Lateral effects

[ttmott]

I'm in the throws of finishing the suspension on my 70 Cuda'. The front suspension is an RMS AlterKtion with PS rack, adjustable QA1 shocks and 500# coilover springs. The front anti-sway bar is a 1 1/8" diameter solid bar with a 9.5" effective torsion lever.

The rear suspension is a modified Alston Chassisworks ProStreet 4-Link with Panhard. It is sprung from adjustable VariSpring shocks and 176# coilover springs. The lower bars intersect the road surface at about 200" in front of the rear axle centerline and the upper bars intersect the lower bars at 174" in front of the rear axle centerline. The bars also splay outward from the front pivot to the rear pivot at 3 degrees to set the shocks/springs as far outboard on the axle as possible. The rear stablizer bar will be a splined 7/8" with arms to the outside of the frame rails. As this car will have over 1200 Horsepower and a manual transmission I needed to intersect the bars so the suspension will take a set and transfer weight to the rear upon acceleration rather than jack the rear of the car. However, lateral handling is the primary objective. The chassis is lowered to 8" from the ground measured from the sill bottom rail.

The issue I am trying to understand is the handling effects due to where I had to install the Panhard bar; it is parallel to the axle at ride height but just above the center section (see photo). I realize the roll center is above the axle centerline but don't really understand the overall effect. I'm not opposed to relocating it but will cause havoc with the fuel cell placement.



Thanks
Tom

[wendell]

I wrote a huge response to this post but it didn't load. Oh well. Basically it was...
huge rear swaybar+ crazy high rrc= no lateral grip= very loose
174" IC, maybe an inch off the ground= >5% anti squat ("jacking")= no grip= 3sec 60'

Aside from the p-bar, you can tune what you've got to perform well. Cool project, good luck.

[ttmott]

I wrote a huge response to this post but it didn't load. Oh well. Basically it was...
huge rear swaybar+ crazy high rrc= no lateral grip= very loose
174" IC, maybe an inch off the ground= >5% anti squat ("jacking")= no grip= 3sec 60'

Aside from the p-bar, you can tune what you've got to perform well. Cool project, good luck.

I here ya... The lower bar forward connect points will be moved up 2 inches. This should get the anti-squat up around 60% but looks to induce a possible slight oversteer condition (I believe). I need to drop some plumbobs off the axle to validate.

the question I have is; what is the consequence of having the panhard up so high?

[Norm Peterson]

It will cause the rear to dominate the distribution of lateral load transfer, and to limit roll itself - possibly to the point where you can't develop enough of that for the front suspension's greater roll stiffness to shift the overall vehicle balance back over to mild understeer. Closely related is that it makes the vehicle much less responsive to efforts aimed at shifting the handling balance.

You can sort of "get away with" a PHB in this general location in a car with large x huge x soft rear tires and little skinnies up front due to the big difference in grip, or on a dirt track where a massive oversteering cornering attitude (and wild amounts of opposite-lock steering) is normal. Perhaps too, on a solid front axle car with its front roll center somewhere up around axle height.


Norm

[ttmott]

Thanks that's what I needed and not what I wanted to here. Rear rubber will be 335-35-17 and front 245-45-17 so the front will have grip. The software I have (SusProg3d) is not really resolute on the PH bar. I'm really concerned exiting the corner and having a bad oversteer problem induced due to antisquat less than 100% and the lower bars angled upwards rather than parallel to chassis. I can always minimize this problem with stiffer spring rates and stiffer roll control bar at the sacrifice to other elements.

I'm going to cut off the PH bar and locate at the El. of the axles.

?? Is there a problem with the PH bar not being parallel to axle in the "Y" axis (looking down on the suspension)? If not a problem this would free up some critical real estate.

[Norm Peterson]

The lower bars can still be angled up without driving things into (vehicle) roll oversteer. You just have to keep the elevation of their virtual intersection lower than the height to the midpoint of the PHB, ideally maintaining it there under at least the most commonly encountered range of chassis rake and suspension positions.

Anti-squat is not necessarily a cure-all for forward traction issues. There's at least one nationally competitive A-Modified Solo driver/builder who firmly believes that letting a PHB-defined roll center drop under acceleration (implying less anti-squat) will increase the amount of grip available for acceleration by restoring some of the rear grip that was lost to lateral load transfer up through mid-corner.

I think the matter of a plan-view-skewed PHB (or what would essentially be a PHB) was partly the subject of one of Mark Ortiz's fairly recent "Chassis Newsletters", but I don't recall any particulars other than the discussion involving circle track cars of some sort. It's around here, somewhere, or maybe on my computer at work . . .


Norm

[Marcus SC&C]

Fab work looks nice. I agree the PHB really should be moved. If I were you I`d move it just behind the coilovers and fab mounts with multiple mounting holes or NASCAR style slot/serration mounts for vertical adjustment with possible heights between say 12"-15". It`ll mean a lot more bracketry but it`ll be worth it in tunability. Make the bar as long as humanly possible while you`re at it. The lower you move the PHB the more spring and swaybar rate you`ll need to balance the car. WIth the 4 link I`d set up the lower arms level at ride height to minimize roll steer and adj. antisquat with the uppers. It looks like you have a number of holes to work with on both the uppers and lowers so you should be fine. Get the whole car together,find (or rent) a place where you can seriously test and tune (savagely thrash?) the car and go from there. Honestly some of the finer points are probably going to be lost on this car. The horsepower will dominate the car. You won`t have traction or anything that remotely resembles it on 35 series street tires and oversteer will always be a toe twitch away so I`d concentrate on making it`s handling characteristics as benign as possible to let you focus on driving rather that agonizing over minute details on paper/computer. It should be quite a ride! Mark SC&C

[ttmott]

Here is the direction I'm going.....braces for the frame mount are not yet fabbed. The bar will not be parallel to the axle in the "Y" axis however. Also this location is throwing a wrench in the stablizer bar mount location.

The horsepower is completely tunable as it's turbocharged, intercooled, and injected. I'll be able to move the torque around quite a bit with a laptop and up to four preprogrammed driver switchable settings the FAST ECM provides. The plan is to set up the ECM for 91 Octane street (around 2 BAR boost maximum) and switch to higher grade fuels / power levels as desired up to 2.8 BAR . In reality the wastegates can be set to any boost setting. So trackability can be optimized; it should be very predictable.

Thanks for the help!

[Norm Peterson]

If this is to be anything resembling daily driven, I'd plan on building in some throttle possession. Too much throttle too quickly in the beginning will make smooth driving with 1200 hp potential difficult (to say the least).

I agree, mostly, with the notion of "benign" handling, but not to the point where everything is heavily understeer-dominated. Too much axle roll steer tends to turn around on you on lift-throttle and braking, where the axle now either steers less in the vehicle understeer direction or actually flips over to vehicle oversteer. I guess it's the rate of change in the roll steer that's as important as the static ride height value here.


Norm

[Norm Peterson]

Here is the direction I'm going.....braces for the frame mount are not yet fabbed. The bar will not be parallel to the axle in the "Y" axis however. Also this location is throwing a wrench in the stablizer bar mount location.

I'll try to find that "Chassis Newsletter" article. I think it was Sept or Oct of this year.


Norm

[ttmott]

I'll try to find that "Chassis Newsletter" article. I think it was Sept or Oct of this year.

Thanks Norm; much appreciated.

One of the big challenges is there will be two giant 90mm throttle bodies causing a large MAP drop with the slightest opening of the butterflys resulting in very jerky off idle conditions. This is fixed by using highly progressive throttle linkages, delaying the opening of the second throttle body for about 10 degrees of the primary, and some software tweeks. This engine, in fact, will be very streetable; low compression, mild cam, etc. Without boost it will be in the high 500 to low 600 HP range. If you are interested in the engine combination I'll post some data in the Engine Misc Forum.

The suspension, I believe, is the make or break on streetability and also have the capability to play at Sebring or other tracks; this is the area I'm most concerned and need to get it right.

Tom

[Davezz28]

I thought I would post a picture on how I make the PHB bracket on my car. When building or designing stuff, I like to look at as many designs as possible before welding.

[pav8427]

I tried to find the 'Chassis Newsletter" also,and am not having any luck.
Can someone point me in the right direction.

Thanks, Doug

[Norm Peterson]

I tried to find the 'Chassis Newsletter" also,and am not having any luck.
Can someone point me in the right direction.

Thanks, Doug

You request to be added to the subscription list and it is e-mailed to you as a Word file attachment. Send an e-mail to [email protected]

As for back issues, I did stumble across a fairly complete collection a couple of months ago, in pdf format. While I don't recall offhand the site, I did find just a moment ago that I did put the collection on this computer.

The PHB article is in the August 2006 issue, which I do have. Around 50k in either Word or pdf, and since Mr. Ortiz actually encourages reproduction for free use I believe that I can e-mail any number of copies out as long as I don't request payment. I'd prefer to send the pdf, though.


Norm

[ttmott]

The PHB article is in the August 2006 issue, which I do have. Around 50k in either Word or pdf, and since Mr. Ortiz actually encourages reproduction for free use I believe that I can e-mail any number of copies out as long as I don't request payment. I'd prefer to send the pdf, though.

Norm, sent private message.

I modeled my 4-link on ProE showing the PH bar skewed in one model and parallel on the "y" axis in the second model. Then moved the suspension both up and down and simulated 4-inches of total differential side to side. There was virtually no change in hub positions between the two models. The only difference is the skewed bar added some stress to the 4-link bars at the side of the axle where the PH bar is attached when side load is applied (in my case, compression on a left hand turn and tension on a right hand turn). The load is obviously proportional to the angle of the bar. For very small angles the effect will probably not be adverse. For larger angles depending on the configuration of the 4-link one side of the chassis (where the PH bar is attached) may want to rise or drop. I'm back to making it parallel......

Tom

[ttmott]

First - Norm, the article was a great help, Thanks.

I ended setting the panhard bar parallel in all planes and at the same elevation as the axle centerline but adjustable 4 3/4" up and 1 3/4" down. The geometry works well at all chassis positions. Not an easy fabrication effort to say the least.

Now time for the sway bay. Looking at the Coleman Racing or Speedway Engineering 3/4" or 7/8" 43 spline bars. The arms will be adjustable between 7 1/2" and 13 1/2" and the bar will be 43" between bearings (46" total length). I calculate the 3/4" bar has a rate of 110# at 7 1/2" and 62# at 13 1/2" at 5 degree deflection. The 7/8" bar is 193# at 7 1/2" and 110# at 13 1/2" at 5 degree deflection. The car will weigh right around 3800# with a 60/40 distribution and CG at about 24".

Your thoughts on diameter?

Thanks for the advise.

[ttmott]

To the top for any help on rear sway bar diameters...


Thanks...

[wendell]

I think there are way to many variables (at least for me) to suggest a sway bar. I personally can't understand why your from scratch suspension should need one. If it was my world I'd have concentrated on providing pbar adjustment bellow axel center line instead of above. A lover RRC could have mitigated the need for a sway bar.

Regardless. Drive the car and use your driving impressions to determine the sway bar.

[Norm Peterson]

Ummm, lowering the rear RC generally increases the need for either more rear bar, more rear spring, or some of each.

But the final proof IS in the driving . . .


Norm