4 replies to this topic

[Andy69]

I've made some more changes to the car over this season and I'm still in the process of sorting things out.  The car is faster, but inconsistent.  I would appreciate any input from the experienced guys here.

 

1987 Si with stock 1.5, 27.5 mm front, 500 lb rear with GC coilovers, stock front ARB,  22 mm rear ARB.  Wheels and tires are 13x9 Keizers with 255 A7s, but recently I switched to a set of similar sized Avon slicks because I can get a used set of Avons with 98% rubber for $280.  It now has the Medieval Pro dampers.

 

Last year I took 175 lbs out of the car, including the roll cage from the car's IT days.  That alone affected the way the car handles in ways I am still figuring out.  The car now weighs under 1700 lbs empty.  

 

I also corner balanced the car using the cross weights.  It was faster at the last Mid South event in August and I beat my closest competitor for the first time this year, and I was only a second and a half behind Karl and his crazy prepped C5 vette.

 

But then I made more changes and I'm still sorting things out. 

 

I spent many days measuring the car for the purpose of modeling the suspension. I used several different calculators and they all gave me different results but the one I am relying on most is the Dynamics Calculator at DG's Autocross to Win.  It's simple, and I'm getting the basics down first.  It told me that my spring selection was as close to what the author believes to be optimal for autocross as I can get given the limited spring selections for these cars.  They recommend suspension frequencies of 2.1Hz f and 2.4Hz r, and mine is 2.4 and 2.8.

 

The process also revealed that my front end was too low, resulting in positive camber change in turns, and the dampers I was using, Illuminas, were insufficient.  So I raised the front so the torsion bar arm was about 3/16" lower at the ball joint than at the torsion bar, as measured along a line through the center of the bar and through the two outer bolts that mount to the control arm.  I also picked up the Medieval Pro dampers valved for the 27.5mm bars front and 400 lb springs rear.

 

I spent the first event with the new dampers and tires getting the pressures right on the tires and playing with the settings on the dampers.  I found that my best bet is probably to set the front in the middle of the range and the rears at full stiff, which matches them to the springs.  So, I think I have the tire pressures and the damper settings pretty close to where they should be.  

 

The thing is, now the car pushes like crazy.  It's making me slower.  And while I did do well this last weekend at Columbus AFB on Saturday - I made the podium against some very competitive drivers - Sunday I was quite a bit slower than the rest of the field.  Still trying to figure that one out, but temps were lower and we ran the course backwards from the previous day which I think accentuated the understeer.  I was a full 4 seconds behind KArl and the vette on Sunday.  I can't throw the car into corners like I used to since it no longer rotates.  I used to be able to just stab the brakes and turn the wheel and  count on the rear coming around but it doesn't do that now.

 

I've been doing some thinking regarding the changed balance of the car.  I think what happened was I changed the front height but not the rear, so I inadvertently increased understeer.  I have adjusted the rear height upward about 3/8" to now be slightly lower in the rear as measured from the pinch weld to the floor.  But I'll have to admit my inexperience means I don't know exactly how much understeer will be induced by raising the front and not the rear.

 

Today's project is to corner balance the car again.  I'll see what that does WRT the oversteer/understeer.  

 

Beyond that, I may need to add a front splitter.  I already have 2 of the 3 pieces of the factory Honda chin spoiler, and perhaps I can add a spitter to the bottom of that.  But of course, I want to test the corner balancing first so I'm not chasing multiple changes at the same time.

 

Thoughts?

 

 

[cbstdscott]

First off, are there any more sets of Avons in that condition and price?

Did you completely disconnect your ARBs before corner weighing?

How do you measure tire pressure other than a guage? Tire surface ltemps? Chalk on tread sides?

No mention of camber plates. If you do not have them, you need them. About 1% negative camber works for me. If no camber plates you can "slot" the strut upper mount.

Because the car is nose heavy, shifting weight backwards is always a help. In profile, lowering the rear to the point that the front of the car is higher shifts weight back.

Rotation. There is a very easy way to get gobs of rotation in these cars. Remove the internal rear sway bar and throw it away. If you have a locking plate, throw it away also. You only need a conventional rear sway bar.

Once you have freed up right rear trailing arm you will probably want more roll stiffness in the front to counter all the new rotation that you will find.

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[Andy69]

First off, are there any more sets of Avons in that condition and price?

Did you completely disconnect your ARBs before corner weighing?

How do you measure tire pressure other than a guage? Tire surface ltemps? Chalk on tread sides?

No mention of camber plates. If you do not have them, you need them. About 1% negative camber works for me. If no camber plates you can "slot" the strut upper mount.

Because the car is nose heavy, shifting weight backwards is always a help. In profile, lowering the rear to the point that the front of the car is higher shifts weight back.

Rotation. There is a very easy way to get gobs of rotation in these cars. Remove the internal rear sway bar and throw it away. If you have a locking plate, throw it away also. You only need a conventional rear sway bar.

Once you have freed up right rear trailing arm you will probably want more roll stiffness in the front to counter all the new rotation that you will find.

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Yeah, call John Berget racing tires (www.jbracingtires.net).  One thing to consider is that used racing tires are a hit-miss proposition, and you may not be able to get the same tire every time, which will require that you experiment with tire pressures constantly.

 

Yep

 

I chalk the sidewalls. I shoot for the wear hitting shoulder just right.  Once I have it I don't usually mess with tire pressures during an event since if the wear pattern doesn't hit the shoulder of the tire in the right place, you're slower.  That said, I may have benefited in the short term this last weekend by putting some air in the rear tires to make them stick less and therefore reducing the understeer, but inducing LESS grip anywhere on the car is not a recipe for long term success.  You are much better off adding MORE GRIP to the part of the car that is slipping rather than taking grip away from the part that is gripping. 

 

I have camber plates set for max neg camber and caster

 

More weight to the back will increase understeer, so it's a tradeoff.  Raising the rear relative to the front will shift weight forward, making the front heavier and exacerbating the already unbalanced front-rear weight distribution, but it will increase front grip and reduce understeer. The trick is finding the point where you have raised the rear too much and upset the balance of the car.  After reading around RPR, I think that point is an even height at all four corners.

 

Haven't considered that but I do have an HF rear axle to install.  It's probably a wash vs removing the internal bar wrt weight.  I know some think the articulated trailing arm on the Si helps with rotation but I don't think it articulates enough for it to matter.  I think the articulation is just for the internal rear bar.  One thing to consider is removing the internal rear bar is not a legal modification re the SCCA Solo rules.

[cbstdscott]

"More weight to the back will increase understeer,"

You may want to review this statement.

Add the HF axle if that is the way to stay legal. You will thank me later.

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[Andy69]

I checked the corner weights yesterday, and the balance was off - 46/54. I had to drop the RF and LR considerably to even it up. I messed with the ride height in the front and rear over the last few months and neglected to think about that. This imbalance is the reason I was so slow on Sunday - ran the course backwards and it was mostly right turns.

So much to keep straight, you can drive yourself nuts.


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