Front suspension geometry?

devilbrad · Post by **devilbrad** » Sun Oct 22, 2006 7:06 pm

Aside from my Dakota I am drag racing, I also have a 67 Dodge D100 short bed truck I am going to start work on soon. Plans call for a "pro touring" stance and road race suspension. Very few (like maybe 2) companies make an IFS kit to replace the straight axle. They are MII based kits and I have seen guys adapt late Vette IFS. But I have been looking at some stock car suspension control arms and spindles/brake combos for the front using coil over shocks. My question is......what are acceptable ranges of mounting the control arm front to rear pivot angles, both lower and upper?? If I were to fab up all the tabs for the instalation, I know that will be of utmost importance. I know they arent parallel to ground. Also it will be a front steer rack and pinion setup, just need to find a rack with proper inner tierod pivots to match the control arm pivots for near zero bump steer, or should I be looking for a narrower rack to help with ackerman angle? This is unchartered territory for me, I've always been a drag racer. Thanks in advance guys!

Nwguy · Post by **Nwguy** » Sun Oct 22, 2006 11:42 pm

Art Morrison Ent., Fife WA sells a nice mustang II based setup. If you are more of the Do it yourselfer type, look at their catalog and copy their ideas.

Mark Workman · Post by **Mark Workman** » Mon Oct 23, 2006 8:43 am

The angle the upper and lower make with one another is more important than the angle with the ground. I don't know of anyone altering the angle of the LCA(side view). Ours are parallel with the ground(in side view) at ride height, as are all of the ones I have seen. You for sure wouldn't want the back to be higher than the front. Any angle would mean a wheelbase change during suspension movement, and that introduces variables into the handling that can be difficult to deal with. Now, the UCA can be at an angle (closer at rear) to the LCA, if anti-dive is desired, or parallel, if no AD is wanted. In circle track racing, the LCA placement and angle(in front view) is a very critical setup adjustment. Very small changes make a big difference in other areas. Once it is set, it is usually not changed unless you are totally out of the ballpark.

Now, this is from a circle track perspective. For street or drag racing, you may be able to get away with things that would have you not making the show (or worse), but good suspension principles are worthy goals, no matter the application.

devilbrad · Post by **devilbrad** » Mon Oct 23, 2006 11:16 am

Thanks Mark. So lower level and what would be a good starting point in degrees for upper? This will mainly be street driven and pushed through turns pretty good.

Mark Workman · Post by **Mark Workman** » Tue Oct 24, 2006 9:06 am

devilbrad wrote:Thanks Mark. So lower level and what would be a good starting point in degrees for upper? This will mainly be street driven and pushed through turns pretty good.

I have had plans for a similar project rolling around in my head for quite awhile. Smoothed removable body, tube frame, engine set-back, coil-overs, big bar in front, Watts linkage in back,dry sumped 370" Ford Cleveland, wide-five hubs with 12" tires all around, quick-change, low,low,low, etc. I had a similar car about 15 years ago, but very mild. The reponse at car shows was very good, but it couldn't really compete with the radical Pro-Street cars it was usually up against. A streetable Late-Model would change that.

For UCA mounting, I can't give you an exact angle, there are too many variables. I would put the LCA level, and angle the upper enough to obtain about 20% anti-dive. But I would also build in adjustment to go either way if 20% don't cut it. That's easy enough to do. 20% is defined as projecting lines from the control arms to where they intersect. Now draw a line from that intersect point back to the tire contact patch. This line will cross a plumb line dropped from the CG at a point that is 20% of the length of the plumb line above the ground. This is all conducted in side view.

I would recommend several books on the subject. For basics, "Paved Track Stock Car Technology"(not too sure of the name here) by Steve Smith is good for very basic info and definitions, but pay no attention to the conclusions that are drawn(they are mostly wrong, IMO), and there is too much "This is what it SHOULD be", and very litle, if any, "why so". More advanced info is available from "Tuning To Win" by the late Carroll Smith, and "Competition car Suspension", by Alan Staniforth. They both explain "why", as well as "what". both are dated in some respects(tires and shocks), and concentrate on Formula-type single seaters, but are excellent reference material. I still reread both a couple of times a year. For software, Performance Trend's 'Roll center Calculator' will be a big help.

Here's a link to a car of ours bare, shows details of front suspension:

http://www.workmonster.com/id45.htm

Notice the rack ends are directly in line with both LCA pivot points, for bump steer considerations....Don't allow ANY toe-in in bump. Zero is best, a LITTLE toe-out is acceptable(.020). Ackerman is hard to get a lot of with a front-steer setup, and is of debatable need anyway. We set it by adjusting the length of the left spindle arm(not good for turning both ways). Usually set at zero. Spindle design is important as well. Usually a builder will pick a spindle, and stay with it. Randy Sweet makes a variety of them. Long LCAs are good(as long as you can get), uppers are available in many lengths and offsets. Decide on ride height and track width first, then design accordingly. Lefthander Chassis also makes a very good setup booklet for their chassis, that has good info in it for ANY coilover setup, and scale drawings to boot. Costs about $10, get the one for the later chassis(has a panhard bar as opposed to a J-bar).

devilbrad · Post by **devilbrad** » Tue Oct 24, 2006 10:59 am

Wow, awesome! Thanks Mark!!

Ed-vancedEngines · Post by **Ed-vancedEngines** » Tue Oct 24, 2006 11:32 am

I agree pretty much with everything that Mark has said with one exception.

That exception is about available component selection. For streeters for driving, I believe that the late C4 Corvette aluminum front suspension with the Vette Aluminum caliper brakes is as good as it can get for a street ride. Both of the Jay Leno Buicks have the Vette C4 and C5 suspensions on them, for what it is worth. You can make them to be coil over, air ride, or use the factory leaf very easily. They have tons of anti-dive and positive castor built in the design. If you set them up like the factory setup is you will find a near perfect ackerman. You can buy the complete crossmember section and keep all components all on it and install that, like some of the steet rod chassis shops are doing, or you can build a tube front clip to mount it all on, or you can modify your factory front frame to mount the individual components on. The lightest and most time comsuming will also be the best end result. The easiest way and quickest is to install the complete crossmember suspension assembly.

Ed

BillyShope · Post by **BillyShope** » Wed Dec 20, 2006 5:46 am

Neither anti-dive nor Ackerman are necessary.

Anti-dive wasn't even available before ball joints and does not improve handling performance. And, at first, some manufacturers were reluctant to use it. Ford products of the sixties will just about scrape the front bumper on braking. This was the result of little or no anti-dive and a softly sprung car. Since ride deteriorates as anti-dive increases, Ford engineers opted for a smoother ride. Certainly, with a stiffly sprung car and a low center of gravity, the presence of anti-dive becomes difficult to detect.

As for Ackermann steering, it was developed...according to Maurice Olley...primarily to avoid disturbing the gravel on the driveways of wealthy owners of early motorcars. To gain the optimum grip from the lightly loaded inside tires, Formula One designers have even used reverse Ackermann.

ADR · Post by **ADR** » Sun Dec 31, 2006 6:35 pm

Wow thanks for that Billy....I was recently told that when I buy struts for my latest drag truck project..."I have to buy the ones with Ackerman built into them" Doesn't sound that important now and may save a bunch of money. I guess I don't mind pushing a little gravel in the pits

Dale

devilbrad · Post by **devilbrad** » Sat Jan 20, 2007 6:51 pm

Still researching this a little, found some specs for those interested. A Mustang II/Pinto front end has the UCA at a 3 degree angle to LCA and the Miatas I always have in the shop are at 5 degrees. I need to go back through this thread and write down the books and hit up Amazon.

Masher Manufacturing · Sat Jan 20, 2007 7:56 pm

Ackerman is used to make the tire on the inside of the turn go through a tighter arc then the outside tire.

Anti dive uses the braking action to push the down ( and as a result the front of the car up ) to counteract the forward weight transfer. However this isn't a something for nothing as it gives the same effect as stiffer springs which will cause the car to understeer under braking.

There are no easy answers for a question that has been asked countless times before.

The books others have suggested will help in getting the theory down.

The Pinto stuff on a large truck is going to be pretty flimsy. How about using a front frame from 80's Dodge? I think the front cross member unbolts as a complete unit. GM 2 WD trucks from 73 to 87 have a complete front suspension cross member that unbolts. However the Chevy and Dodge frames are differing widths.

Or even using a frame stub from a Fullsized Ford or GM car? Weight will become a issue using the truck stuff but the fullsize car GM from 77 to 96 is pretty light.

devilbrad · Post by **devilbrad** » Sat Feb 10, 2007 4:47 pm

Masher Manufacturing wrote:Ackerman is used to make the tire on the inside of the turn go through a tighter arc then the outside tire.

Anti dive uses the braking action to push the down ( and as a result the front of the car up ) to counteract the forward weight transfer. However this isn't a something for nothing as it gives the same effect as stiffer springs which will cause the car to understeer under braking.

There are no easy answers for a question that has been asked countless times before.

The books others have suggested will help in getting the theory down.

The Pinto stuff on a large truck is going to be pretty flimsy. How about using a front frame from 80's Dodge? I think the front cross member unbolts as a complete unit. GM 2 WD trucks from 73 to 87 have a complete front suspension cross member that unbolts. However the Chevy and Dodge frames are differing widths.

Or even using a frame stub from a Fullsized Ford or GM car? Weight will become a issue using the truck stuff but the fullsize car GM from 77 to 96 is pretty light.

I've looked into the frame grafting options, but the ugliness of the factory crossmembers and stuff just turns me off. I would rather do a custom square tubing crossmember on the factory frame rails (frame will be fully boxed) and tubular control arms and make it pretty, light, and functional.

Bubstr · Post by **Bubstr** » Sat Feb 10, 2007 7:14 pm

I agree that ackerman and anti dive are not wanted for the best handeling. What is importiant is what they call king pin inclination, caster and camber stability and the elimination of bump steer. These are the thing that will steer your car for you or make it feel like it is. Both are unsetteling. The king pin inclanation works with the amount of caster to provide a car that has no tendancy to steer it's self, but not be hard to steer. It has a more important job of changing wheel weight depending on where the wheels are pointed. This gains or loses traction on that front wheel by changing the distance from chassis to ground. This is a built in , oversteer, understeer corector. A little toe out is good for stability, It eliminates the darty feeling. I would sugest copy something from a tried and true chassis builder, although an education on suspention may help you knowing what you want.

techinspector1 · Post by **techinspector1** » Tue Mar 27, 2007 11:53 pm

I was hellbound and determined to design and build my own IFS on a T-roadster project several years ago. I read book after book and learned nothing about how to go about it until I read Carroll Smith. He's the only person on the planet who put it all together for me with his "paper doll" approach. I successfully completed the construction of said project with 9 1/2" of suspension travel at zero bump steer and 3 1/2 degrees of camber gain at full bump with minimal scrub.

Now that I've done it, I'll never do it again. Building all the jigs and fixtures to make everything right was a pain in my butt and in my pocketbook. Not to mention all the planning, scaling, choosing materials, ad infinitum. I think everybody should do it once, just so they can appreciate the engineering that goes into it.

I strongly suggest getting all of Smith's books and absorbing them fully before starting out on this quest.

Pete · Post by **Pete** » Thu Apr 12, 2007 12:01 am

In addition to the advice above you might also get to as many truck , hot rod , modified car shows as you can and see if anyone has been done the road you are thinking about taking.

This will give you some background on the companies to avoid as well as those who actually know their stuff.

Hopefully you won't have top learn the same lessons the hard way !

Good luck , Pete.