24 replies to this topic

[JeffBaker]

Need a quick eval path:

Car has been extremely fast, competitive for several years. It's now understeering horribly.

ITA Miata with Prather/Afco spring-shock setup 750/400, ISC offset solid spherical bushings, ISC bars front/rear, R7 on 7" rims, 2515weight, perfect 50/50 cross.

-3.5 front, 6.0 caster, 0 toe, -3.0, 0 toe rear.

Recent alignment set the rake at 12.5" front, 13.5" rear.

I've messed with tire pressures, checked toe, checked camber, loosened front swaybar. Still understeers on turn entry, in pretty much every turn, but most noticable on tight, slower entry.

Race this weekend, any ideas?

thx so much,

JB

[Peter Olivola]

Front suspension binding.  Sway bar links free?  Any evidence of chassis damage that could cause dynamic camber/caster changes?

 

Also check the rear for a broken sway bar mount defeating roll stiffness in the rear which would cause a significant increase in rear grip and reduction in front grip on turn in.

[JeffBaker]

Good thought on rear, thanks. Front mounts, bushings and shackles are all good.

[Bench Racer]

Need a quick eval path:

Car has been extremely fast, competitive for several years. It's now understeering horribly.  During this time period have you always been the driver? Did the same guy do the current set-up as compared to when the car was extremely fast/competitive?

ITA Miata with Prather/Afco spring-shock setup 750/400, ISC offset solid spherical bushings, ISC bars front/rear, R7 on 7" rims, 2515weight, perfect 50/50 cross.

-3.5 front, 6.0 caster, 0 toe, -3.0, 0 toe rear.  Presuming nothing within the suspension is out of sorts, give each front 1/16 toe out and each rear 1/64-1/32 toe in.   

Recent alignment set the rake at 12.5" front, 13.5" rear. Don't know where your measuring. Flatten this out. If you have a 1/8 inch lower at front of pinch rails than at rear of pinch rail, that'd be a good start.

I've messed with tire pressures, checked toe, checked camber, loosened front swaybar. Still understeers on turn entry, in pretty much every turn, but most noticable on tight, slower entry.

Race this weekend, any ideas?

thx so much,

JB

 

Give Jesse a call, he's excellent support to his customers/shock set-up.

[JeffBaker]

Thanks for the feedback. I've always been the driver. New setup, and the guy who did the setup is very fast as well, but I'm concerned about the rake setting. I've always had zero rake.

This whole thing started because I raced NHMS and consistently hit the underside of the fender with the tires. I wanted to raiss the car a bit. My normal setup guy ws not available.

Measurement is from center of axle/hub to edge of fender.

I'll try working the toe, but at Watkins, I need all the straightline speed I can get...

[MPR22]

Set car back to level.
Check ball joints.
1/16 toe out front and 1/32 toe in rear will not cost you any speed. You are racing at WGI, if the car pushes you are screwed, handling will be more important than any minuscule speed scrub from toe adjustments.

[Tom Sager]

Thanks for the feedback. I've always been the driver. New setup, and the guy who did the setup is very fast as well, but I'm concerned about the rake setting. I've always had zero rake.

This whole thing started because I raced NHMS and consistently hit the underside of the fender with the tires. I wanted to raiss the car a bit. My normal setup guy ws not available.

Measurement is from center of axle/hub to edge of fender.

I'll try working the toe, but at Watkins, I need all the straightline speed I can get...

 

There are so many possibilities.  You've changed the tires?  You might look for something in the front that is bent or loose or binding that won't show up in a static measurement.  You're sure the tire is no longer bottoming on fender?  Also your toe setting might be more a culprit than you think.  For one, zero toe is not the least drag when you are running -3.5 degrees camber.  You've got camber scrub for sure at that setting and running 1/8" - 3/16" total out will provide less drag.  Also, the NA cars suffer from bump steer at racing ride height and max caster assuming stock steering rack position and other stock arms, knuckles, tie rod ends, etc...  That means that when you turn the wheel you are likely losing ackerman (toe out) rather than gaining it so at zero toe in a tight corner with more steering wheel angle you might actually be toeing in slightly which will cause a push for sure.  The NA cars IMO need slightly more toe out static than the NB's. 

[callumhay]

In checking Carroll Smith's book ("Tune to Win"), the front being too low will cause corner entry understeer. (Presumably from too much weight on the tires on braking causing the slip angles to reach and exceed the maximum grip too quickly..so you don't have time to catch the car?). I also wonder if the car is cross weighted at 50/50 with too much rake, that if that causes the center of gravity to change. Maybe it goes rearward? (and up) If so, as soon as you release brakes in the corner, the car is going to settle on the rear and unload the front, compounding the understeer. Probably on the tighter corners the weight transfer is much quicker..so more likely to see the issue in those types of corners.
Does the new guy for set up work on a lot of miatas? If he is used to bigger higher hp cars maybe what works for those won't work as well in a Miata?

[JeffBaker]

Great thoughts on these, and I hadn't thought about camber vs toe that way. I do know that the camber settings are .5 degree more negative since I started running R7 over R6, and my times dropped every track, with tire temps coming in perfectly.

The car has the corrective tie rod ends and a '99 subframe/rear end. The shocks are not bottoming, for sure. The alignment is at my historical settings, save for the ride height which is 1/2" higher front/1" higher rear. This did correct the tire rub in the fender wells - I ran tape across the insdie radius of the fenders on all four sides and no marking or removal of the tape.

I was wondering about the cg. Your description matches my impression of the feel of the issue.

New guy is a Miata racer and a good fast one. He sets up both AX and road race cars.

Thank you all for diving in, it's what is great about the miata community!

[Caveman-kwebb99]

This could also be a shock issue, I have found in the past that alot of the revalved shocks out there make the car handle in this way. So I would also think that a bad shock could cause this as well. Just my thoughts

[Danica Davison]

Jeff, what is your size? Are you a big guy?

[JeffBaker]

I'm 6 foot 155lbs. Wiry.

[JeffBaker]

BTW, Caveman, Watched you get your Hard Charger award.

Respect.

[JeffBaker]

Jeff, what is your size? Are you a big guy?

About 155, 6 feet. Taller than Elivan, but lighter

[Caveman-kwebb99]

BTW, Caveman, Watched you get your Hard Charger award.

Respect.

 

 

Thanks, Jeff, now if I could only figure out how to qualify!

[ner88]

cant trust the fenders for ride height.

You may have too much front bar.

Tire pressures??

If youre using the rear subframe and uprights from a 99' and NA subframe up front then you may have more track width in the rear.

[granracing]

"New setup, and the guy who did the setup is very fast as well"

 

I wonder if this is a part of your issue.  Certainly something I've heard many times from racers who get the "fast guy setup".  I know when Kessler sets up my car, he doesn't give me the same setup as his pro driver whose used his Miata's. 

 

Are you now back to the set-up you were super excited about at Thompson?

[luvin_the_rings]

I'm still getting to know these car setups, and I'm fairly new.  However, I have formal education in vehicle dynamics.  I'd say that the rake is something to look at.  With too much rake, the roll axis of the car can become too tilted forward and cause a difference in the kinematic and kinetic total lateral load distribution ratio front to rear.  Basically, with the front so much lower than the rear, you could be running into a situation where the cornering force in the front is transmitted through the spring more so than the rear,  this could cause too much front spring deflection and make the front of the car "fold over" causing under steer.  This is because the distance from the roll center to the CG is vastly different front to rear.  We run into this problem a lot when dealing with cars that have MacPherson front suspension with double a-arm or trailing arm setups in the rear.  

 

Also, like said above, the fender's are not a reliable way of judging ride height.  It is a good sanity check after an off course excursion, but for setups, most people use the pinch weld heights.  When this isn't feasible due to pinch weld damage, the pro's use shock length to measure ride height.  I like to use a digital level running across the pinch welds on both sides.  

 

Final thought: 6 deg of caster seems like a whole shit ton.  Are you running offset bushings in the front?  Doesn't the NA caster max out around 3-4 deg? That's all we were able to get out of it.   Incorrect caster will cause terrible bump steer.  Something that I am still battling with 2 years later on my NA.  

[JeffBaker]

"
Are you now back to the set-up you were super excited about at Thompson?

So yes, I found a shop who reset everything back to what was so fast at TMSP in the spring, and has proved excellent through the season, then we went back to trying to solve what got all this started - wheel to fender rub. We raised the height 3/8" and the put all the camber/caster back in.

I'll tell you Monday if it worked...

[JeffBaker]

Best Answer

Final thought: 6 deg of caster seems like a whole shit ton.  Are you running offset bushings in the front?  Doesn't the NA caster max out around 3-4 deg? That's all we were able to get out of it.   Incorrect caster will cause terrible bump steer.  Something that I am still battling with 2 years later on my NA.

Thanks for the feedback. Fwiw I've never had a problem getting caster. This car does have offset spherical bushings, with the Mazda corrective tie rod ends. The car will attain 7.2 degrees on both sides. I've always run 6, but now that you bring it up I will look at it and do some testing.

At the track last week, I did loosen the front swaybar and that definitely helped - but i loosened it from a setting that was previously working very well. So my feeling was that I was using lateral roll rate to compensate for some other porblem - root cause was still out there.