Total Lateral Load Transfer Distribution

[stab6902]

All indications point to the need for more rear stiffness or less front stiffness. Since the front is decently soft (1.5Hz undamped) I have opted to stiffen the rear (currently 1.2-1.4Hz undamped).

Are those measured ride frequencies, or calculated? When I actually measured my ride frequencies, I was surprised to find they were quite a bit higher than calculated, especially in the rear. My car (1968 Firebird, 3410 lb, 58% front) has a rear ride frequency of 1.75 Hz with stock 5 leaf rear springs, which have a published rate of 110 lb/in.

To measure the ride frequencies, I took the shocks off one end of the car, put the other end on full stiff (to minimize coupling effects), and bounced it. It'll be kind of like pushing a swing - it's fairly easy to feel the natural frequency. The quick and dirty way is to count bounces in like a 30 sec timespan, but I got fancy and used the accelerometer on my phone and then analyzed the data in Matlab. I think it's worth measuring (if you haven't already) to give you a baseline.

[Sleeper68]

Are those measured ride frequencies, or calculated?

Calculated, using measured values - namely corner weight, motion ratio, and spring rate (normalized) using the std. equation for natural frequency ω=√(k*MR/m) , where ω is the natural frequency of the sprung mass, k is the spring rate, MR is the motion ratio, and m is the sprung mass.

When I actually measured my ride frequencies, I was surprised to find they were quite a bit higher than calculated, especially in the rear. My car (1968 Firebird, 3410 lb, 58% front) has a rear ride frequency of 1.75 Hz with stock 5 leaf rear springs, which have a published rate of 110 lb/in.

To measure the ride frequencies, I took the shocks off one end of the car, put the other end on full stiff (to minimize coupling effects), and bounced it. It'll be kind of like pushing a swing - it's fairly easy to feel the natural frequency. The quick and dirty way is to count bounces in like a 30 sec timespan, but I got fancy and used the accelerometer on my phone and then analyzed the data in Matlab. I think it's worth measuring (if you haven't already) to give you a baseline.

That is very interesting. Your measured value and theoretical value for natural frequency vary quite drastically in the rear. My calcs show you should have been in the 1.56Hz range for rear ride frequency (assuming a motion ratio of 1.0), however with a motion ratio of ~0.7 (the actual leaf spring simplified motion ratio in roll) you get 1.1Hz. This brings up an interesting point - the wheel:spring motion ratio for a solid axle changes depending if the sprung mass is moving in heave or roll, or some combination of the two. This may be why you got a higher measured value than "expected", you were moving the sprung mass in heave. Additionally, as you probably already know, leaf springs have a progressive rate when installed with shackles like G1 and G2 F bodies have. This may have contributed to the higher measured ride rate/frequency as well.

To be clear, my estimate of my rear ride frequency (1.3Hz) actually uses the measured "pure" roll motion ratio of ~0.7:1. Using a heave motion ratio of 1.0:1 I get 1.85Hz rear ride freq for my car.

I will check the rear ride frequency using your method on my car when I have a chance and report back.

[stab6902]

Calculated, using measured values - namely corner weight, motion ratio, and spring rate (normalized) using the std. equation for natural frequency ω=√(k*MR/m) , where ω is the natural frequency of the sprung mass, k is the spring rate, MR is the motion ratio, and m is the sprung mass.



That is very interesting. Your measured value and theoretical value for natural frequency vary quite drastically in the rear. My calcs show you should have been in the 1.56Hz range for rear ride frequency (assuming a motion ratio of 1.0), however with a motion ratio of ~0.7 (the actual leaf spring simplified motion ratio in roll) you get 1.1Hz. This brings up an interesting point - the wheel:spring motion ratio for a solid axle changes depending if the sprung mass is moving in heave or roll, or some combination of the two. This may be why you got a higher measured value than "expected", you were moving the sprung mass in heave. Additionally, as you probably already know, leaf springs have a progressive rate when installed with shackles like G1 and G2 F bodies have. This may have contributed to the higher measured ride rate/frequency as well.

To be clear, my estimate of my rear ride frequency (1.3Hz) actually uses the measured "pure" roll motion ratio of ~0.7:1. Using a heave motion ratio of 1.0:1 I get 1.85Hz rear ride freq for my car.

I will check the rear ride frequency using your method on my car when I have a chance and report back.

I was assuming you were talking about heave ride frequencies all along, so things make more sense now. I'm not used to people talking about roll ride frequencies, but I can see why you were thinking along those lines given the problem you're trying to solve.

For what it's worth, my measured front ride frequency was also higher than calculated, as were the ride frequencies on both ends of my 1972 Skylark. The biggest delta was with the leaf springs though. They are progressive of course, but I couldn't measure a difference between light bouncing (+/- 0.5") and heavy bouncing (+/- 1"). I'm using the same standard equation as you, but always for heave as I'm trying to dial in ride comfort. More caveats - I'm running rubber bushings everywhere , which could add a very slight amount of wheel rate, and there's some estimation involved in my unsprung weights (I'm not taking into account half the weight of the control arms etc), but I think they're close.

Anyway, I'm curious what you measure. Sorry I'm not more help on your specific issue - I'm kind of on the opposite end of the spectrum, trying to improve handling performance on street cruisers while still maintaining a comfortable ride.

[Sleeper68]

Thank you for the contribution Ryan, I appreciate it.

I was assuming you were talking about heave ride frequencies all along, so things make more sense now. I'm not used to people talking about roll ride frequencies, but I can see why you were thinking along those lines given the problem you're trying to solve.

For what it's worth, my measured front ride frequency was also higher than calculated, as were the ride frequencies on both ends of my 1972 Skylark. The biggest delta was with the leaf springs though. They are progressive of course, but I couldn't measure a difference between light bouncing (+/- 0.5") and heavy bouncing (+/- 1"). I'm using the same standard equation as you, but always for heave as I'm trying to dial in ride comfort. More caveats - I'm running rubber bushings everywhere , which could add a very slight amount of wheel rate, and there's some estimation involved in my unsprung weights (I'm not taking into account half the weight of the control arms etc), but I think they're close.

Anyway, I'm curious what you measure. Sorry I'm not more help on your specific issue - I'm kind of on the opposite end of the spectrum, trying to improve handling performance on street cruisers while still maintaining a comfortable ride.

I have not been able to measure the front and rear heave or roll rates yet. I will do so as soon as I can. I am still unsure how to easily measure the front and rear roll rates, those are a little more tricky.

Good news: the aforementioned leaf spring front eyelet bushing change did seem to net a benefit on course. I tried it out Sunday at a new (to me) venue with the Highlands Sports Car Club HSCC. This was a tight course. I found that front grip while braking, turn in feel, roll response rate (total), yaw rate (velocity), and front tire outer edge wear were improved. These improvements were really only qualitative in nature. This was the first time the car had been on this surface and temps were lower than usual. Next weekend I have an event on a familiar surface so we should be able get some more quantitative data. I can say this, the car definitely feels better than it was, and maybe better than it ever has been. Part of that may be driving, part of it may be real , and part of it may be placebo. It is also important to note that I used the new-stiff front jounce bumpers at this event.

Here is the video from this last Sunday, 5th PAX, 6th RAW:

https://youtu.be/a_rXDVMWofE

My theory is that the TLLTD (Total Lateral Load Transfer Distribution) and the roll couple were not playing well together after I moved the front roll center up (a consequence of making the front camber gain curve more aggressive). What I mean by this is that the front roll rate was too high compared to the roll couple to allow proper front suspension compression while accelerating laterally and by extension reducing dynamic camber to the ground plane (car pivoting on outside front tire). By moving the front roll center up, the car experienced increased front jacking forces, increased front load transfer (also reduced rear load transfer), and flattened the roll axis inclination. Other dynamic effects such as roll understeer induced by leaf spring rake may have affected this as well since rear roll increased due to the stiffer front axle. Stiffening the rear and reintroducing inclination to the roll axis increased rear weight transfer and reduced front weight transfer. What I have yet to be able to check is front/rear roll suspension compression ratio. Setup 2, 2.5, and 3 exhibited little front suspension compression. I would like to see what the front end is doing now. It feels like it is compressing more and the car is starting to yaw more quickly after steering input but I need better data.

For completeness, the suspension tuning progression:

SETUP 1: The first significant front RC change I did was achieved by going to a 0.9" tall upper ball joint from a factory one on otherwise stock geometry (raised roll center ~ 1.25" and made camber gain negative instead of positive - 0.3°/in). This was in 2015. -3.0° camber , +6.0° castor , 0.0" toe

SETUP 2: The second front RC change was done in 2020 by lowering the inner pivot of the upper control arm about 1/2" rear hole 5/8" front hole (raised front roll center ~1.1" and made camber gain more aggressive 0.8°/in). This also had the effect of reducing front anti-dive since it is fairly excessive in first gen camaros. The reason for the second change was because I found I had to run alot of front camber (2.7°-3.0° neg) to keep the front tires happy but this came at the cost of poor front end grip in braking. I attributed this poor braking performance to the excessive static camber after playing with brake pressure proportion (ended up with full bias to rear tires in braking, still locked up front tires with even small amounts of trail brake). The idea was to increase camber gain and thus reduce the need for static camber. The inadvertent effect was a stiffer front end due to the higher front roll center. -1.5° camber , +6.5° castor , 0.0" toe.

SETUP 2.5: Rear RC moved up to 13.5" from 12.5". Rear ARB endlinks changed to rod end style from polyurethane-bushed style. -2.0° camber , +6.5° castor , 1/8" total toe out

SETUP 3: The third front RC change was performed in 2021 by going to a 0.5" tall upper ball joint from 0.9" tall (lowered front roll center ~0.5" and made camber gain less aggressive - 0.6°/in). This change was made in an attempt to "split the difference" between change one and two. -2.1° camber , +6.7° castor , 1/8" total toe out.

SETUP 4: Change to softer front jounce bumpers. Increased front damper bump travel (reduced rebound travel) by moving the lower damper mount down in relation to the lower control arm. -2.1° camber , +6.7° castor , 1/8" total toe out

SETUP 5: Front eyelet bushing of leaf springs changed to aluminum/delrin/steel bushing with thrust washers from steel/rubber/steel bushing. This eliminated any appreciable compliance in the bushing and forced the rear leaf springs to comply with roll, effectively increasing the rear roll rate. -2.1° camber , +6.7° castor , 1/8" total toe out.