Clean Sheet Front End

[ccracin]

Well folks here is my issue. I am designing the chassis for my 68 C-10 pickup. I have several 2 D layouts done and I am going back and forth on the direction I want to go. I am from a circle track racing back ground that says aggresive neg. camber gain is good. About 1.5-2 deg. per inch of travel. This has a RCH aroung 5" above the ground and will not move more than an inch in dive and roll. It also does not move r to l excessively.This is using a 60" track width with 18" lower arms and approx. 8" uppers at abot 15deg angle up to the ball joint. I set this up so far using a circle track pinto spindle with 10 KPI. Then I read some more about street and road race stuff and it seems that .5-1deg. per inch is what I am finding that arena. When I set things up to do that, my RCH is around 2" off the ground and moves all over the place up and down and right to left. I know from experiance that when the roll center moves alot left to right the handling becomes very unpredictable. I also am looking at the AFX spindles after seeing them on this site. I am going to be talking with ATS soon. Any comments on a clean sheet front end. I have seen posts in the past but I am hoping to guide this discussion towards my goals. I know there are a million things to look at and I am willing to give any imput I can. I am hoping through discussion to make some decisions. Thanks alot.

[novanutcase]

CCracin:

As qualified as many people on this site are you may want to point this question at the jolly old souls over at Cornercarvers.com. They seem to thrive on this type of thing and I think they probably have more people over their that can answer this question with more certainty. Can it be done here? Certainly! but I just thought I would give you another avenue in case you weren't aware!

John

[ccracin]

Thanks John. I'll try over there. I agree that I've seen some good stuff here as well. Hopefully someone will be willing to spout some opinions. Thanks again.

Chad

[ccracin]

Wow 129 views and no replies other than one telling me to look somewhere else. I know the knowledge and opinions are there, come on folks.

[camcojb]

Originally Posted by ccracin

Wow 129 views and no replies other than one telling me to look somewhere else. I know the knowledge and opinions are there, come on folks.

Well, the advice to go to CornerCarvers is very good. Just buckle down and do a LOT of searching before asking any questions. If you ask about something that has been asked many times before they will blast you.

However, with all that said, they have some extremely good suspension guys there, that's what they do.

Jody

[Mean 69]

There are clearly a ton of factors that go into a front suspension setup, it's way harder thanthe rear setup due to the need to steer as well. To make things really interesting, virtually everything is coupled to something else, in a negative way: push this paramter a bit, something else blows up. It is REALLY a big exercise in compromises, and because you can't have it all, you need to pick your battles. Sounds like you already have a huge headstart in terms of the circle track experience, just learning the lingo takes forever.

A few things to keep in mind, at the forefront, are things like the amount of suspension travel you will be looking at, it'll typically be a lot more than a high speed oval track setup, so insuring things don't go whacky in terms of travel is very, very important. As you noted, the circle track cars can and do run very high camber gains, but in general, this is WAY too much for a road course/street car (by the way, the two are about as closely related in terms of design requirements as there are, if you design for road course, you can detune the setup by softening up rates, etc, and it will almost always work exceedingly well on the street, which is why we take this as a design approach basis for our goodies. It's also the hardest of the suspension "biases" to solve, unfortunately).

I can give general pointers for the priority list, as I see/design things, but I am sure that others might do things differently. You have to start somewhere, or you'll go nuts, the first thing I do is to insure that the lower control arm is parallel to the ground at ride height. At some point you will run into packaging issues, but if you think you can run an 18" lower, that is really good, plenty long enough for a 60" track width. You already figured out that the main thing that affects camber gain is the ratio of the upper arm length to lower, this is also very important in terms of the roll center migration, though not the only contributor. Make the upper, and it's mounting as a start to get the camber gain somewhere around .7 degrees per inch of bump, over the first two inches, and start playing. A suspension program is extremely useful. Alter the upper pickup points up and down, play with the length of the arms, and see how it all affects the roll center height in bump/droop, and also pay very close attention to lateral migration in roll (2 degrees of roll is about all you need to consider). Then you get to start picking your battles.

It's all really complex, as I noted, but it is super cool to play with and learn about. You won't get too much specifics other than general pointers from folks that have it "figured out" because it likley took them a long time to do so, and they will probably be reluctant to share the secrets they found. I know I won't! One dude that di, is Herb Adams, grab his book called Suspension Engineering, it is an easy read, and has a lot of good info in it. Warning, don't copy his front suspension design that is listed in the book, I am not sure if he put it there to throw people off, but it's not terribly impressive and can be greatly improved.

One more point to your specific application, anti-dive. Trucks have a notoriously long polar moment, high center of gravity, and usually have a high front weight bias (static), in braking there is a good amount of weight transfer, exaggerates the whole deal.

Have fun, good luck!
Mark

[ccracin]

Jody,
You are quickly becoming my favorite on this site. Anyway I have already applied for membership to the almighty corner-carvers. I'm not rying to be a smart a**, but I know alot more about suspension geometry than super charging and I am intimidated just from reading the rules. I do admit from browsing the forum they know what they are talking about. I'll see how long it takes me to get blasted.

Mark,
Thanks alot for the info. I do have a suspension program and I have read the book several times. I even have it highlighted and tabbed. That's pathetic. Packaging may be an issue, but since I am building the chassis from scratch I have a bit of freedom. I felt the same way about his example.

One od the biggest down falls for me so far is the scrub radius. Since this is essentially a street machine we want some dish to the wheels and with a 10deg KPI its tough. I was thinking of running a 2" dropped heidts spindle to help with scrub radius. I can correct the bump with the placement of the rack.

I am only designing for 2 deg roll. But when I try to get the camber gain down to where you suggest the roll center migration right to left is terrible. I am still crunching, but it is challenging. To get this my upper arm is almost 50% of the lower and it is at about 5 deg. Is the .7/inch in the first 2 inches really adequate for the flat corners that street driving gives? I guess it is or you wouldn't have said it. With this set up when I am at 1 deg roll the roll center moves 7 inches. At 2 deg it is 15 inches. That to me is unacceptable. If I maintain the same arm lengths and change upper arm angle to 15 deg I get about 1.7 deg -camber gain per inch. But when I roll 1 deg the roll center only migrates 1.9 inches and at 2 deg. it's only 3.9". To me that is pretty good. With the latter setup the roll center is at 5.2" above ground and only moves .375" in dive and roll. This helps with the taller polar moment that you spoke of allowing me to run softer springs with less roll. With the first example the RC is 1.8" and moves down 1" in dive and roll. What do you think or am I missing something? The problem I do see with the aggresive camber gain is tractive effort under hard braking. I could have up to -6 deg under full dive.

I will have some anti-dive built into the front, especially running a big block with power adder.

Any comments based on the above are appreciated.

By the way, I am also considering the ATS AFX spindles, but I need to call and get some dimensions I can layout. Any thoughts on these for a clean sheet application?

Thanks,
Chad

[novanutcase]

Yes Chad!

Mark and Jody are QUITE correct in their posts! Do LOTS of searching before you post and be VERY careful in your wording of your post as the guys(and gals) at CC take their suspensions VERY seriously! Make sure that you use the proper lingo as they are very sensitive to this and are constantly on the look out for someone who is not on their game. I personally don't think it needs to be that harsh but it is their realm and they run it as they please. Not my bag!
I made the mistake of asking a question that was too broad and was lambasted for it!(I think I actually hold the record for the longest flame!) From reading your posts you definetly seem to know a thing or two about what you want to do but certainly, as Mark said, no one over there is going to give up hard earned knowledge so if your looking for them to design it for you it won't happen but the questions that I have seen you post here seem to be right up their alley so just be careful so you don't get flamed!

John

BTW FYI......Post #4? BIG NO NO at CC. That's what got me sent down the corridor of the beat down!

[ccracin]

John,
I'm going to give it a shot, but the first time I get flamed that will be it. I am not willing to play the game. Just because you have a bit more experience in an area does not mean you can belittle and be rude. There are plenty of awesome people on this site and others. I think people with the knowledge should be willing to help others to promote this hobby or what ever you choose to call it. Disease maybe! Plus, I can handle the suspension design. I just think the "been there done that factor" is very helpful. Most of my concerns with the front suspension are generalities. Mainly as I have posted camber gain is my question. I know what worked going circles but the street is a different animal. Some of the only times it is acceptable to me to be strict in not giving away specific information is when that information is how you make a living or in competative racing situations. Other than that how do we educate new people with regard to the automotive life and keep these awesome cars coming? Anyway, thanks for the heads-up and if it doesn't work out for me at corner-carvers I look forward to many discussions here at Lateral-g.

Chad

[Mean 69]

Chad, it's really simple at cc. Read their rules before you post, and follow them. The issues everyone complains about there is "how they treat the newbs," and how rude they are. To some there, it IS a bit of a game, there is a lot of internet bravado that I am certain wouldn't be present if was face to face, but so be it: that's the sign of the times. When you post, be specific, give details, and ask specific non-open ended questions. Once you get a bit of earned respect there, you can ask about anything and not have to worry. Don't expect them to design it for you, they won't and it's clear that you aren't looking for that. Small, specific questions.

Now, to your thoughts. Scrub radius is very important, too much, and you will generally chew up your steering gear, especially with wide, sticky tires. Too little, and you lose steering feedback. I'd suggest somewhere between an inch or two, no less than an inch. Look at it dyncamically too, make sure it doesn't go "positive" then "negative" in practice (which will be unlikely if you keep it over/around an inch or so). BUT, since you are already looking at scrub radius, you have already selected your spindle, which I think you noted is the MII unit. Ditch it. If you are starting fresh, start with the spindle, it's the hardest damn thing to find for the setups, and unless you are REALLY good at fabrication and engineering stress analysis, use one that's already made, this is a CRITICAL component. MII's have been used for decades now, originally because you could use a rack with the setup pretty easily. The MII spindle of today is the "C5" Corvette piece, and just like the MII, it can be used, but both require compromises, some may say minor, I say that they are major. When you try to get it "all" in a front SLA, WITH a typical V8 engine IN a typical muscle car/truck, you find that packaging the engine relative to the rack and pinion unit is a complete PITA. So much so, that for us, with our forthcoming "Extreme" suspension systems, we are designing and fabricating our own spindle. It ain't cheap, but if you're going to do something, do it right. Anyway, you might be forced to compromise scrub radius if you want to acheive a certain "look" with the wheel selection, which often times is really important. The AFX spindle (or rather, the completely bitchin' AFX spindle) was designed as a bolt in upgrade for the first gen F body cars, and as such, expect the scrub radius of that piece, as applied to that situation (which is very similar to yours in terms of track width) to the same as a stock F-body car. It is a very nice piece, and it may, or may not fit into your build, that's all on your shoulders.

The lateral migration numbers you noted, show you pretty clearly how darned sensitive all of this stuff is. Again, you are going to have to pick your battles, but this is where the design really takes form. You can make it such that the roll conter moves laterally less than an inch, it's really not a problem. If that's the main concern, it can be done. I had a former employee that was obssessed with this, and while it is cool in itself, if you aren't looking at the rest of the picture, it can be misleading as to how good the design truly is. And you are right, roll center height, camber gain, roll center migration, all of that stuff is related. Without question, excessive camber gain will kill you in very hard braking, espceially if run some static negative camber to begin with (this is a general statement, by the way). So yes, the approximate gain of between 0.6 to 0.8 or so per inch "seems" to be the best range when all things are considered.

Two more points on this. One, when do you really need camber gain? In roll? Pure roll? Or do you need it when you're turning? Point two, what's the inside tire doing during all of this? Yes, in a hard turn, weight transfer happens, so there is less loading, but that doesn't mean you can ignore it!?

Fun eh? Oh, and it's all just begining still, wait 'til you move on to the steering....

Mark