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View Full Version : Suggestions for increased caster angle.



SShep71
12-20-2004, 03:55 PM
Im makin a protouring 70 chevelle and I am creating and fabing the chassis,suspension,and driveline. I am thinking about in creasing the caster angle 1.5-2 deg from -1° to something around -2.5° to -3°. I am going to fab the upper and lower control arms for this adjustment. Do you think I should go more or less. I was planning on going to a closer ratio steering box, and maybe changing the steering arm length.
Any suggestons you can help me with are greatly appreciated. I am getting as much input before I even start drafting the arms.


Thank You
Tom Sheppard

dennis68
12-20-2004, 04:27 PM
You are fabbing your own chassis? I strongly suggest doing some homework first, increasing caster would bring in the + side of things, not further negative. I was going to write I could not think of a good reason but instead am going to write there is no good reason to decrease caster further than -1.0. In fact anything less than 2 or 3 degrees positive was a waste of an alignment.

Wow getting into modifying steering arm length too, what arms are you going to change and why?

Maybe we should start over with what you would like to accomplish and why to better determine an appropriate course of action. Threads starting with I want fabricate my own chassis but don't really understand suspension/steering geometry make folks nervous.

By the way….welcome to the site.

Salt Racer
12-21-2004, 06:47 AM
I'm nervous alright. People need to understand that they can't just put a bunch of links together randomly and call it suspension. Not only he'd get himself much closer to death, but he'd also endanger others who share the highway. Having good fab skill doesn't necessarily mean you're qualified to design suspension.

Here is the list of BARE MINIMUM REQUIREMENT readings before you even think about altering/designing suspension geometry in your backyard...

- Herb Adams' Chassis Engineering
- Fred Puhn's How to make your car handle
- Don Alexander's Performance Handling
- Forbes Aird's Race Car Chassis
- Paul Van Valkenburgh's Race Car Engineering & Mechanics

All of these can be found at Amazon.com (note I don't work for Amazon).

Good luck with your project, but PLEASE, PLEASE do your homework before you start cutting metal. When you get to the point where you can ask questions like "What kind of SVSA length and SVIC height should I shoot for?", "What's a good compromise of KPI/Scrub Radius/big caliper dilemma?", etc, you will at least be able to design a safe suspension.

SShep71
12-21-2004, 02:20 PM
Sorry guys I shoulda Checked where my head was when I wrote that. Im not looking to change the caster angle any, Im looking at changing the camber angle. I guess thats what writing right before I went to bed at the end of a long cold work day (-10F to be exact). As far as changing the steering (pittman) arm length I wasnt planning on cutting and welding an arm, I was referring to changing it out. I was discussing the ratios achivable with my chassis teacher (Im a motorsports technology major). I can only change the steering box ratio soo much due mainly in part to what is available. If I install a shorter or longer arm, I am hoping to get different ratio (I.E. 16.5:1 instead of 16:1).
I guess i best stard re-reading my question when I post em. I didnt wanna make any one nervous, thats what the holidays are for!!


-Shep

dennis68
12-21-2004, 02:58 PM
Changing the pitman arm length will change your overall turning radius but won't do anything for your steering ratio. You can easily get 12.7:1, 14:1, 16:1, and 20:1 guts for 800 series Saginaw box. What ratio do you want and why is .5:1 increase worth the trouble? I don't think you would even notice a difference.

You should be able to pull -3.0 camber out of the stock chassis just using different control arms.

SShep71
12-21-2004, 06:51 PM
The pitman arm swap was only a thaught, I wasnt planing on doing it unless there wer definite gains from doing it. I know there are a bunch of companies that make aftermarket control arms, but Im a believer in making things myself. Its just more fun and interesting.


-Shep

Salt Racer
12-22-2004, 06:57 AM
Changing the pitman arm length will change your overall turning radius but won't do anything for your steering ratio.

Denny,
Think how longer pitman arm changes turning diameter. Longer pitman reduces turning diameter b/c tires turn more for a given steering wheel input. The pitman arm converts rotational motion of steering gear box into linear displacement of drag link. So for a given input angle at the splined shaft, the output linear displacement of drag link is greater with longer pitman arm. So it indeed changes overall steering ratio (steering angle @ tire per one rev @ steering wheel).

Conversely, steering arm on knuckle will revert the linear motion back into rotational motion. So shorter the steering arm, greater the steering angle @ tire you'd have, again for a given drag link displacement.

Of course, when you mess with one thing, something else will be affected just like any other things in suspension/steering system. You'll have to take stuff like stress in linkages, change in Ackermann angle, effort if manual steer, etc, into account.

dennis68
12-22-2004, 10:30 AM
OK, I suppose but changing pitman length has more of an affect on turning circle I think, we run into this on the lifted Jeeps. Usually to change the ratio and nothing else you would just change the box or least rebuild with a different box ratio.

The longer the pitman and idler arms the more linear motion per degrees of box rotation/steering wheel rotation.

Salt Racer
12-22-2004, 10:42 AM
...The longer the pitman and idler arms the more linear motion per degrees of box rotation/steering wheel rotation.

Exactly. But like you mentioned, it's easier to change the box ratio.

I happen to own a Jeep P/U that I use for rockcrawling. It's quite an experience getting behind the wheel of my Riv right after I drive the Heap (or vice versa)
https://static1.pt-content.com/images/pt/2004/12/104_04541-1.jpg

SShep71
12-22-2004, 08:20 PM
Salt racer, You made a good point as to the effect changing the steering arm would have concerning stresses on the rest of the steering components. However when I was originally contemplating the swap I was thinking along the lines an increased arc circle as per the swing of the new arm in relation to the original arm would exxagerate any binding of the steering components. Do you think that is correct? Dennis68 and yourself have got the gears turning once again, and I am starting to think more about it. If I was to do any swap of some sort, I would start at the box/pitman arm and design moving outward toward the spindles.

Thank You
-Shep

dennis68
12-22-2004, 09:44 PM
Don't see too many of those anymore Katz, cool truck.

Salt Racer
12-23-2004, 09:40 AM
...when I was originally contemplating the swap I was thinking along the lines an increased arc circle as per the swing of the new arm in relation to the original arm would exxagerate any binding of the steering components. Do you think that is correct?...

Yes, bind certainly becomes an issue especially if you run HD rod ends but that's what misalignment bushings are made for. Typically bind won't be an issue with OE style tie rods because of their much greater range of angular misalignment than rod ends. If you make such drastic changes that OE style tie rods bind, you probably should worry about other stuff.


If you have a running car, try swapping in a box w/ quicker ratio first. A 12.7:1 box in A/G-body feels quite quick, and is more than sufficient for occasional track use. Depending on the PS pump you have, spool valve torsion bar in the range of 0.210 ~ 0.240" diameter gives you nice steering feel for street. I have 0.235" t-bar with 1250psi TC style pump. It needs two hands to turn smoothly in parking lot, but feels really nice on highway and track.


Dennis,
Thanks (it's nowhere near nice, acutally). There are quite few Comanches up here. I got mine in NorCal last year. It was cheap, has 4.0L, short box (shorter WB), Dana44 rear axle and 5sp stick - just what I was looking for.