Roll Centers

[7uptruckracer]

I figured this is where I will get the best response. I'm looking to raise my Roll Center and shift it laterally as well. The car has slotted lowers on both sides and I can get Upper Ball Joints with taller studs in a few increments as well. I have also been told different things as to how the lowers should be angled. Should the lower outer ball joint be higher or lower then the inner pivot point if its anything other then level and what happens if its both ways being I have slotted lowers and not sure where to start with ride heights.

[Dirtrunner35]

Do you want to move it static or dynamic ?

[7uptruckracer]

Lets start with static and save dynamic for later after testing! Help me with each effect. The lower arm positions as far as the outer ball joint in relation to the inner pivot point. Which way is correct. and adding longer ball joints on LF upper LF Lower and the RS etc.

[53003jason]

Google "Bob Bolles" and read some of his articles.

[7uptruckracer]

any of the google stuff just sends you to links to buy his books......that's why I posted here for a good breakdown.

[mab475]

http://dirtracingforum.com/ Go here and ask. You'll get some really good responses.

[hpmaster]

Why? I have been working with front end design and designing my own frt. ends and assisting in designing and testing race car frt. ends for builders for over 4 years now. The more I learn the more I find out how little is know by people who have studied this in depth from an engineering problem stand point most of their lives. With an engineering and racing background of over 30 years I have also taken classes specifically about frt. end suspension design and worked with several car designers and several engineers.

First of all get yourself a couple of computer programs and see what cars that run well have for readings. Now I am sure I will be told I don't know what I am telling you by someone but at my age I really am getting used to that and don't care. Roll center is a theory. While it is an indicator it is not the do all answer, all that and a bag of chips some people think, in MHO. Moment arm length, center of gravity location and instant center locations along with camber gain in roll plus initial camber settings AND their complex relationship can be more important in MHO. In other words roll center is in my OPINION an average of ALL those factors. I have built and adjusted suspensions with all sorts of roll centers and roll center movements BUT it is all at the cost of the other factors I listed. This weighing of importance or prioritizing which are most important to handling is what makes a car consistent or inconsistent under all driving circumstances IMHO. Now with that said I have found in MOST cases a roll center from 2" to 4" off the ground and slightly right of center seems to be a good starting, static point. Then again when you start the dynamic changes or increasing the roll center movement you WILL have to sacrifice other things.

Mark Bush designed the Rocket Black frt end that has won a ton of races and it's roll center has almost NO movement on roll, or dynamically. I have built and raced frt ends that would send the roll center over 20 inches to the right and maintained the roll center at 2" above ground through out roll BUT it was at the expense of camber gain and instant center locations. Ok so with all that said unless you are ready to get into spindle design and the compromise in that design factor you are limited by whatever spindle you use also. Compromise is what it is all about. So with all this going on you have bump steer that needs to be as close to 0 as you can keep it, IMHO. In closing welcome to the deep dark hole.

[7uptruckracer]

I'm very familiar with all of this and before i spend the money on the programs i'm looking for some more general answers. I run both dirt and asphalt and they both require a different package. Dirt we ran about 8" left of the centerline and 4 inches off the ground with my lowers level and the uppers at around a 6 degree split. etc etc. what I'm working on now is a G body Metric. we are limited to what we can do I can't modify mounts it is running a drop spindle. we have a 6" ride height rule. My question is should i run my lowers level or should the inner pivot point be higher or lower. I need to ultimately raise my roll center and shift it to the right it I know has a low low roll center as it is and it might not even be offset right with its current setup which make it lay over really hard on the RF and i don't want to go the Big Bar route like the rest it makes it too tight and causes clearance issues but regardless I just am looking for a general lower control arm rule of thumb and what running extended ball joints in my lowers will do I plan on running an extended upper on the right upper arm to help shift my RC to the right as it is now i could run a shorter but still extended upper of the Left upper if it also helps raise me RC just need some guidance being I don't want to spend if it isn't needed

[hpmaster]

That's the problem without the programs you are in for a lot of time plotting this on paper. For me to suggest anything would be a shot in the dark not taking into account all the other measurements. I would spend the $100.00 to $200.00 for a program and plot your frt end and then make theoretical adjustments until you get what you need. I still believe plotting a good car's frt end is where I would start then adjust yours close. From that starting point your experiments will have a repeatable base line to go back to when things do not work out. Racewise, Mark Bush's company, has a frt. end program that is 3-d and seems pretty cool, Auto Ware is one of the ones I have and seems ok to me. If you want to talk about it message me and I will try to help more.

[53003jason]

Bob Bolles has written tons of articles for Circle Track and other publishers its on the net you just have to look a little harder.
You are going to need to purchase a software program to locate and adjust your roll center. You can plot it out on paper but is a waste of time because you dont know whats going on with it dynamically you cant simulate roll and g-force on paper. I would buy either of those two programs they work fine, also if you like puzzles trying to design roll centers with stock parts will be fun.

[Ghopper]

My question is should i run my lowers level or should the inner pivot point be higher or lower. I need to ultimately raise my roll center and shift it to the right it I know has a low low roll center as it is and it might not even be offset right with its current setup which make it lay over really hard on the RF and i don't want to go the Big Bar route like the rest it makes it too tight and causes clearance issues but regardless I just am looking for a general lower control arm rule of thumb and what running extended ball joints in my lowers will do I plan on running an extended upper on the right upper arm to help shift my RC to the right as it is now i could run a shorter but still extended upper of the Left upper if it also helps raise me RC just need some guidance being I don't want to spend if it isn't needed

Changing the kinematic roll center to decrease the car from "lay(ing) over really hard on the RF" adds a mechanical stiffness to the suspension during cornering. This stiffness is created by a sideforce applied to the tire acting through the instant center of the suspension. The resulting force vector has lateral (side) and vertical components. The vertical vector component is the force that resists movement of the suspension. Move the instant center, increase/decrease this force. The closer the IC is to the wheel the more effect it will have. Moving the roll center up or to the right is usually done by moving the IC closer to the wheel.

Whether the car is "lay(ing) over really hard on the RF" or not, the Left to Right weight transfer is nearly the same. Front roll stiffness and rear roll stiffness will determine which tires are gaining the most weight. YOU are increasing the front roll stiffness when you move the roll center.

Roll center
- a cornering force dependent roll stiffness. The force vector that resists suspension movement is larger with a larger cornering force
- the same force vector resisting movement also effects ride-rate (ride stiffness)
- the car is usually tighter on exit with a rollcenter to the right (increased wedge)

Swaybar (front only)
- changes roll stiffness only WITHOUT any dependence on cornerning force at the tire.
- ideally the ride-rate is NOT effected
- the car is usually tighter on exit with high front roll stiffness (increased wedge)


So back to the sway bar option..... you said you dont want a front sway bar, because by its self the car is tighter, but if a rear sway bar is added at the same time the weight transfer could be the same.

Track banking will determine if you should change RC or swaybar. High banked tracks with have a large vertical acceleration and lateral acceleration, versus flat tracks that have large lateral accelerations. That will highlight the importance of ride-rate changes between RC and Swaybars.




Ghopper

[7uptruckracer]

No I said I didn't want to run the huge sway bar because there are to many clearance issues and the people run them to overcome the G-bodys low roll center so. I want to try to run a smaller bar and raise the roll center to something more normal. I need these questions answered. Which is the correct way for the lower arms to be mounted with the inner or outer pivot points higher. And what does running a longer lower ball joint from what's there now do? Will it do anything at the same ride height and my thought is I should not run it If my arms are flat now that would drop my lower below the inner pivot point and cause weird camber change issues if I lowered my heights to compensate the issue wouldn't that just lower my roll center? And does raising both upper angles raise roll center I know offsetting the angles changes my lateral location which I want to to just dunno if I should leave one stock or raise one and little and the other more so I still get my lateral shift but still raise it

[Ghopper]

.....I need these questions answered. Which is the correct way for the lower arms to be mounted with the inner or outer pivot points higher. And what does running a longer lower ball joint from what's there now do? Will it do anything at the same ride height and my thought is I should not run it If my arms are flat now that would drop my lower below the inner pivot point and cause weird camber change issues if I lowered my heights to compensate the issue wouldn't that just lower my roll center? And does raising both upper angles raise roll center I know offsetting the angles changes my lateral location which I want to to just dunno if I should leave one stock or raise one and little and the other more so I still get my lateral shift but still raise it



Ok, forget the golden ticket answer above of why and how to make this change.

Answer you seek -> Don't change ride heights and add longer ball joints to top/bottom - Done


Ghopper

[drtrkr244]

I have used Bob Bolles software now for 3 years. It is spot on. I have used it dlm's, mods., tq late mods. On the tq setup , the driver has not lost a race in 2 years. It takes a little time at first, but it gets easier the more u use it. I talked to Bob last year at VSP, he told me every major dlm car builder is now using his software. Matching the roll angles front to rear only seems logical when u think about it. It will make your car more consistent. Beware, you still have to make the proper adjustments according to the track cond., but your car seems to stay consistent!!

[Dirtrunner35]

Bob Bolles lol sorry i had to laugh. I have his program , a lot of dust on it. Auto Ware has great stuff, should attend his webinars. Ever seen any data from a Data acquisition ??

Great reading-- Auto Ware

Let me start by saying the RC is the result and not the cause of the motion or forces. So, it is best to think about RCs in reverse. Regarding force, lateral position of an applied force has no impact on the resulting vertical forces (thanks to Mr. Newton's Principle of Transmissibility) So, saying the lateral location of the RC changes the vertical loading in the tires is just not true. Chances are anyone that has observed a change in handling and attributed it to the RC location, just missed something else that really caused the change.

Again the RC is the result and not a solid object like a Heim joint. The RC is you are thinking is no doubt from the "wheel position analysis method" where lines are projected from control arms. The method is almost 100 years old and has many simplifying assumptions that have long been forgotten. A newer method roughly 25 years old is much more accurate, but somehow its popularity never caught on like the old method. But again many aspect of vehicle dynamics never flourished like RC (for example when was the last time you heard anything about static margin, elastic conjugate points or any of the dozens of other parameters)?

Granted if you want to think in terms of kinematics the RC is a convenient point to sum moments, and a higher RC does result in positive or negative jacking. However, shocks, springs, bars and even bushing all contribute to the resulting RC (that's why I like the new method better).

Okay, now that I went on about how to build an atomic clock and you just asked about the time;) let me say, use your suspension geometry needs to net desirable dynamic camber and if you are un happy about weight transfer do something else, not mess up the alignment. imho

[let-r-eat]

The position of the upper mount/lower mount and tie rod inner pivot point is what your looking for. These guys are talking but are not taking into account the steering pivot points. You can't talk suspension geometry without talking steering/upper/lower pivot points. These softwares are berries and cream but you have to understand what your looking at. If you change the offset on a front wheel your changing suspension geometry. The center of the contact patch is where its at. Visualize a line through the lower inner pivot/upper inner pivot and inner steering pivot. You need to be able to draw a straight line threw them. Then worry about the arm angles and placement of the tie rod end on the spindle. Where are they measuring the 6" ride height?

[Ghopper]

Dirtrunner35 -
If you are mentioning force-based roll centers, with calculation of a virtual swingarms. Unless you know another way? I am a believer in weight jacking term (you also mentioned) as described in my previous post and a component of the force-based RC. Looking at normal force change during a parallel lateral force test, checking this at a few roll and pitch angles give. If you iterate the solution on each side of the test body and load condition, calculate the virual swingarm by the point that the 3 force vector cross, then look for the intersect of results of the two sides.

Static Margin is difficult to bring into a discussion on increasing roll stiffness as it is usually described with 2D bicycle models and lumped front/rear axle tire cornering stiffness. However a good way to explain understeer gradient with respect to CG fore/aft placement.

I am drawing a blank on elastic conjugate points. Can you explain further?


Let -r -eat -
If 7up just changes upper ball joints, he will not have to worry much about steeringing geometry changes....it may bump out a little more. I would just bolt on new upper and lower ball joints. He may have to add some shims on the outer tie-rod to lower the pivot point.



Ghopper

[7uptruckracer]

I have to be careful what I do because of the rules. I was told I can get bump back by moving steering components. I know adding the joints can cause me to bump out more. I just have to make sure I can get it back within spec the others cars I have are no problem but this one has gearbox steering so its going to be more a hassle while skirting the rules (using that favorite color of racers) I just needed clarification because I was told the outer lower control are pivot point should be mounted BELOW the Inner pivot point and that goes against what I thought because wouldn't that have implications on my camber curves that are undesirable? And being it has a 6" lowest point ride height rule I can get to 6" but geometry is more important so i'm trying to maximize my heights, raise my RF and shift it to the right of the CL. Being this particular car is a G body metric car and has a already way low roll center I know I want to up it. and it does have drop spindles which has lulled the competition into thinking it rolls less but it hasn't changed the RC Just the Center of gravity height so they seem to think it doesn't roll less persay but they are still running the huge bars for a 2700lbs car and they are cutting arms and doing unsafe things to run them id rather run a smaller bar which should also compensate for might RC shift to keep the car balanced and maybe even add some balance to it! sorry for the rambling

[Dirtrunner35]

I was just giving that example for different things that effect front end geometry, I have bob bolles program about getting the roll center the same front and back, to many questions you need to guess at in the program. I have 3 programs from Auto-ware.com and they work great.

Since you are limited on what changes you can do to your car, I would put the inner control arm points a little lower than the ball joints. Can you elongate the upper control arm mount holes or cut them off and make them lower ?

I ran a stock sway bar on my metric clip car for 2 years on dirt.

[drtrkr244]

I was just giving that example for different things that effect front end geometry, I have bob bolles program about getting the roll center the same front and back, to many questions you need to guess at in the program. I have 3 programs from Auto-ware.com and they work great.

Since you are limited on what changes you can do to your car, I would put the inner control arm points a little lower than the ball joints. Can you elongate the upper control arm mount holes or cut them off and make them lower ?

I ran a stock sway bar on my metric clip car for 2 years on dirt.

Yea, I agree. You have to call Bob to get too many inputs. It needs to be more user friendly.

Have you compared the two sftwares results, were they close or totally different?

[Dirtrunner35]

Yes I agree, roll center height , left to right, center of gravity height. The program is about roll angle and not weight transfer. I guess if you know how a car works you could use it with success but then why would you buy it? I have his set-up secret book and bought his newer one and 2/3rds is the old book so I returned it. He contradicts himself in his book, I did learn some things from his book but more at Auto-ware.

Have you compared the two software results? Were they close or totally different? I did years ago and that's why I quit using the roll angle program. My computer crashed and I don't have the measurements for Bob's program, if I did I would tell you what was different. I would hate to see the roll angle on my dirt mod, lol.

[drtrkr244]

Yes I agree, roll center height , left to right, center of gravity height. The program is about roll angle and not weight transfer. I guess if you know how a car works you could use it with success but then why would you buy it? I have his set-up secret book and bought his newer one and 2/3rds is the old book so I returned it. He contradicts himself in his book, I did learn some things from his book but more at Auto-ware.

Have you compared the two software results? Were they close or totally different? I did years ago and that's why I quit using the roll angle program. My computer crashed and I don't have the measurements for Bob's program, if I did I would tell you what was different. I would hate to see the roll angle on my dirt mod, lol.

Im sure its not the "end all' software, but i do feel it helped my racing program.Its a little difficult to use, but i think its a good investment for $150.At least, it gets you measuring every suspension pick up point, and thinking how they work together. I do know i used it on a homemade car, built on a garage floor and it turned into an unbeatable car. Good thing it had bolts instead of ball joints as i used about one foot of 3/4 tubing for spacers on the front end.I do believe it will get u in the ballpark...JMO

[7uptruckracer]

NO we can't cut anything or elongate holes other then a repair issue, Kinda a catch all thing and way of ruling they use....Nascar what can ya say......I have a gameplan I'm going sunday to set my arm angles and heights with stock ball joints and see where I end up height wise with the right geometry then see how much more i can adjust. I was going to get extended lower Ball joints but after looking tonight my outter pivot points on my lowers are already lower then my inners and I only have an Inch till I'm at the minimum allowable ride height so I might only be doing my upper ball joints. I might use a .500 inch longer stud on the RF and .100 on the LF thoughts? If i can get what i want outta the lowers i can do up to 1" lower ball joints and get them up right and get my 6" minimum.......input? right track here?

[drtrkr244]

NO we can't cut anything or elongate holes other then a repair issue, Kinda a catch all thing and way of ruling they use....Nascar what can ya say......I have a gameplan I'm going sunday to set my arm angles and heights with stock ball joints and see where I end up height wise with the right geometry then see how much more i can adjust. I was going to get extended lower Ball joints but after looking tonight my outter pivot points on my lowers are already lower then my inners and I only have an Inch till I'm at the minimum allowable ride height so I might only be doing my upper ball joints. I might use a .500 inch longer stud on the RF and .100 on the LF thoughts? If i can get what i want outta the lowers i can do up to 1" lower ball joints and get them up right and get my 6" minimum.......input? right track here?

Definitely, putting longer balljoint studs on a "stock" car is necessary.They generally have too low of a roll center for circle track racing.Putting longer stud on rf wii shift rc to the right.Technically, you can draw out your pickup points on graph paper, but if you need to make changes , youll have to start over.To start changing things without knowing where you are at might cost you more in the long run.

[cgs]

does anyone have a geometry program laying around the want to sell at a reasonable price? :)

[1Blacksheep]

Chassis pivot point should always be lower than the the outer pivot point JMO !!! Increasing the upper A angle moves the (theoretical ) RC closer to that corner and raises it at the same time with No other changes. RC ht and lateral location walks hand in hand with front spring rates and rear wt %. To answer original Question .