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Suspension Dynamics.
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Author:  BillyShope [ Mon Apr 27, 2009 7:10 am ]
Post subject:  Re: Suspension Dynamics.

shrinker wrote:
[T]he tyre contact patch is indeed the force exertion point upon the ground BUT it is not the force exertion point into the chassis. The axle centerline is actually the place where the reaction of the tyre contact patch is distributed into chassis. The chassis does not react about the tyre contact patch it reacts about the axle. This is my biggest bone of contention to the books written on the subject. There are other things I think are wrong in the books too but that's a good one to start discussion with.

The chassis does not react "about" anything. It reacts to the forces and moments imposed upon it. In this case, it reacts to the force vector at the tire patch as it acts along a line through the instant center.
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Author:  BillyShope [ Mon Apr 27, 2009 7:20 am ]
Post subject:  Re: Suspension Dynamics.

Bubstr wrote:
If you are getting compression in the rear, that is a spring storing weight/ energy and not putting it at the tire contact patch.

If a spring is compressed, the forces at BOTH ends are changing. There's no need to concern yourself with energy, the compressed spring indicates increased down force on the tires.

Weight transfer is always going to find its way to the rear tire patches. There's no way to stop it. It isn't possible to "store" it somewhere as "energy" and extract it later. With 100% antisquat, all the weight transfer is carried through the links. With values other than 100%, compression or extension of the rear springs indicate they are playing a part in the weight transfer.
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Author:  shrinker [ Mon Apr 27, 2009 7:29 am ]
Post subject:  Re: Suspension Dynamics.

Billy; the first column is the gearbox input torque, second is the axle lifting force(the weight that's resisting the torque) you might view it as antisquat, the third is the total weight transfer of the car. It doesnt print out as it is on my spread sheet. If I knew how to do that it would be clear and I could post the rest of the info. Beretta posted the gear ratios etc.
It wont resolve to a constant CGH as you go down the rows because the CGH rises as the car moves. As the car lifts, the fourlink geometry changes IC etc.

How do you explain a theoretical car where the rear axle weight is zero, there would be no torque imposed upon the CG by the acceleration inertia of the axle, so in that situation there would only be a forward force imparted to the body at the axle height and a need for axle torque reaction as well. If the rear axle height is above the CGH then explain that please.

Also our car has an instant center of 35.5" and 7.75" high at rest and I dont know anyone with a ladder bar car that short with 870HP and 2385 lbs weight. That ladder bar car would severely jerk off of the line and bounce the tyres etc, yet the fourlink doesnt. We use light resistance shocks too, we dont tie it down with shock resistance. In fact the shocks wore out once and were virtually useless and the car was fine. It was only because of dynoing the shocks before and after rebuild that we found out they were way off. The car is not slow either it 60's at 1.151 (current best). So what gives? I think a fourlink is totally different deal to a ladder bar.

Author:  BillyShope [ Mon Apr 27, 2009 8:08 am ]
Post subject:  Re: Suspension Dynamics.

shrinker wrote:
The reason why I do all the measurements from the ground and from the axle centerline is the axle is the thing that pushes the car forwards and applies a torque reaction that has to be resisted by the four link. The centerline of the axle is where everything starts from. There are 2 places on the car that the physics start from, one is the axle centerline and the other is the crankshaft.
I dont mean to upset anyone but I argue that the concept of working out thrust lines or antisquat etc from the tyre contact patch is not correct. The tyre thrust occurs at the axle, thats where the forces come into play, not at the tread.
I always find it useful to exaggerate the design or shape of things in order to better understand them So consider this, you take the rear wheels off your drag car and install monster truck wheels. You also install lower gears in the diff so that the final torque applied to the tread is the same as you have now. Then you alter the fourlink so that its still got the same IC. Now who recons that car will launch the same and have the lift etc?

As my posts immediately above would indicate, I've been looking back at early posts in this thread. (Bubstr alerted me to this thread long after its beginning.)

The post quoted above is particularly interesting. The problem appears to be that Shrinker attributes more intelligence to the chassis than would I. I would maintain that the chassis has no idea where the axle housing is located. How could it when its only connection with the rear axle assembly is either 2 (ladder bar) or 4 (4link) pivot points? All the chassis "knows" is that it is receiving forces at these pivot points.

The rear axle assembly is experiencing many internal forces as the car launches. The ring gear teeth are engaging those of the pinion gear, the housing is deflecting, the bearings have increased loads, etc. But, all of these internal forces are canceled (reacted) within the assembly and do not influence analysis. The rear axle assembly's contacts with the outside world are through the shocks and springs, the suspension linkage, and the tire patches. Since the shocks and springs are incapable of lateral forces, it follows that the entire rear axle assembly reduces to a simple link, with one pivot point at the tire patch and the other at the instant center. (For the ladder, the IC would be the pivot point.) The tire patch force vector, then, acts on a line through the tire patch and the instant center. This line is further described as a line of constant percent antisquat. (If the inertial force, generated by the weight of the rear axle assembly, is taken into account, the line of constant percent antisquat passes an inch or so below the rear tire patch.)

The preceding comments reflect the views of suspension engineers, established, now, for a century. This is not to say that Maurice Olley...and others of his ilk...could not have been mistaken in all of this, but this should give some indication of the work that lies ahead of you if a century's work is to all be overturned.
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Author:  shrinker [ Mon Apr 27, 2009 8:30 am ]
Post subject:  Re: Suspension Dynamics.

This is an interesting discussion but Just because there is an accepted way of doing things doesnt mean its right. I stand humbly ready to be corrected on any issue as thats the best way to learn I think.

I once built a test rig for sprintcar wings. We mounted it on a car and drove at race speeds on the road. we measured the downforce directly onto scales. The wing manufacturers used a NACA 4412 design and that predicted a downforce of 230 lbs for our test conditions. The real downforce was 125lbs. So i designed my own profile. It produced 335lbs with a small flap and with less drag than the naca 4412 fitted with 60 degree flaps so as to max its downforce at 225lbs. (we had to flap the NACA 4412 it to get it to the theoretical downforce of an unflapped wing) we continued to use our design from that day onward, we won lots of stuff. We even had to tame it down as too much downforce wasn't always good. So I am well aware of how some things can be incorrectly applied.
Billy please consider this and fully explain where I go wrong in your or others opinion please----If both fourlink bars are facing downward at a considerable angle (but a sensible IC is created) the forward thrust of the axle will force the body down will it not??? That force must be accounted for in the final sprung mass reacted position.

Author:  BillyShope [ Tue Apr 28, 2009 12:24 am ]
Post subject:  Re: Suspension Dynamics.

I've just added a page (Page 38) to my site which should serve to clarify matters. The spreadsheet allows the user to place the IC anywhere, adjust the link spread and rear pivot locations, and change the link lengths. But, the tire patch force vector is shown to ALWAYS pass through the instant center.
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Author:  Scott Smith [ Tue Apr 28, 2009 12:21 pm ]
Post subject:  Re: Suspension Dynamics.

Is it possible to post a column of numbers etc? There is a lot more other factors to see than this but it wont make sense if it just comes out the way it is for me at the moment.

Surround your colums with the code brackets.
*code* */code* (Repalace the * with [ ]
Like this:

Psn  Driver               Vehicle                            ET   Speed
   1. Greg Anderson        Summit Racing Equipment GXP     6.574@ 210.93&
   2. Johnny Gray          Johnny Gray Racing Stratus      6.599  209.14
   3. Jason Line           Summit Racing Equipment GXP     6.600  210.05
   4. Allen Johnson        Mopar/J & J Racing Stratus      6.607  208.78
   5. Jeg Coughlin Cobalt                 6.613  209.72
   6. Greg Stanfield       Attitude Apparel GXP            6.617  209.43
   7. Warren Johnson       K&N Filters GXP                 6.628  209.98
   8. Larry Morgan         Lucas Oil Stratus               6.629  208.71
   9. Kurt Johnson         AC Delco Cobalt                 6.632  209.10
  10. Justin Humphreys     RaceRedi Motorsports GXP        6.648  207.66
  11. Steve Spiess         Spiess Construction Cobalt      6.656  207.66
  12. Ron Krisher          Valvoline Cobalt                6.658  209.30
          --------- Not Qualified ---------
  13. Rodger Brogdon       Charter GXP                     6.661  208.04
  14. Tom Hammonds         Race For Achievement Cobalt     6.663  206.45
  15. V. Gaines            Kendall Oil Stratus             6.669  206.83
  16. Ronnie Humphrey      Troy Humphrey Stratus           6.670  207.56
  17. Dave Northrop        Nitro Fish Wear Stratus         6.751  205.91
  18. Erica Enders         Dragstar Racing Mustang         6.765  203.71
  19. John Gaydosh Jr      Gaydosh Performance GTO         6.864  203.58
  20. Rickie Jones         Quarter-Max/RJ Stratus         10.961   80.81
  21. Mike Edwards         Young Life/Penhall GXP         12.406   63.37
 @ = Track ET Record

Author:  Beretta [ Wed Apr 29, 2009 5:51 am ]
Post subject:  Re: Suspension Dynamics.

Beretta wrote:
Bubstr wrote:
Barry, Are you going to share what adjustments you made to see improvement and what kind of ET did you gain?

Dennis, Last week it had the 4.30 gears in the rear and did work well as it just touched the wheelee bars and short times were better. So this week I put the 4.57 gears in and short times were better and tightened up the extension and the compression up on the rear shocks and both were run with a IC of 55" and 4" off the ground.....

The chassis/motor likes the lower bar a 4 1/2* down and 6psi of air any less air and I got tire shake....The air was in the 2500 range as last week it was 1800 range...Last week the short times were in the 1.25 range but this week in two rounds they were 1.239 and 1.242.........With the 4.57 gears....I am going to try a longer and shorter IC with trying to keep the height the same....

No Comments???

Author:  shrinker [ Wed Apr 29, 2009 6:17 am ]
Post subject:  Re: Suspension Dynamics.

torque                 axle lift   w/ trans
0                       0.0   0.0
50                      66.9   99.6
100                      138.4   204.6
150                      215.4   316.0
200                      298.7   434.6
250                      389.5   562.0
300                      489.6   699.9
350                      597.6   840.6
400                      721.5   1004.6
450                      732.5   1184.8
500                      854.4   1108.0
550                      970.9   1108.0
600                     1095.7   1108.0
650                     1229.7   1108.0
700                     1374.1   1108.0
750                     1530.5   1108.0
800                     1700.5   1108.0
850                     1886.2   1108.0
900                     2090.3   1108.0
950                     2316.0   1108.0

Ok this is the best way i can figure to post the numbers. I have to do it in sections Boy is this tedious. i cant get it to post the columns how they appear for me .
Anyway to explain these numbers.
The column torque is the gearbox input shaft torque You can see it ranges from zero to 950 ft lbs. If its a converter car it can get to that with 600 ft lbs engine on the stall. I assume beretta's engine is about that power.
The axle lift column is the upward force of torque reaction at the geometry the four link would be at within the dynamic movement of the car. If the engine is outputting 500 ft lbs the car will accelerate and the forces involved will cause certain movements and this is what I think will be the result.
The weight transfer column is the total weight transfer of the car. I assumed 14"cgh. you gotta start somewhere.

Author:  shrinker [ Wed Apr 29, 2009 6:30 am ]
Post subject:  Re: Suspension Dynamics.

torque   G's   body drop   front lift   separation
0   0   0.000   0.000   0.000
50   0.134918617   0.126   0.181093619   0.155
100   0.269837234   0.264   0.372064773   0.328
150   0.404755851   0.418   0.574466006   0.522
200   0.539674468   0.590   0.790237325   0.739
250   0.674593085   0.784   1.021849749   0.987
300   0.809511701   1.003   1.272524355   1.269
350   0.944430318   1.275   1.528309593   1.612
400   1.079348935   1.567   1.826548867   1.993
450   1.214267552   1.351   0.139657138   1.837
500   1.349186169   1.861   0.153483004   2.348
550   1.484104786   2.201   0.441192618   2.687
600   1.619023403   2.569   0.747704532   3.055
650   1.75394202   2.968   1.07531499   3.454
700   1.888860637   3.402   1.426739582   3.889
750   2.023779254              1.805219174   
800   2.15869787              2.214660087   
850   2.293616487              2.659821623   
900   2.428535104               3.146569239   
950   2.563453721               3.682218211   

Ok the torque column is the gearbox input shaft torque again, the G's column is the g force of the car.
The body drop column is the distance the body at the rear axle point will get closer to the ground, this is the total of the tyre squash and spring reaction etc.
The front lift column is the lift at the front axle location, the row 400 ft/lbs is 1.82" then the next row is 0.13". this is because the front wheels are still on the ground at 400 ft lbs but at 450 ft/lbs they will be just in the air by 0.13". If the front shock extend more than necessary to achieve zero weight at full extension then you wont see the front wheels in the air at this point of the run.
The separation column is the distance of diff to body separation. Its the extension of the shocks on beretta's design rear end.

Author:  Beretta [ Wed Apr 29, 2009 6:36 am ]
Post subject:  Re: Suspension Dynamics.

Shrinker my logger shows 2gs just after hit...

Author:  shrinker [ Wed Apr 29, 2009 6:46 am ]
Post subject:  Re: Suspension Dynamics.

torque   traction%   start traction%             axle torque          spring load
0         0.0%            0.0%   0                1053
50         10.1%           11.1%                  402                  1019
100         18.5%           22.2%                  804                    981
150         25.6%           33.2%                 1206                    938
200         31.4%           44.3%                 1609                     890
250         36.1%           55.4%                  2011                    836
300         39.9%           66.5%                  2413                    774
350         43.1%           77.5%                  2815                    698
400         45.3%           88.6%                  3217                      615
450         48.6%           99.7%                  3619                     619
500         54.0%         110.8%                  4022                     537
550         59.4%         121.8%                  4424                      462
600         64.8%         132.9%                  4826                     381
650         70.2%         144.0%                  5228                     293
700         75.6%         155.1%                  5630                     197
750         81.0%         166.1%                  6032                      93
800         86.3%         177.2%                  6435                     -22
850         91.7%         188.3%                  6837                      -149
900         97.1%         199.4%                  7239                      -289
950        102.5%         210.4%                  7641                      -446

The torque is the gearbox input torque.
The traction% column is the tyre traction used up when the car has its weight transfered. Note the car gets to 100% around the 950 FT/lbs point. I used a traction coefficient of 2.5 as this is near the best I have seen on our car. So this car doesnt have enough engine torque to spin the wheels once the car has moved off the line unless the torque converter multiplication kicks in. If the traction is less then it would turn the tyre a bit.
The start traction column is the tyre traction when the car is relaxed on the line before any power is applied. Note how its 100% at 450 ft/lbs so this car can easily snap the tyre into wheel spin if the power is too violent. Our car has better start line traction than this car and we even have narrower tyres. Its interesting to compare results like this.
The axle torque column is the torque at the axle,
The spring load column is the weight supported by both springs.

This data is what I use to work out our car, it works for me and I see the car do what these calcs are etc.

Author:  shrinker [ Wed Apr 29, 2009 6:56 am ]
Post subject:  Re: Suspension Dynamics.

Things like diff separation can be different to these calc due to shocker dampening. i cant work out that stuff and how to put it into the spreadsheet.
Beretta's 2 g just after launch is what i would expect from his type of car.
I have adjusted our car by altering the ride height at the rear .100" and the car responds with answers that i can predict with my spreadsheet. The driver certainly feels the differences.
I hope what I have posted is interesting to others. I dont proclaim it to a definative answer but I use it as a tool. The way most drag racers do it is to look at the IC and where it moves around to under dynamic loadings. What I do is approach it this way, its different to see the info in the style that the data logger and video shows.

Author:  shrinker [ Wed Apr 29, 2009 5:16 pm ]
Post subject:  Re: Suspension Dynamics.

Beretta has posted that his car does a peak of 2 g just after hit. What i suspect is that the g stops there because the tyres start to spin excessive. Thats exactly what i see on our car, the G peaks and the tyre starts spinning. The car reduces G's then and the torque converter starts to loose multiplication as the output shaft speeds up and the tyre regains grip. When i use traction coefficient of 2 in my prog it shows the tyre maxed out the traction at 741 Ft/lbs input shaft torque in first gear (Thats easily achieved with a torque converter), it shows the front wheels at 1.74" in the air and a spring load of 115 lbs in the rear springs. The diff separation is at 4.28" but that would be dampened by the shock resistance so Its not reaching that probably.
A video of the front and back wheels at launch is good have and confirm things that are happening.

Author:  ADAMSVEGA [ Fri Sep 14, 2012 1:15 pm ]
Post subject:  Re: Suspension Dynamics.

after ten pages of this and i'm still shakin my head .wow theres more than one guy throwing the box away besides me :-}
heavy short springs are for super gas guys to go red!
by what the car is saying to me stick with the lighter spring rate and set ride height .
its saying it wants more torque application for the geometry set up.
do you know what converter k numbers and tq. multiplication your getting ,is it spragless?
do you have room for a steeper rear or first gear ?
if not then it looks like adding some weight back and higher will help .
as long as your not folding the tire and running over sidewall. or its squatin bad when weight transfers .you could start with 20 # as far back as you can.
if better add 20# to cross bar,seat brace. or x brace high is ideal.
we found in the summer time when we need more bottle and more timer it was 75 h/p progressive to 125 right off the brake . this made our car dance a straight line and 60ft .were mirrors
we mounted 2 bottles opposite of seat . the added weight worked it beter with the shot and we just kept it on 75 shot and used timer to make index .
that little bit of weight and added tq. really made it consistent.

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