M3SQRD]

Originally Posted by Track/S

Yes, rear is full coilover, 140nm front and 150nm rear is too harsh for street use.

3way coilovers, 20 clicks for low speed and 20 for high speed, 18 for rebound.

Front and rear sway bars H&R, 2 point adjustmen on front and 1 on rear.
Front and rear GT4 kinematics, don't necessary more than -2 of camber for street use.

This morning I did tests, stiffer rebound helped a lot, I think the problem may come from the tires since apart from the understeer it doesn't pull like before, so I understand that I should go back to the tires I used before, 285/35/18 (different brand and compound) This reminds me that it also happened to me with the cup2 in the rear...


With the previous tires I didn't have the problems that I have now so the problem comes from there I think.

You’ll definitely notice a change in effective gearing going from a 285/35 to a 285/30. Sometimes the effective shorter gearing actually helps with increasing speed but not always.

I was suggesting a higher neg front camber to get more grip out of it. As long as the inner to outer temps are uniform then the actual magnitude of camber doesn’t matter.

I’d say the difference in compound has a larger effect on balance than a shorter sidewall. If you made the change at only one end, it would alter the front-to-rear ride heights but you swapped both together. Your use of the 275/30 on the GTS supports it’s not a shorter, slightly stiffer sidewall (sidewall stiffness is dominated by internal pressure, not structure excluding RFTs) that’s altering turn-in balance. You’re probably overdriving the different brand 285/30 a bit expecting it to provide the same grip level as the 285/35.

[Track/S]

Originally Posted by RugbyBro

What are all the tires in question? I know one of them is AD09s but don't know about the others.

Originally Posted by M3SQRD

You’ll definitely notice a change in effective gearing going from a 285/35 to a 285/30. Sometimes the effective shorter gearing actually helps with increasing speed but not always.

I was suggesting a higher neg front camber to get more grip out of it. As long as the inner to outer temps are uniform then the actual magnitude of camber doesn’t matter.

I’d say the difference in compound has a larger effect on balance than a shorter sidewall. If you made the change at only one end, it would alter the front-to-rear ride heights but you swapped both together. Your use of the 275/30 on the GTS supports it’s not a shorter, slightly stiffer sidewall (sidewall stiffness is dominated by internal pressure, not structure excluding RFTs) that’s altering turn-in balance. You’re probably overdriving the different brand 285/30 a bit expecting it to provide the same grip level as the 285/35.

Now comes the most interesting part, the tires that I used up on the M2 are the Goodride Sport RS, the same compound that I use on GTS on front.

And the rear tires were in this condition when I changed to ad09, in that condition I had more grip than now with ad09... and also before I had -1.8 rear camber now -1.5.

[RugbyBro]

Originally Posted by Track/S

Now comes the most interesting part, the tires that I used up on the M2 are the Goodride Sport RS, the same compound that I use on GTS on front.

And the rear tires were in this condition when I changed to ad09, in that condition I had more grip than now with ad09... and also before I had -1.8 rear camber now -1.5.

Sounds like it's a perfect combination of these three factors:

1. AD09s have a stiffer construction overall = more stiffness
2. Moving from a 35 to 30 profile tire = more stiffness
3. Moving to a stickier compound = rear is gripping more which has the knock on effect of feeling more understeer

Since you're running pretty low compression as it is, probably the best pragmatic approach is to try to adjust front rebound up slightly (sounds like you've done this) and/or drop the front sway-bar to the softer position if that is an option. Don't take my advice as the gospel though, those are just the things i'd look at messing with first.

[Track/S]

Originally Posted by RugbyBro

Sounds like it's a perfect combination of these three factors:

1. AD09s have a stiffer construction overall = more stiffness
2. Moving from a 35 to 30 profile tire = more stiffness
3. Moving to a stickier compound = rear is gripping more which has the knock on effect of feeling more understeer

Since you're running pretty low compression as it is, probably the best pragmatic approach is to try to adjust front rebound up slightly (sounds like you've done this) and/or drop the front sway-bar to the softer position if that is an option. Don't take my advice as the gospel though, those are just the things i'd look at messing with first.

The sway bar is in soft position, I have never used stiffer position.

I agree with points 1 and 2, ad09 added a lot of stiffness
about the third I never experienced anything strange, I had a lot of grip in turns and corner exits

[jfritz27]

I forget if I posted this previously somewhere, but when I entered the world of adjustable suspension, to help wrap my head around what different things do, I made this flowchart algorithm on addressing understeer & oversteer, broken down by which phase of the corner things are happening. This was made directly by copying the approach outlined by suspensionsecrets.co.uk. I've read other resources that cite somewhat different approaches, but I like how this one tries to break things down.

https://drive.google.com/file/d/1iCt...ew?usp=sharing

If people would like I'm happy to modify if there's a consensus about a different way to approach some of these scenarios

[M3SQRD]

Originally Posted by RugbyBro

Sounds like it's a perfect combination of these three factors:

1. AD09s have a stiffer construction overall = more stiffness
2. Moving from a 35 to 30 profile tire = more stiffness
3. Moving to a stickier compound = rear is gripping more which has the knock on effect of feeling more understeer

Since you're running pretty low compression as it is, probably the best pragmatic approach is to try to adjust front rebound up slightly (sounds like you've done this) and/or drop the front sway-bar to the softer position if that is an option. Don't take my advice as the gospel though, those are just the things i'd look at messing with first.

Tire stiffness is considerably higher than your spring stiffness in terms of wheel rate. As an example, the front main spring with a rate of 700 lbf/in has a wheel rate of 0.96*0.96*700=605 lbf/in. Tire stiffness ranges for a street performance tires between 1800-2200 lbf/in. So the effective combined stiffness of the main and tire springs in series are:

Ksoft_tire = (Kspr_w*Ktire_so)/(Kspr_w+Ktire_so) =
(605*1800)/(605+1800) = 453 lbf/in

Kstiff_tire = (Kspr_w*Ktire_st)/(Kspr_w+Ktire_st) = (605*2200)/(605+2200) = 475 lbf/in

So for a 22% change in tire stiffness, the wheel rate changed by 4.8%. Some spring manufacturer spring rate tolerance is higher than 5%. It requires a MASSIVE change in tire stiffness to have a significant change on wheel rate. I doubt the two tires in question have an order of magnitude difference in stiffness. Does tire stiffness matter? Absolutely. But for the spring rates we’re using, a realistic change in tire stiffness has a small effect on wheel rate and/or handling balance. Tire ultimate grip, squirm and slip angle have a much larger effect on handling balance.

[M3SQRD]

Originally Posted by Track/S

Now comes the most interesting part, the tires that I used up on the M2 are the Goodride Sport RS, the same compound that I use on GTS on front.

And the rear tires were in this condition when I changed to ad09, in that condition I had more grip than now with ad09... and also before I had -1.8 rear camber now -1.5.

So you were originally using sport rs, switched to ad09 and then you developed understeer or were using ad09, switched to rs sport and then encountered understeer? First sentence suggests you were on sport rs whereas the second sentence suggests the opposite. It’s probably my reading comprehension skills

[Track/S]

Originally Posted by M3SQRD

So you were originally using sport rs, switched to ad09 and then you developed understeer or were using ad09, switched to rs sport and then encountered understeer? First sentence suggests you were on sport rs whereas the second sentence suggests the opposite. It’s probably my reading comprehension skills

First sport rs, switched to ad09 and then i developed understeer.

Today i try +9clicks rebound on front and 4 rear, low speed and high speed compression at minimum and have been able to go quite fast and I found myself quite comfortable, I also tried increasing rear compression, I don't like it, it becomes quite harsh.
Front tires ended up drowned







I don't want to imagine how much additional grip I would have with this amount of rebound clicks and sport rs tires, if the grip increases proportionally it has to be awesome.

[M3SQRD]

Originally Posted by Track/S

First sport rs, switched to ad09 and then i developed understeer.

Today i try +9clicks rebound on front and 4 rear, low speed and high speed compression at minimum and have been able to go quite fast and I found myself quite comfortable, I also tried increasing rear compression, I don't like it, it becomes quite harsh.
Front tires ended up drowned

I don't want to imagine how much additional grip I would have with this amount of rebound clicks and sport rs tires, if the grip increases proportionally it has to be awesome.

From what I could find online about the Goodride Sport RS is it’s a 180 TW tire designed for drifting and the track. The TW suggests grip >= ad09 grip. Did you use the same cold pressures on the ad09 that you use on the sport rs? I’m assuming you did unless you say otherwise. Did you adjust cold/hot pressures throughout the day? Check tread temps across the tire?

I had heard the ad09 was supposed to be a decent track tire but you’ve shown the opposite to be true. I’m surprised by how much on-track understeer you experienced with the ad09. The ad09 must like a different hot temperature/pressure range as well as a different cold starting pressure from the sport rs. It probably has lower overall grip than the sport rs. However, I’d look at it as a blessings in disguise! Using it showed you the true balance of your M2C was biased towards understeer which was being masked by grippier tires.

However, as you’ve made suspension changes on the street, the post-track rebound adjustments using the ad09 have shown improvement in both balance and cornering speed as well as increased comfort/confidence. Confidence is a big part of getting a setup right. You won’t be fast if you aren’t confident in the car. At 100 mph, saying to yourself what the hell is this thing going to do as you approach the corner means you won’t attack the corner! I’m thinking you probably overdrove the ad09 on track by expecting/hoping it would provide the same level of grip as the sport rs but you’d get more and more push. This just drove up the temperatures/pressures of the ad09 even more quickly which only exaggerated the understeer.

The rebound changes you’ve been making with the ad09s on the street should carry over unchanged to the sport rs on track. It may require a change or two to account for grip differences at elevated temperatures and the differences between the ad09 and sport rs. The good thing is you’re dialing out the understeer and feeling more comfortable in the car. Looking forward to hear how things go on your next track day.

[M3SQRD]

Originally Posted by jfritz27

I forget if I posted this previously somewhere, but when I entered the world of adjustable suspension, to help wrap my head around what different things do, I made this flowchart algorithm on addressing understeer & oversteer, broken down by which phase of the corner things are happening. This was made directly by copying the approach outlined by suspensionsecrets.co.uk. I've read other resources that cite somewhat different approaches, but I like how this one tries to break things down.

https://drive.google.com/file/d/1iCt...ew?usp=sharing

If people would like I'm happy to modify if there's a consensus about a different way to approach some of these scenarios

Edit: I misread the charts. New comments coming

[Track/S]

Originally Posted by M3SQRD

Tire stiffness is considerably higher than your spring stiffness in terms of wheel rate. As an example, the front main spring with a rate of 700 lbf/in has a wheel rate of 0.96*0.96*700=605 lbf/in. Tire stiffness ranges for a street performance tires between 1800-2200 lbf/in. So the effective combined stiffness of the main and tire springs in series are:

Ksoft_tire = (Kspr_w*Ktire_so)/(Kspr_w+Ktire_so) =
(605*1800)/(605+1800) = 453 lbf/in

Kstiff_tire = (Kspr_w*Ktire_st)/(Kspr_w+Ktire_st) = (605*2200)/(605+2200) = 475 lbf/in

So for a 22% change in tire stiffness, the wheel rate changed by 4.8%. Some spring manufacturer spring rate tolerance is higher than 5%. It requires a MASSIVE change in tire stiffness to have a significant change on wheel rate. I doubt the two tires in question have an order of magnitude difference in stiffness. Does tire stiffness matter? Absolutely. But for the spring rates we’re using, a realistic change in tire stiffness has a small effect on wheel rate and/or handling balance. Tire ultimate grip, squirm and slip angle have a much larger effect on handling balance.

When I went from Cup2 to Sport RS I had to stiffen the rear rebound because the rear felt like it had flat tires, I increased 4 clicks and solved the problem, my car never had as much grip as the day when i put Sport RS, when I installed ad09 I thought it would have more grip, but from what I'm seeing I was wrong.

Originally Posted by M3SQRD

From what I could find online about the Goodride Sport RS is it’s a 180 TW tire designed for drifting and the track. The TW suggests grip >= ad09 grip. Did you use the same cold pressures on the ad09 that you use on the sport rs? I’m assuming you did unless you say otherwise. Did you adjust cold/hot pressures throughout the day? Check tread temps across the tire?

I had heard the ad09 was supposed to be a decent track tire but you’ve shown the opposite to be true. I’m surprised by how much on-track understeer you experienced with the ad09. The ad09 must like a different hot temperature/pressure range as well as a different cold starting pressure from the sport rs. It probably has lower overall grip than the sport rs. However, I’d look at it as a blessings in disguise! Using it showed you the true balance of your M2C was biased towards understeer which was being masked by grippier tires.

However, as you’ve made suspension changes on the street, the post-track rebound adjustments using the ad09 have shown improvement in both balance and cornering speed as well as increased comfort/confidence. Confidence is a big part of getting a setup right. You won’t be fast if you aren’t confident in the car. At 100 mph, saying to yourself what the hell is this thing going to do as you approach the corner means you won’t attack the corner! I’m thinking you probably overdrove the ad09 on track by expecting/hoping it would provide the same level of grip as the sport rs but you’d get more and more push. This just drove up the temperatures/pressures of the ad09 even more quickly which only exaggerated the understeer.

The rebound changes you’ve been making with the ad09s on the street should carry over unchanged to the sport rs on track. It may require a change or two to account for grip differences at elevated temperatures and the differences between the ad09 and sport rs. The good thing is you’re dialing out the understeer and feeling more comfortable in the car. Looking forward to hear how things go on your next track day.

I have tried different pressures and nothing helped to improve the grip, with sport rs i go 2.0bars on cold and try not to go over 2.6/2.7 on hot.

I would like to try ad09 with hottest asphalt, maybe it works better at higher temperatures, sport rs works great in cold, hot and wet conditions, everything I am testing is on street, on track does not matter to me since this year I will not attend no event.

I also just remembered with sport rs I had a total 4mm rear toe in and I could go fast in slow corners, now with ad09 and 2mm toe in I have understeer...

[M3SQRD]

Originally Posted by Track/S

When I went from Cup2 to Sport RS I had to stiffen the rear rebound because the rear felt like it had flat tires, I increased 4 clicks and solved the problem, my car never had as much grip as the day when i put Sport RS, when I installed ad09 I thought it would have more grip, but from what I'm seeing I was wrong.




I have tried different pressures and nothing helped to improve the grip, with sport rs i go 2.0bars on cold and try not to go over 2.6/2.7 on hot.

I would like to try ad09 with hottest asphalt, maybe it works better at higher temperatures, sport rs works great in cold, hot and wet conditions, everything I am testing is on street, on track does not matter to me since this year I will not attend no event.

I also just remembered with sport rs I had a total 4mm rear toe in and I could go fast in slow corners, now with ad09 and 2mm toe in I have understeer...

Different tires can definitely have an effect on feel and response. Some tires also have more/less damping (max is around 10%, of critical, damping and min is < 1%, of critical, damping) which has a small effect on feel as well. Even with a tire having 3000 lbf/in of equivalent spring rate, it only changes wheel rate by ~10%. You could swap main spring rates by 50 lbf/in without telling someone (excluding professional racers, especially F1 drivers who can feel a 10 lbf/in well rate change) and 9/10 would not even notice the change. That’s why changes are typically recommended in 100 lbf/in increments and why 50 lbf/in springs are few and far between. Like I said, there’s a well-known spring manufacturer that has a 5-10% variability in spring rate so a lot of people are driving around obliviously with different rates at all four corners from what they think they’re running. Hyperco used to etch the actual spring rate on the very end of the top flat coil. They were always +/-1-2% of labeled spring rate.

The sport rs sounds like it’s a great performance tire that’ll also get good mileage out of a set. How cold was it? I was at a track event in 2004 where the ambient temperature was 14 F and my PSC’s would come off track barely warm to the touch. It was a great day for working on car control. Road temperature definitely impacts the available grip you can get out of a tire. It’s fair to give them another try on a warmer day. 4 mm total rear toe is more understeer-ry than 2 mm total rear toe so it definitely points to the tires and/or temperatures. I’m glad you’ve found a street setup that’s both comfortable and confidence inspiring.

[Track/S]

Originally Posted by M3SQRD

Different tires can definitely have an effect on feel and response. Some tires also have more/less damping (max is around 10%, of critical, damping and min is < 1%, of critical, damping) which has a small effect on feel as well. Even with a tire having 3000 lbf/in of equivalent spring rate, it only changes wheel rate by ~10%. You could swap main spring rates by 50 lbf/in without telling someone (excluding professional racers, especially F1 drivers who can feel a 10 lbf/in well rate change) and 9/10 would not even notice the change. That’s why changes are typically recommended in 100 lbf/in increments and why 50 lbf/in springs are few and far between. Like I said, there’s a well-known spring manufacturer that has a 5-10% variability in spring rate so a lot of people are driving around obliviously with different rates at all four corners from what they think they’re running. Hyperco used to etch the actual spring rate on the very end of the top flat coil. They were always +/-1-2% of labeled spring rate.

The sport rs sounds like it’s a great performance tire that’ll also get good mileage out of a set. How cold was it? I was at a track event in 2004 where the ambient temperature was 14 F and my PSC’s would come off track barely warm to the touch. It was a great day for working on car control. Road temperature definitely impacts the available grip you can get out of a tire. It’s fair to give them another try on a warmer day. 4 mm total rear toe is more understeer-ry than 2 mm total rear toe so it definitely points to the tires and/or temperatures. I’m glad you’ve found a street setup that’s both comfortable and confidence inspiring.

I have tested sport rs from 0°C to +30°C, 275/30/19 treadwear is 240, 285/35/18 is 180. I have ordered a set of 285/35/18 want to try them with the current suspension settings

[M3SQRD]

Originally Posted by Track/S

I have tested sport rs from 0°C to +30°C, 275/30/19 treadwear is 240, 285/35/18 is 180. I have ordered a set of 285/35/18 want to try them with the current suspension settings

Interested to read about how the rs 285/35 with the revised damper settings perform compared to the ad09 on the same settings.

[jfritz27]

Originally Posted by M3SQRD

I haven’t gone thru mid and exit flow charts yet but I disagree with their corner entry changes. First, they don’t discuss changing rates. If your rates are far off then no matter what damper, ride height, brake bias, etc. changes you make they will not address your issues. Spring rates have an effect on balance in all parts of a corner. Second, they say to reduce front ride height AND/OR reduce rear ride height. Increasing forward rake (lowering front or raising rear or both) changes handling balance towards oversteer. So lowering the rear ride height decreases forward rake and, therefore, increases understeer. Third, they say to reduce front brake bias AND/OR reduce rear brake bias. Which one is it? Typically, you’d want to either reduce front bias or increase rear bias (both are doing the same thing) to help get the car to rotate.

Not sure, but I think you might be inadvertently flip-flopping between reading the chart for oversteer and the one for understeer?

[M3SQRD]

Originally Posted by jfritz27

Not sure, but I think you might be inadvertently flip-flopping between reading the chart for oversteer and the one for understeer?

Yes I was and I deleted my original comments. That’s what happens when you’re doing it on your phone where you can see only one section at a time.

[jfritz27]

Originally Posted by M3SQRD

Yes I was and I deleted my original comments. That’s what happens when you’re doing it on your phone where you can see only one section at a time.

Hah, no worries.... the one thing for me that doesn't make sense in particular is the rec for corner entry oversteer under hard braking, where it says to soften rear rebound. Seems to me (and my own experience backs this up) that INCREASING rear rebound would be the solution, slowing the forward weight transfer to keep weight on the rear to improve rear grip.

[M3SQRD]

Originally Posted by jfritz27

Hah, no worries.... the one thing for me that doesn't make sense in particular is the rec for corner entry oversteer under hard braking, where it says to soften rear rebound. Seems to me (and my own experience backs this up) that INCREASING rear rebound would be the solution, slowing the forward weight transfer to keep weight on the rear to improve rear grip.

I’m not quite sure why you’d reduce rear LSR, or just R on 1W and 2W dampers, because that speeds up cross-weight transfer to the front outside wheel which would make it oversteer more, not less. This is an adjustment you’d make to address understeer. Increasing/maintaining rear LSR/R keeps the weight off of the outside front wheel longer; however, increasing rear LSR/R may also provide more confidence because the chassis feels more settled after braking/trailbtaking and transitioning to corner entry. Maybe it’s supposed to say reduce rear LSC/C?

[M3SQRD]

Originally Posted by jfritz27

Hah, no worries.... the one thing for me that doesn't make sense in particular is the rec for corner entry oversteer under hard braking, where it says to soften rear rebound. Seems to me (and my own experience backs this up) that INCREASING rear rebound would be the solution, slowing the forward weight transfer to keep weight on the rear to improve rear grip.

I’ve given this more thought and I don’t see any advantage to reducing rear LSR/R if you have corner entry oversteer. Reducing LSR/R would transfer weight to the outside wheel faster but it’s not likely enough to make a difference. I’ve looked at a couple of other setup references and increasing LSR/R wins. Reducing LSR/R helps minimize understeer so making just that one adjustment rarely solves both understeer and oversteer.

[jfritz27]

Originally Posted by M3SQRD

I’ve given this more thought and I don’t see any advantage to reducing rear LSR/R if you have corner entry oversteer. Reducing LSR/R would transfer weight to the outside wheel faster but it’s not likely enough to make a difference. I’ve looked at a couple of other setup references and increasing LSR/R wins. Reducing LSR/R helps minimize understeer so making just that one adjustment rarely solves both understeer and oversteer.

Yeah, that's the logic I keep coming back to also.

Given your suspension knowledge, when you have a minute, I'd be interested from a theory standpoint in hearing your Cliffs notes version of how you think about addressing mid-corner issues. In theory, this is the phase where the car has taken a set and the weight has stopped moving around -- so if that is true, damper adjustments should largely be moot/unhelpful, no? The flowcharts kind of reflect this, in that the majority of interventions listed for mid-corner under/oversteer are non-damper-related. But there are a few damper interventions listed for some scenarios....

[M3SQRD]

Originally Posted by jfritz27

Yeah, that's the logic I keep coming back to also.

Given your suspension knowledge, when you have a minute, I'd be interested from a theory standpoint in hearing your Cliffs notes version of how you think about addressing mid-corner issues. In theory, this is the phase where the car has taken a set and the weight has stopped moving around -- so if that is true, damper adjustments should largely be moot/unhelpful, no? The flowcharts kind of reflect this, in that the majority of interventions listed for mid-corner under/oversteer are non-damper-related. But there are a few damper interventions listed for some scenarios....

Dampers require piston rod velocity relative to the damper body to generate force (excluding friction/stiction, seal drag, gas pressure, etc.). No relative velocity, no force. However, if you’ve ever tried to compress a piston of a damper by hand, you quickly realize it takes a decent amount of force to move the piston rod into the damper at a velocity close to zero. The force you’re pushing against is the gas pressure lifting force from the internal N2 gas chamber (either in the main damper body for NR dampers and in the remote reservoir for dampers with external reservoirs) which acts like a compressive preloaded spring in parallel to the main spring (any load carried by this gas lifting force offloads the main spring compressive load but has no effect on spring rate). We’re going to ignore the gas lifting force and preloaded spring. So a damper generates force proportional to velocity.

To answer the first question, yes. For a smooth midcorner without curbs, the damper R and C forces are zero because there’s no relative motion/velocity. Answer to the second question also is yes. Dampers cannot be used to alter the balance of steady-state cornering. If the weight transfer from braking and cornering are done and the corner is smooth then the dampers are no longer part of the equation. Steady-state cornering balance is altered by making spring rate changes, ride height changes, rake changes, changing front track width, changing rear track width, adjusting front and rear tire pressures, changing tire compounds, changing tire widths, width, altering suspension alignment front and rear, etc. but this all changes as soon as you encounter one bump midcorner! Not too many perfectly flat, not-a-single-bump corners left on race tracks anymore.

[Track/S]

Sport rs mounted, rear toe-in moved to 4mm and understeer problem solved.