bump stop modification? (NC Miata/RX8)

[Nathan Atkins]

Hey everyone, just seeing if any of you NC folks have experience with perforating or shortening bump stops to balance handling at 10 tenths.

My understanding is that the NC and RX8 both have minimal shock travel to bump stop engagement so that they can use soft springs for daily driving and then have tall very progressive spring rate bump stops for hard cornering. Shaikh from FCM elaborates on this a bit and the theory is that because the front ones engage first and less progressively, that's the cause of the terminal under steer that plagues my car.

If i can just go drill some holes in the heel of my front bump stops to soften them up then that's infinitely less costly than a stiffer rear sway, and i should walk away with more ultimate grip at the expense of a margin of safety required by the average (read:terrible) Joe motorist, but that we 10-foot-tall and bulletproof autocrossers laugh at.

So let me know if you have experience, or not, either way, I will post my findings and perhaps if you too bear the pox of abominable under steer then mayhap my experience can guide you.

[Areddi]

http://www.racingbeat.com/RX8/Suspensio ... geRX8.html

Do it!

[Nathan Atkins]

You know why the racing beat suspension kit is so awesome?

this:http://www.racingbeat.com/manuals/14026-sprg-shock.pdf

See step 6 and Photo 4

[Nathan Atkins]

Notice that they say "sway bars are designed to reduce chassis-roll in turns and increase the RX-8's sensitivity to steering input, thereby improving maximum cornering power" The improved cornering is a result of sensitivity, not grip. Sensitivity only makes you faster if your car is a handful and needs to be tamed, mine suffers only from understeer. It's maximum grip is adequate before the understeer sets in which would still occur without modifying the bump stops. A stiffer rear sway would balance out the understeer by reducing grip in the rear by shifting the behavior of the rear to more closely replicate a solid rear axle, that ultimately makes you slower if you drive to the limit of the car. Stiffness may give a sporty feel, but it isn't demonstrably faster

besides, I'm not getting the racing beat kit, but getting a full set of FCM coilovers as a graduation present to myself

[Areddi]

I am buying Chad's V3's as you know. Yet again it is on between us this year, and yet again the stakes are raised!

[Nathan Atkins]

Well that said, no coilovers for me until i at least get my student loans paid off, so it's your fat-tired, modded FRS vs my stock RX8, It'll be a bloodbath.

I'll have to bring Al's BRZ out to stand a chance.

[hufflepuff]

Realize this is a dead thread... but what about swapping the front and rear bump stops? Then you have softer bump stops up front and stiffer ones in the rear.

http://fatcatmotorsports.com/igallery/g ... ngRate.GIF

[Dan133]

Notice that they say "sway bars are designed to reduce chassis-roll in turns and increase the RX-8's sensitivity to steering input, thereby improving maximum cornering power" The improved cornering is a result of sensitivity, not grip. Sensitivity only makes you faster if your car is a handful and needs to be tamed, mine suffers only from understeer. It's maximum grip is adequate before the understeer sets in which would still occur without modifying the bump stops. A stiffer rear sway would balance out the understeer by reducing grip in the rear by shifting the behavior of the rear to more closely replicate a solid rear axle, that ultimately makes you slower if you drive to the limit of the car. Stiffness may give a sporty feel, but it isn't demonstrably faster

besides, I'm not getting the racing beat kit, but getting a full set of FCM coilovers as a graduation present to myself

I'm obviously not an engineer but my understanding of stiffening the rear sway to decrease understeer is thus:
A stiffer rear sway will decrease body roll in the back, thereby decreasing the amount that the inside rear corner will lift, thereby decreasing the amount that the opposite corner will dive, which will preserve negative camber and increase grip in the front?

[hufflepuff]

Swaybars increase weight transfer at the end they are working at, which decreases weight transfer at the other end. A stiffer rear bar actually causes the inside rear tire to be pulled up or jacked up. So you're actually loading the outside tire MORE heavily with a stiffer swaybar, but you get less weight transfer at the other end of the car. I.E. The left rear corner is using the right rear corner's spring for compression since the bar is pulling up on the right. (This is why a stiffer rear bar on an open differential RWD car would tend to increase wheel spin.) The jacking effect is evident; watch FWD cars with really stiff rear bars and they lift their inside rear tire well into the air. The rear bar is actually preventing that tire from dropping and making contact with the pavement; the stiff rear bar makes the rear end "less independent" (obviously, since it connects the two and tends to make them move in unison).

So for a car that has understeer, going to a larger rear bar diminishes rear grip while slightly adding to front grip.

now going stiffer at the front and rear in unison, you can preserve the car's balance while adding roll resistance. But, this MAY still result in more overall weight transfer to the outside tires and an overall reduction in grip??? This is something I've never been clear on - the effect of swaybars that dominate the car's overall roll stiffness (contribute more than the springs). My RX8 and Miata are both on stock springs and beefy sways, so they fall in this example. They get less roll, but am I losing grip compared to running stiffer springs and softer sways?

regardless of how you go about it (stiffer sways, stiffer springs), your goal is to obtain optimal tire contact patches with enough roll resistance for the vehicle to respond well to transients. Cars with good SLA suspensions don't need as much roll stiffness to achieve the former compared to McStruts.

[hufflepuff]

total load transfer is a function of the CG and the track width, no other suspension related factors. so if you increase the front and rear swaybar stiffness proportionally, you are not increasing weight transfer at any wheel, and the vehicle stays flatter. since total load transfer is not affected by swaybars, what you are doing is increasing weight transfer at the stiffer end and decreasing weight transfer proportionally at the softer end.

http://www.eng-tips.com/viewthread.cfm?qid=254265

So this also tells us that stiffer springs are not reducing total load transfer, either. The swaybars and springs should only be stiff enough to help preserve the desired contact patches and improve transient response sufficiently. Any "stiffer" beyond that, and you're probably just reducing grip on bumpy surfaces (unless the stiffer springs allow you to lower even further, but then suspension geometry and roll centers may suffer).

http://honda-tech.com/road-racing-autoc ... e-1465882/
this one's not 100% accurate, but most of the discussion is about right.

[Nathan Atkins]

Yep that's pretty spot on Matt, my only beef with sway bars is that your damping has to come from the regular dampers coaxial to the coil springs, so you have to compromise your damping in a straight line to gain more motion control in roll.

Ideally you would use no swaybars and have supple soft springs but we need to meet in the middle for the sake of responsiveness. (OR use hydraulic interconnect like a formula one car or the McLaren vehicle line)