CSG and RCE tag-team suspension Q&A thread! Ask us anything!

[solidONE]

Originally Posted by 7thgear

on a smooth road you wouldn't feel the transition

but next time you take an off/on ramp at speed, aim for a pothole with your loaded front tire.... if you get kicked up harshly with almost no delay then you know you've had no more travel left for that bump.

I was thinking more of what the front end does while trail braking into a turn. So it's not as abrupt as hitting a pothole mid corner.

[7thgear]

Originally Posted by solidONE

I was thinking more of what the front end does while trail braking into a turn. So it's not as abrupt as hitting a pothole mid corner.

if you are at the limit and on the bumpstops, you are no longer suspended and the tire is now doing all the work. Any additional bumps will have to be absorbed by the tire sidewall but most likely will force the chassis up, upsetting the car and having the front lose traction.

so if you have smooth inputs and no bumps, it's not so bad.. but def no optimal.

edit: i think i've finally understood your question.

as mentioned below, chances are oyu are in fact on your bumpstops during any serious cornering.. and so long as your inputs are smooth and the ground is smooth, you'll never feel the abruptness... but only when you encounter a sudden bump.

[Wepeel]

Originally Posted by solidONE

Bumpstops... I wonder how much of the 'at-the-limit' handling characteristics I attributed to springs, but were in fact bumpstops. I always assumed that once the bumpstops were engaged in the the compression stroke the effect would be very very abrupt. I don't think this is the case with our 86's stock bumpstops.

I can't remember what your suspension setup is.

On the stock setup, the bumpstops are very close to being engaged, maybe within 1/4"-1/2" or so. Bumpstops today aren't like bumpstops on older cars, they are very progressive so the loading isn't that obvious (except maybe when you hit a big bump like mentioned above). I think most modern cars have progressive stops and some actually rely on them to provide a lot of the spring rate - if you look at the coil spring rates of BMW you will notice they are very, very, very soft. They rely on the "bumpstops" to provide a lot of their rate. And in general BMW have good handling.

I know on my coilovers, at a 1" drop, I have a little over 1.5" of travel before the bump is engaged. But that bumpstop is really firm, it doesn't have near the gradual ramp up as an OEM style one.

If I did the math right for our track width, 1.5" bump on the outside wheel and 1.5" droop on the inside wheel results in ~2.9 deg of roll. Stock BRZ's will maybe a little over 3 degrees / g, so most modded BRZs will roll less than that. I would think being that I'm about 64% stiffer than stock in roll in my setup that during roll, at my ride height and coilovers, I shouldn't be on the bumpstops in the front.

I think.

[wparsons]

Originally Posted by ZDan

That does not happen. No fore/aft acceleration, zero fore/aft weight transfer.


No, you're not.

Fore/aft weight transfer is longitudinal acceleration in g's multiplied weight multiplied by the c.g. height and divided by the wheelbase.

No braking or acceleration, no fore/aft load transfer.

So you're claiming that a FWD lifts it's inside rear tire (but not the inside front) because more weight is transferring to the outside rear alone?

Unless the chassis has a rotating pivot in the center, that just can't happen. To lift the inside rear the outside front has to compress more (from more weight transferred to it). If weight were just transferring laterally then you'd be likely to lift both inside tires, not just the one at the stiff end.

If you put an RC car on 4 kitchen scales and lift one of the wheels up, watch where the weight goes, it's the one diagonally opposite the one in the air. Sure that isn't the exact same way the lateral forces work, but at the end of the day that inside tire isn't lifting without weight going to the opposite corner.

[Racecomp Engineering]

Yeah the standard bumpstops are progressive, so you may not feel the initial contact. They just ramp up to higher rates. On big bumps, well that's going to be abrupt because you're out of travel and/or into the really high rate portion of the bumpstops.

It's not bad on a stock car, but when you lower you're already sitting on the higher rate portion and it only gets stiffer from there.

From the factory it gives a soft initial ride that firms up when cornering. It can also build in factory engineered safe understeer. Plus I have some theories about what it lets them do with regards to shocks but they're just theories.

- Andy

[Wepeel]

Originally Posted by wparsons

So you're claiming that a FWD lifts it's inside rear tire (but not the inside front) because more weight is transferring to the outside rear alone?

Unless the chassis has a rotating pivot in the center, that just can't happen. To lift the inside rear the outside front has to compress more (from more weight transferred to it). If weight were just transferring laterally then you'd be likely to lift both inside tires, not just the one at the stiff end.

If you put an RC car on 4 kitchen scales and lift one of the wheels up, watch where the weight goes, it's the one diagonally opposite the one in the air. Sure that isn't the exact same way the lateral forces work, but at the end of the day that inside tire isn't lifting without weight going to the opposite corner.

Some FWD cars, depending on the setup, can lift the inside in mainly pure cornering, when not trail braking. Of course trailbraking will exacerbate the problem and can hike up that tire even more.

It's not that the weight transfers to the outside rear alone, but if the front rolls more than the rear (think super stiff rear swaybar), it's the roll of the front that's picking up the rear, because the chassis is stiff. Weight is transferring to the outside on both front and rear axles but more so at the stiffer end.

And springs won't really affect fore/aft steady state weight transfer, as ZDan said that's more based on wheelbase and cg height. But it can effect transiently how quickly the car responds in pitch.

[Shankenstein]

Originally Posted by Wepeel

Some FWD cars, depending on the setup, can lift the inside in mainly pure cornering, when not trail braking. Of course trailbraking will exacerbate the problem and can hike up that tire even more.

It's not that the weight transfers to the outside rear alone, but if the front rolls more than the rear (think super stiff rear swaybar), it's the roll of the front that's picking up the rear, because the chassis is stiff. Weight is transferring to the outside on both front and rear axles but more so at the stiffer end.

And springs won't really affect fore/aft steady state weight transfer, as ZDan said that's more based on wheelbase and cg height. But it can effect transiently how quickly the car responds in pitch.

From my experience, the rear sway bar is the major contributor. I was 3-wheeling a RAV4 around the autox track... but I've seen Jettas and Mazda3s and Minis do it too.

FWD guys use a stiffer rear bar to combat understeer. While it does make the car behave more neutrally, there's the side effect of force distribution. Compression from the outside tire gets sent to the inside. If you have the right combination of stiff chassis, compliant suspension, and a beefy bar... you'll disengage the road surface.

Most people think the sacrifice is worth it, since that tire isn't seeing much downforce (?) anyways. I got better performance with a rear bar than without (RAV4.1's weren't equipped from the factory, which was the primary reason they were notorious for flipping).

Regarding fore-aft transfer, springs make a difference... but good geometry makes a bigger difference. Dive and Squat are "moment" reactions of your car. If the force of deceleration (placed at the center of gravity) is sent through a path that's not in plane with your reaction force, you'll get some dive/squat. That applies to the direction (horizontal) and location (height).

MacPherson struts are simple and effective enough... but they don't manage these forces terribly well. Typically, you'd tilt the upper wishbone's points to reduce the moment. This will do wonky things to the rest of your setup... but generally this compromise is worthwhile, since braking and cornering work best on 4 wheels instead of 2. (see discussion of 3 wheeling above). MacPherson strut doesn't have a true upper wishbone... so you end up fiddling with the points on your lower control arm, which cause a similar amount of wonkiness. If you do too much, toe curves get nasty and bumpsteer gets bad. Neither of which are good conditions for a sports car... so we get to deal with dive.

Springs will do the job... but a small change in stiffness yields a large change in driver comfort. #BecauseRacecar won't see much dive on his 10k springs.

[solidONE]

Originally Posted by Wepeel

I can't remember what your suspension setup is.

On the stock setup, the bumpstops are very close to being engaged, maybe within 1/4"-1/2" or so. Bumpstops today aren't like bumpstops on older cars, they are very progressive so the loading isn't that obvious (except maybe when you hit a big bump like mentioned above). I think most modern cars have progressive stops and some actually rely on them to provide a lot of the spring rate - if you look at the coil spring rates of BMW you will notice they are very, very, very soft. They rely on the "bumpstops" to provide a lot of their rate. And in general BMW have good handling.

I know on my coilovers, at a 1" drop, I have a little over 1.5" of travel before the bump is engaged. But that bumpstop is really firm, it doesn't have near the gradual ramp up as an OEM style one.

If I did the math right for our track width, 1.5" bump on the outside wheel and 1.5" droop on the inside wheel results in ~2.9 deg of roll. Stock BRZ's will maybe a little over 3 degrees / g, so most modded BRZs will roll less than that. I would think being that I'm about 64% stiffer than stock in roll in my setup that during roll, at my ride height and coilovers, I shouldn't be on the bumpstops in the front.

I think.

I had on a set of swift BRZ spec spring @ about 25mm lower and 3.8k front and 4.5k rear springs. I also cut off the first notch of the bumpstop (the softest lower portion) when I did the install. I was experiencing some pushing at corner entries, but it was not at all abrupt, so I attributed that to the upped spring rates at the time. I though I could have dialed it out with minor alignment adjust, but now I'm not so sure.

I've since reverted back to stock springs and superglued the soft notch of the bumpstops back on upon reinstall. Lmao! I hope the glue holds decently. Lol

[Fleischwagen]

On my GTI I used to lift the inner rear wheel all the time after I upgraded to a rather large rear anti-sway bar. It went from understeering horribly to moderately over steering in a very fun fashion. Trail braking would usually over load the front tires (already wallowing like a child) and it would then snap oversteer if you weren't quick enough at the steering wheel. It sure was a lot of fun hah

[SubieNate]

As fuel to the fire on the weight transfer while cornering thing, remember cornering is acceleration. You're changing the direction of the velocity vector of the car which is the definition of acceleration.

[ZDan]

Originally Posted by wparsons

So you're claiming that a FWD lifts it's inside rear tire (but not the inside front) because more weight is transferring to the outside rear alone?

As cornering g's increase, the outside wheels are loaded up and the insides are unloaded. Of course both the outside front and the outside rear are seeing increased load. How much each sees is dependent on the roll stiffness distribution.

If roll stiffness is somewhat biased to the rear, the outside rear will load up more and the inside rear will UNload more vs. the fronts. As cornering g's increase, eventually the outside rear will be supporting ALL of the weight at the rear of the car and the inside rear will lift off. From that point on the outside front loads up more and inside front unloads more.

Unless the chassis has a rotating pivot in the center, that just can't happen. To lift the inside rear the outside front has to compress more (from more weight transferred to it). If weight were just transferring laterally then you'd be likely to lift both inside tires, not just the one at the stiff end.

No, you'll lift the inside at the stiff end first. The softer front roll bar allows the inside front to continue to droop.

at the end of the day that inside tire isn't lifting without weight going to the opposite corner.

AFTER you've totally unloaded the inside rear, the *rate* of increasing load on the outside front does go up, because under pure cornering the outside rear can't load up any more than the total rear weight of the car and the inside can't unload beyond zero of course!

But the fact remains that the stiffer end of the car is the end that loses grip first for reasons already discussed: It loads up its outside rear more in cornering.

[7thgear]

I concur with Zdan


weight transfer in this case is not the same as lets say water or electricity "flowing" along the path of least resistence.

[solidONE]

Originally Posted by normancw

]

My goodness, that beautiful! Dem hats..

[7thgear]

omg i just have to post this,

[ame="https://www.youtube.com/watch?v=mZzfIFBXx5Q"]Car Suspensions: "Spring Harmony" 1935 Chevrolet Auto Mechanics - YouTube[/ame]