New project-reducing bumpsteer

[RebekahsZ]

I am on slip plates, btw. I'm considering taking the car to an alignment shop between the 1/2-mile and the mile. But, we always assume that the shop's stuff is more accurate than ours. And that isn't necessarily so. They have calibration issues too. Most of the time there is at least something I have to fix once home from a laser alignment. Last time there were two lock nuts loose. Ive found that just torquing a lock nut can induce as much as 1/8" change.

[JMortensen]

Totally agree on all points. MM rear arms had no triangulation and big squishy rubber bushings on the outers IIRC.

 

Bad calibration on that Hunter machine was what led me to run 1/2" toe in and wear out a brand new set of tires in a week. Same thing happened on a customer's car and burned up her rear tires in short order.

 

As to the measuring at home vs laser, I tried to do a string alignment with limited success as well. I think slip plates would really help, particularly with measuring caster. I tried drawing the angle on my shop floor and then measuring camber and it was not repeatable and the car pulled hard left. Can't really do string alignments with the car sliding left to right though. I suppose could set camber and rear toe, then measure caster and set front toe on the slip plates.

[RebekahsZ]

You got it right on the MM LCAs. I got them back when I was running 9" drag slicks inside stock fenders and they are the only version around that can be made shorter than stock as I needed to narrow the rear to maximize fender space. One phenomenon that validated my bumpsteer reduction was that the slip plates used to torque when I lowered the car. Now they just slip laterally with no torquing. So I do think I'm getting somewhere. I just took a nap and feel so much better (I used to be able to work 'til 2am every night!). But my assistants are all napping so I'm gonna work on a mounting pad for the spoiler til I have someone to turn the steering wheel for caster. I'm betting all my bumpsteer work from yesterday goes in the toilet when I add caster. Then it is back to the drawing board! Oh, bye, 450 springs are stiff as hell-I'm afraid my windshield is gonna pop out when I pull into my driveway! Sorry about having to twist your screen, but here is a photo dump.

Edited September 4, 2016 by RebekahsZ

[heavy85]

Wait are you checking bump steer with weight on the tires and rubber bushings? If so I'm guessing that's you issue with measurements. Should measure with weight off the suspension to avoid any binding from the tires needing to scrub across the ground. If you don't have dial indicators then just clamp some angle iron or something to the rotors and measure with tape measures.

[RebekahsZ]

Do you not think that slip plates handle the binding well enough?

[RebekahsZ]

My buddy James helped me til midnight last night and my other buddy, Evan helped tonight 'til 2am. Building off data from the past 2 nights, we started by increasing caster to 7 degrees positive. Then we ran the car thru the travel, measuring toe every 1/2" within the anticipated bump and compression range. It was great using the new 4-post alignment lift and doing chin-ups off the sway bar to settle the suspension! Then we plotted the curve that resulted. It looked like we needed a mix of the 4.5 caster setting and the 7 caster setting, so we went out and set caster to 6 and went thru it again. Son-of-a-gun, that was the ticket! Going thru the travel range and re-measuring to yielded the following results:

 

6": 1/8 out

5.5": 1/32 in

5": 1/32 in

4.5": 1/16 in

4": 1/32 in.

 

I set ride height at 4.75" measured at an airdam support bracket and called it done!

 

Final Landspeed alignment setting is:

 

Front: 1/16" toe in, -.3 camber, +6.0 caster, stock inner tie rods, stock crossmember LCA pivot hole location, first-generation TTT LCAs adjusted to shortest length, 450# coilovers. Initial Koni rebound setting will be full stiff. 24.5" MT Front Runner tires at 50psi. 1-1/8" sway bar.

 

Rear: zero toe, -1.5 camber, 400# coilovers. Initial Koni rebound setting will be fill soft. 275/60/15 MT Drag Radial Pro at 44 psi. No rear sway bar.

 

Tomorrow is test drive and centering the steering wheel. And hopefully that finishes suspension. Or, we might be starting over completely!

Edited September 5, 2016 by RebekahsZ

[Chickenman]

You may want to re-think the no rear bar setup. A rear anti-roll bar actually increases the straight line stability of a car. And with a high CG like you have, you want as much roll stiffness as possible. The front bar alone can't do all the work. Side winds are one area where a rear bar will really help with car stability. Even the stock one would help. But a 7/8" would be better. You're not turning corners, so don't confuse the oversteer/understeer balance issue associated with turning corners like in Autocross. On a Landspeed Attack car or Drag car the Traction Circle is only limited to a straight line. There are no, or minimal cornering forces generated. 

 

Running on Highways can pose problems. A Highway has road crown and dips and wallows. Additional roll stiffness will really help the car maintain a level attitude on uneven roads. Dragstrips and Airport runways are better... but Airport runways can often have gusty side winds. Add side winds to a high CG and you have a white knuckles experience.

 

Edit: As a side note.  A few years ago I helped " Heavy Chevy " at ThirdGen.Org set up his 1988 Camaro Super Stock car. He always ran it with no rear bar and car was inconsistent launching.  Addition of a rear anti-roll bar  transformed the car  and it launched dead straight every time after that. Production based Drag cars that lift their front wheels on launch need a rear anti roll bar. Rear springs alone are not enough to handle the body roll. Front anti-roll bar does nothing when the wheels are off the ground.  IRS cars are better at launches than Beam axle cars, but there is still Torque induced roll present. A proper Racing Four link or Three link  design is a different ball game.. but production rear suspensions have big compromises.

Edited September 5, 2016 by Chickenman

[Chickenman]

Eventually you may have to make a decision on which way you want to compete with this car. A high CG on a Drag car is somewhat desirable because of weight transfer at launch. ( Although Pro Stock cars have a surprisingly low CG ).  On a Land Speed car you want the CG as low as possible for stability and minimal wind resistance. . 

 

I've heard you mention Autocross suspension settings. Do you Autocross this car as well or do you have a separate Autocross car? If only one car, then that is a big challenge to run all these different types of events.  A lot of compromises. Drag Car setups and chassis design are just too different from Autocross or Road race designs and setups to be realistically compatible.

 

Tires are a big factor for straight line stability.  Drag slicks have very little sidewall stiffness. Even when pumped up to 44 psi or more. They simply aren't designed for lateral loads and don't handle things like side winds well.   Something like Nascar tires or GT1 Road Race tires have much stiffer sidewalls. These will handle the high speeds needed while providing lateral stability and allowing you to lower the CG. Personally if I was building a car for these Land Speed attacks on public roads or even Airport Runways, I would be leaning heavily towards a Nascar chassis design and setup.  However, I can understand the dilemma if you want to Drag Race and Land Speed attack. It's all a compromise...   Just my .02c. GL and race safe.

Edited September 5, 2016 by Chickenman

[RebekahsZ]

Thank you! Tons of good stuff in there. I'm gonna think thru all your suggestions and work-in what I can. Glad to have you riding along. Honestly, a Z-car is a horrible Landspeed candidate. But, if I got a 240sx or an Rx7FD, I couldn't hang out on this forum-and I love you guys! So I'm gonna keep beating on this old chassis til one of us gives up-the chassis, or me.

Edited September 5, 2016 by RebekahsZ

[RebekahsZ]

Dave at AZC has produced a bumpsteer kit for our cars and I just bought one. Since I spent all that time getting my front alignment "right," these will sit on the shelf for a while while I work on other aspects of the car. But here is a photo of the kit. Note: the inner tie rods are not part of the kit. These are Rare Parts PN: RP26575, which are both RH threads, so you need to request RH thread outer tie rods from Dave when you order your bumpsteer kit. I have these inner tie rods on my 240z and they fit great. Also of note: the bumpsteer adjuster is made too long on purpose with the expectation that you will trim off excess threads for wheel clearance after bumpsteering the car. The quick steer knuckles are sold separately.

Edited November 24, 2016 by RebekahsZ

[clarkspeed]

Another great product from Dave! Solves a problem AND saves weight.

[260DET]

At really high speed it's all about aero, particularly where the track undulates or has peaks, anything that lifts the front of the car up. Alignment is only relevant when the car and particularly the front is on the deck and particularly when it comes back down from a aero fail induced front end lift incident. Then it is crucial and can make the difference between a safe 'landing' and disaster. 

 

Ask me how I know that from experience. Ask me also how aero is king, it's always about keeping the car with a bit of rake on the deck no matter what the track is trying to do to upset that crucial balance.

Edited December 2, 2016 by 260DET

[JMortensen]

OK, how do you know that from experience?