5 replies to this topic

[Andy69]

After experimenting off and on for a couple of years, I've decided that having a rear spoiler makes the car faster.  That's probably a DUH moment for a lot of folks, but for a while I wasn't convinced, and I still suspect it slows me down at higher speeds.

 

Anyway, last autox I noticed the spoiler changed the car from a slight tendency to oversteer to a slight tendency to understeer.  I suspect most of this was due to the fact my A7s were worn out, but it got me thinking about the suspension anyway.

 

What, if any, changes has anyone had to make to the suspension to compensate for additional rear grip due to the addition of aero?

 

I'll wait to make adjustments until after the next event where I will have fresh tires, but I'm thinking at most maybe a 50 lb increase in rear spring rate.

 

 

[Sinub]

Late response. I've been told that the 86-87 Crx Si spoiler was actually functional, not just for looks. Honda designed it so it gave the car more stability on higher speeds. Not sure if it actually affects corner handling in anyway, but I would love to hear input from others. 

[Andy69]

Late response. I've been told that the 86-87 Crx Si spoiler was actually functional, not just for looks. Honda designed it so it gave the car more stability on higher speeds. Not sure if it actually affects corner handling in anyway, but I would love to hear input from others. 

From what I've seen, I don't think people retain that spoiler, or they use the HF, which doesn't have one, and then add a spoiler per the SP rules


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[I Like Dry Toast]

Why look for a solution in the suspension?

If you apply an aero aid at one end and it upsets the balance, then apply an aero aid at the other end to reset the balance.

A splitter, a lower air dam, cannards, etc.

[dcostello]

I tested with aero last year and feel its now required on my autocross car.  I first put a rear spoiler on and it made the car so planted that I developed a front splitter for balance.  After all that was completed I did not find a need to adjust any spring rate as the way my car is currently setup I can adjust what I need by mostly by rear tire pressure and shock settings.

 

What I am about to say is my opinion but my opinion does have some experience behind it.  In my opinion a cars alignment, shocks, tire pressures, sway bars, etc… once set are then “perfect/optimal” for one corner only.  For example, if the suspension is set perfectly for a 45 mph sweeper to give near the threshold of tire grip at max G’s then the same set up will be tight/pushing/understeer on a 25 mph sweeper.  Likewise, if the suspension is set perfectly for the same 45 mph sweeper then the same set up will be loose /oversteering on a 75 mph sweeper.

 

With that theory said.  I found the aero broadens that 45 mph “setting” to a wider range of near perfect for corners from 40 mph to 50 mph.  If the suspension is set up perfectly to throttle steer at 45 mph, with aero providing more downforce at 50 mph I can actually still throttle steer at 50 mph at the same rate without worrying about the car being too loose.  I can actually add some spring rate to the car and widen the range where the car is most optimal making it looser and more drivable at lower speeds like 40 mph.

 

Lastly, I will say that if you autocross a lot the courses and corners vary dramatically from speeds all the way down to 20 mph up to 75 mph for street prepared.  Therefore, even with aero you will still push/understeer at 20 mph and be loose/oversteer at 75 mph.  On those you just need to be smooth and possibly give it up a little.

 

Hope that helps.

[Andy69]

I tested with aero last year and feel its now required on my autocross car.  I first put a rear spoiler on and it made the car so planted that I developed a front splitter for balance.  After all that was completed I did not find a need to adjust any spring rate as the way my car is currently setup I can adjust what I need by mostly by rear tire pressure and shock settings.

 

What I am about to say is my opinion but my opinion does have some experience behind it.  In my opinion a cars alignment, shocks, tire pressures, sway bars, etc… once set are then “perfect/optimal” for one corner only.  For example, if the suspension is set perfectly for a 45 mph sweeper to give near the threshold of tire grip at max G’s then the same set up will be tight/pushing/understeer on a 25 mph sweeper.  Likewise, if the suspension is set perfectly for the same 45 mph sweeper then the same set up will be loose /oversteering on a 75 mph sweeper.

 

With that theory said.  I found the aero broadens that 45 mph “setting” to a wider range of near perfect for corners from 40 mph to 50 mph.  If the suspension is set up perfectly to throttle steer at 45 mph, with aero providing more downforce at 50 mph I can actually still throttle steer at 50 mph at the same rate without worrying about the car being too loose.  I can actually add some spring rate to the car and widen the range where the car is most optimal making it looser and more drivable at lower speeds like 40 mph.

 

Lastly, I will say that if you autocross a lot the courses and corners vary dramatically from speeds all the way down to 20 mph up to 75 mph for street prepared.  Therefore, even with aero you will still push/understeer at 20 mph and be loose/oversteer at 75 mph.  On those you just need to be smooth and possibly give it up a little.

 

Hope that helps.

 

Hi Dan.  Thanks for the response.  I didn't see it until now.  I've been getting away from autocross a bit this year since I'm now self employed and I'm working for survival rather than just a paycheck :-)

 

The car definitely is more stable with the rear spoiler.  I have a series of photos from a NASA autocross at the Jetport in Millington that pretty much demonstrated it for me.  One of the other guys has a two part spoiler that he attaches with big Gopro suction cups.  Just for grins he put it on my car during the fun runs.  The guys watching all said I was coming around a right hand sweeper way faster than previous runs when the spoiler part on the left side came off.  I didn't notice inside the car, all I noticed was snap oversteer, but everyone else all saw what happened.  And Jane with her camera (she's there at every event, makes her photos available for free, and only asks for enough donations a year to cover the cost of worn out equipment.  She is awesome) captured it all. 

 

One thing is certain, I need to put some thought into this suspension this winter.  Removing the roll cage and other extra weight (175 lbs worth) has changed the way the car handles and I'm in unknown territory.  The good news is the car weighs about 1750, which is over 250 less than when I got it.  I know these cars can be gotten down to 1650 or so, so I've got some cypherin to do on that.  It's an Si so I'm at a disadvantage over using an HF to start with but I have transfered a lot over from the HP.  The sunroof is next, that should be another 35ish lbs.

 

Details on the car

 

1987 Si, SI transmission with LSD

27.5 mm torsion bars

GC coilovers with 500 lb springs

stock front bar

very large rear bar, not sure what brand, I'd have to go back through the receipts, but it's at least 3/4"

13x9 Keizer wheels currently with old 235 Ecsta V700

I've swapped some lighter HF parts in like bumpers and such

Integra front brakes (which I think are a disadvantage for autocross, since I believe they have shifted the brake bias to the front, causing the fronts to lock up under moderate braking)

 

I've considered running the HF rear axle setup but I was concerned about the durability, particularly with the 255 A7s I was running.

Edited by Andy69, 13 November 2017 - 12:31 PM.