What are the absolute essentials for a good handling chassis?

Sounds like a simple enough question, right? My sedan has been on the road since early 2008 and has about 10,000 miles on it. I've made six trips to the Detroit area and back (over 200 miles round trip) but that's as far out of my zip code as its been. I trailered it to the Hot Rod Cinematic and the last two Pileups. I was thoroughly embarassed watching an A roadster and a shoebox pass me on I-80 last year and not even give my trailer queen a passing glance. The truth is my hot rod is fun around town at 40 mph but out on the express way its an adventure and it really beats you up.

This is my first hot rod and I know I did a lot of stuff wrong. First, its too low at 5" off the ground. The second is the suicide front end. I think not having the weight of the engine and frame over the axle doesn't allow the suspension to compress and it basically skips over bumps. The car with me and a load of fuel is just over 2400 lbs. I could be wrong, but that's how it feels. Third, the tie rod is out front which I think is worse than not having any Ackerman at all? Do I have negetive Akerman with that setup? Is there such a thing? Lastly, my choice of steering is a reversed Corvair which either has too much bump steer, or the steering shaft is way too long. Either way your hands and forearms get quite a workout at 70 mph.

So I'm thinking about building a new chassis over the winter. The only things that are certain is that I will be putting the front axle back under the frame and I will not use a four bar setup. Other than that, what are the key fundamentals for a good handling chassis? Wheelbase? Coilovers vs. Buggy spring? Ground clearance? Steering? Rolling Bones setup with the slight frame kick up and the spring mounted to the wishbones vs. spring over the axle in the stock locale? GVW, Ackerman, bumpsteer, toe in? I know all these things are up for debate but I'd like to hear from the long haulers as to what they think goes into chassis design for a car that can be driven a lot further than 120 miles from home.

Thanks, Scott.

 

IMHO, most important is torsional ridigity, or eliminating frame flex from corner to corner (diagonally). If it does this, suspension will never work properly. Will also lead to body flex, cracking, and in worst case, driveline and steering misalignment.
Make sure to gusset all the corners and try to get some "depth" into the frame and crossmembers when viewed from the side. Look at some aftermarket frames to get better idea of what I'm talking about. Is your frame flexing/moving at the steering box mount? Have seen this in homemade frames. Giveaway to this will be cracked paint and brackets.

 

I don't have the answers, but I congradulate you and wish you luck in starting a new trend away from the reduculous to something that actually drives and handles well.

 

Scott, Order up a new Howe Chassis, mount your body.........done!!!

Frank

 

The biggest mistake ever [ street rods or race cars ] is INCORRECT SPRING choice.
Seconded by incorrect combination of parts

blaming the suicide front end for it skipping over bumps, the springs are to stiff for the weight it is supporting [ moving the axle back wont fix it ]

Suspension should be measured by frequency [ cycles per second ] when you remove weight the frequency goes up.

Stiff suspension causes more chassis flex!!!

My advice here is don't build a new chassis [ unless it is totally fugly ] but spend your efforts setting up what you have

Hang around with a couple of dirt racers, they'll teach you a lot about suspension set-up

The late Bruce McLaren [ McLaren F1 ] said the real cost of racing isn't the parts you use, but the parts you throw away.

You are prepared to build a whole new chassis [ essentially throwing away the one you have ] when all you probably need is an assortment of parts to dial in the one you have.

Building motors is the similar, wrong combinations pruduce poor results. But one in a good state of tune will perform better.

 

i do not have a answer but am glad you asked the question. i look forward to seeing what the experts have to say. driving the car is what makes it for me.

 

Don't try to use your springs to control the stiffness of ypur suspension. Use springs that the leaves don't bind on one another as they move (especially a buggy spring) and control the stiffness with good shocks. Jump up and down on your front perch. If the suspension doesn't move, the spring is far too stiff. Take out a few leaves, grease the rest so they slide on one another, and try again until you get it to bounce just right.

Read up on ackemann... If your tierod is behind the axle it needs to be shorter than if it is in front. There's a formula that measures wheelbase, track, and gives you the right ackemann angles. It's been around since the early 1900's so it ain't exactly rocket science!

Is your frontend correctly aligned? Do the wheels point pretty much straight ahead together, is the axle bent or straight? Are kingpins worn, shackles loose or binding? Do all the basics before you tear your chassis apart.

Good luck, Glen.

 

A quick correction here Glen
Don't try to use your springs to DAMPEN the stiffness of your suspension.
Friction between the leaves dampens the spring

The stiffness springmakers refer to is the LBs/In rating.
a 500lb spring is stiffer than a 250lb spring

A 500lb spring with 2000lbs loading has a softer frequency than a 250lb spring with a 800lb loading

This is where car builders make mistakes because a hot rod is nearly always lighter the the donor suspension

 

The essentials?
A rigid, square frame.
Tierod behind axle.
Springs, front and rear, that provide quite a bit of bounce when the shocks are removed. If too stiff, they will exaggerate road imperfections.
Shocks all round that are adequate rate, mounted for maximum effect, and in good condition.
If using side steer, the drag link must be on a plane that pivots from the same point as the split wishbones.
Correct caster angle, correct toe in, and kingpin bushes, bearings, tierod ends and tires all in good condition.

 

There are many lessons to be learned from Colin Chapman.
Spring rates, shock rates, etc are all documented in ridiculous detail by these guys. Basically, read a ton, measure a ton, and get the exact combination of parts to compliment each other.
http://locostusa.com/forums/index.php?sid=ccd809a30577c71e90a457bbbd549917

 

Lucky77,
How "good-handling" do you want? 1-G on the skid pad?

How traditional do you want to keep it? Solid axles, leaf springs, drums & bias plys?

You'll need to do some soul-searching to define what your requirements are. Good-handling traditional cars have been-and are being built- but I bet they're the exception.

One specific point: Sprung vs. unsprung weight.
The entire car is pretty light, while the traditional suspension (solid axles, drum brakes, steel wheels & leaf springs) is relatively heavy. This makes it hard for the suspension to do it's job- isolate road irregularities from the rest of the car.

Anything you can do to reduce unsprung weight will help the suspension do its job. FWIW, This is a huge challenge for motorcycle engineers/designers.

You've asked an excellent question that I've often wondered myself. Thanks!

 

Thanks so far guys. Probably should've included a picture of my chassis first, so here you go. It's Z'd in front and gusseted on both sides as well as the rear kickup. There's a rearward facing K member as well so its pretty rigid. I had the front end aligned by a very reputable shop and they dialed in 7 or 8 degrees positive caster. So far there are no cracks on the chassis, I do check it often, and went through it pretty well in April when the engine/trans were out.

I don't need to pull 1G but I want to be able to go 80 mph comfortably and come up with a better steering setup, one without so much bump steer. The rear has a 17" kickup and the front is Z'd with a suicide setup. I think it actually would be easier to build a new chassis rather than modify this one. I want to remain traditional, that's why I said no four bars. So buggy springs or coil overs, ladder bars, hairpins, etc. Also, the front end is the kit Speedway sells marketed as "The Classic" its a spring behind the axle with a max vehicle weight of 2500 lbs and I'm running bias plys.

 

Clearly the chassis is nicely built. Good job. You've gotten the right advice above; essentially, it's all about shocks, springs, and alignment numbers. You might gain some by reducing the frame kickup at the rear, but only in increased ride height. Whatever your new frame design is, if you put those shocks, springs, and alignment variables on it, it will drive similarly. Good on you for wanting your car to be a "reacher."

 

Oh, one more thing, and I can't tell what your interior looks like from the pictures - COMFORTABLE SEATS. I see these guys building cars with steel tractor seats or unpadded bomber seats all the time, and I just laugh my ass off. No way in hell those guys are driving any distance like that.

The hobby is about driving the cars. You might call Pete and Jake's, in Peculiar, MO. They've done enough rods over the years to set you straight.

 

You are very correct with your first question. We need to be really honest to ourselves here.

The principles of racing are the same as road cars, it is the demands and loads that change.
Sprung vs unsprung weight is what suspension frequency is all about.
If 90% of the weight was sprung vs 90% of the weight being unsprung requires a totally different spring rating [ yet both have the same weight ]

As a car travels faster the suspension frequency should increase [ if you drive at 30 mph on a race circuit you would wonder why suspension was needed, yet at race speeds they can bottom out ]

A road car [especially one with lighter sprung weight ] definately needs a lower suspension frequency [ softer suspension to layman ]

Another big mistake is roll stiffness, a 1G corner at 30mph has exactly the same weight transfer [ bodyroll ] as a 1G corner at 100mph!
The faster corner is a sweeper, the slower is a hairpin.

The faster corners require stiffer suspension so it needs Less anti-roll bar stiffness
The slower corners require softer suspension so it needs more anti-roll bar stiffness to control bodyroll
Anti-roll bars do not contribute to suspension stiffness only roll stiffness
[ a road car is a big compromise ]

 

Sorry out the last blurb, I was busy 1 finger typing and didn't notice the pictures you posted.
Why would you consider starting again, the chassis you are working with is OK

Firstly, I am not a fan of split bones on a street car, it tries to torsionally twist the front beam during bodyroll [ like a giant anti-roll bar ]
Too much roll stiffness in the front will cause bad understeer [ it'll want to "step out" out mid corner ]
The best fix is obviously 4 bar, but you want to keep it Traditional so consider drilling a shitload of holes in the beam [ between the bones ] to lessen its torsional resistance.

Coil overs on the rear help make it simple to swap springs [ there are 100's of ratings available ] I'm gonna suggest 80lb/in -100lb/in as a starting point [ don't let your ego say more is better, we aint bench racing yet ]

Please ditch the friction shocks, they don't dampen at 2010 road speeds.
Are you using side-steer or cross-steer?
You need some sort of lateral locating medium [ panhard or watts linkage ]

You can only use a panhard bar with cross-steer ,with the bar being identical in length and angle as the steering arm [ the R/H side of the axle should move on the same arc as the steering arm cancelling out any bump-steer ]

The upper triangulated arm on the diff is too short, try lengthening it by mounting the upper centre point behind the diff [ it is no worse than mounting 4-links in front of the diff ] just try and keep the links parrallel and of equal[ish] length [ the kick up creates an engineering challenge ]

Now I am going to try and explain Motion Ratios [ leverage ratios through suspension "arms"]
The formula [simplified ] is the wheel rate [ lbs/ in x motion ratio squared = spring rate ]

If we looked ar your rear suspension ,the coil overs act directly on the axle. So a 100lb spring would yeild a 100lb wheel rate
Now,if you mounted the coil overs half way along the lower links [ yeilding a 2:1 motion ratio ] you would need a 400lb spring to get a 100lb wheel rate

When you need a 100lb/inch force at the wheels, you need 200lbs force in the middle of the lever [ bottom link here ]
Because the bottom link has a 2:1 ratio, when the wheel moves 1 inch the spring only moves 1/2 inch [ 200lbs x 1/2in = 400lbs/in ]
There are two factors here [ force and distance ]
Why am I telling you this, altering the motion ratio is an easy way to soften suspension.

If you mounted the spring hangars further back on the bones [ suicide suspension ] it will soften the ride a bit by altering the motion ratio
So moving the axle forward would be more beneficial [ if you left the spring where it is now ]

 

I'd do a new chassis because it's quicker and cleaner than trying to fix that one, mainly because it sounds like you want major multiple changes.

It also sounds like you want a more traditional look?


I would consider going up to 2x4 tube rather than what looks like 2x3, then you can pie cut the front rails to look a bit more like a real frame rail.

 

this is very important in making a car handle, correct ackermann

 

Order up some Bilsteins. ElPolacko had a hand in the developement of the shocks. In fact, PM Steve on this whole thread.

 

I'd PM Matt aka hotrodrummer from the Ionia Hot Rod Shop as well and see if he can enlighten ya a bit. Perhaps he's heading to Frankenmuth in a couple of weeks and he could not only check your car out in person but go for a ride as well. Him and his dad know their stuff and are great guys.

 

you need a minimum of bump steer , sufficient caster to keep the wheel straight automatically. Sufficient toe in so there is always toe in going straight ahead even when going over big bumps but not so much as to wear tires. You need enough camber so the tire and bearings are loaded properly, Your rear suspension should also have minimal bump steer as that can make some phantom moves that can be surprising. Idealy you do all the steering, not the car itself. After all is said and done and it steers well it should not require excessive steering effort (often caused by exaggerating required steering angles. )
Don

 

I am not a chassis builder but your frame looks like it probably has a lot of flex in the front half, and those friction shocks are never going to work well at highway speeds. There's a reason why the rods in '50s magazines are all converted to hydraulic shocks.

There are others giving more specific advice here, but my experience has always been that soft springs and firm shocks make for a very nice riding combo. (Assuming the shocks aren't TOO stiff - I did manage to set up the rear of my '53 Chevy so that it actually bounced on small bumps until I changed for much softer shocks!)

 

Some other considerations would be what you percieve as movement on the road that you may instinctively correct when it may not be necessary. I don't see a panhard bar out front. I agree with the unsprug weight evaluation too. It's not impossible to overcome excessive unsprung wt but you need to consider it. I'm working under a 37 Packard here at the shop. In the rear it has leaf springs, a stabilizer bar (sway bar) and a Panhard bar, all OEM. The front is OEM IFS with coils and a sway bar. Drives like a dream but weighs more than twice what yours does. Weight aside, the lateral stability was one of their primary concerns. If the ass end moves around the front end has to fight that as well as the road surface. It looks like the rear suspension is pretty stable. Is it? I'm posting these thoughts in an effort to get you looking and thinking about what you have. There's no parts list I can offer, just theory. And yes, all steering angles rule the outcome. I like where you're going and what you want. I hope you keep us informed.

 

Lots of good advice here. Your caster is good and as mentioned earlier, get the ackerman right. You mentioned bump steer in your post. My advice is to pay close attention to your steering geometry. Stiff ride is one thing, but a bump steering front end is an S.O.B. to drive and gets worse with speed. If using a cross steering set up, use a panhard bar, it needs to be as long as possible. Panhard and drag link need to be on the same plane and describe the same arc during suspension movement as much as is possible. Side steer, the drag link needs to describe the same arc during suspension movement as the hairpin / axle arc. Use something such as a dead perch to eliminate side movement of the axle on the shackles. If possible, mock up your suspension with only the main leaf (or no spring) and move it through its range of motion. There should be no (or very, very little) spindle movement.
A car that darts about when encountering road irregularities seems to amplify other issues. Once you can get it to stay on course further tuning will improve ride and such, but no amount of parts changing will cure bad steering geometry.

 

The newbie has said everything that I was planning on saying here with the exception of the fact that moving the axle forward has about the same effect as moving the engine back. For handling you want the weight bias to be as close to 50/50 as you can get.

You mentioned the ride height, for highway handling you want it close to the ground the lower your center of gravity the better.

I get the impression that the thing that bothers you is bouncing around and a rough ride. The only way to fix that is to get your spring rate right as kerry mentioned here.

If you don't want to fiddle with it find yourself a set of scales you can borrow and get a weight on each wheel. Then have someone help you set up springs for that weight. You'll be golden or at least close.

 

Before condemning the chassis to death, I'd do two things. Both utilize parts that you can re-use if you do it over.

First, rework the steering/spindle/tie rod arrangement as mentioned in the previous posts, with the tie rod behind the axle. Get all the angles correct.

Then, properly mount some good quality tube shocks instead of those friction shocks. In my humble opinion, on today's roads, friction shocks have no place, but they look cool.

If you want a third, I'd re-mount the rear coil-overs so the tops are closer together than the bottom, for a little more stability.

 

Professional chassis engineers don't really talk about spring rates as much as deflection. Spring rate itself is really meaningless without the exact geometry and load.
Here is a quick guide.
The amount the car settles on the spring from complete free of ground to full load is the deflection. Deflection of one inch will knock your teeth out. 5 inches will ride like a 56 Buick. Something in the 2 1/2 to 4 range will be livable. Adjust leaves,coilover springs to achieve. Once you find how much deflection you like, you can tune other suspensions.
Note: If coils are preloaded due to being tighten down or used on a front independent where the installed springs are under preload, does not work.
You have to get the installed spring rate by measuring deflection due to an applied load and backing into the equivalent deflection with the total load.
This is more straight forward than some of the natural frequency BS.
Also, Alway run a rear anti-sway bar with coil-overs as the geometry of the angled shocks gives very minor roll stiffness to the rear. You need some rear roll stiffness to loosen the car up as they say on Nascar. It will be super tight otherwise.

 

One more thought: have you ever pulled off the shocks and worked your rear axle up and down from just one side? I think your center link might be binding like crazy when one wheel hits a bump or a hole. (One wheel tips up, the other tips down, and the pumpkin has to shift to the side to allow it all to happen. Your setup allows NO shifting.)

 

Now we're talking Good stuff here, thanks a ton. By the seat of your pants it actually feels like the rear suspension is working and the front is fighting. And with the wheelbase (115") you actually have time to notice the difference. I have a circle track background so I'm familliar with the concepts mentioned but for some reason the unsprung weight situation never came to mind. It has a big old 9" rear end from a 1977 truck, and I'm sure the 40 Ford/Buick drum combo I have up front is pretty heavy as well. The chassis its self is light enough that I could pick it up and flip it around myself while I was welding it up.

The reason I'm leaning towards a new chassis is the front suspension. With the "Z" I can't put the tie rod behind the axle, I tried. The only way would be to cut a hole in the chassis and pass it through. Its 2"X3" material and doesn't leave a lot of room for boring holes through. Secondly, I can't take another leaf out or I won't be able to make it up my driveway approach. I know it doesn't lower the suspension but the chassis will come down slightly in the front and I'm at the lowest point I can go and not get hung up on the sidewalk. I have bottomed out with a passenger in the car before while heading up the driveway.

My chassis isn't bad, especially for a first timer and it does go down the road pretty well. I drive it with one hand at 55-60 mph all the time, maybe I'm expecting too much? I don't think there is any lateral movement in the rear. If there is, its not much. I thought that top wishbone acted like a panhard bar, no? I was following fuelpump in his 32 roadsters around a really twisty road one day. He sailed right through a corner at 50 mph or so and when I tried to do the same, my rear end stepped out and got into the gravel. It was the first and only time it ever got away from me like that. Maybe the rear suspension did go into bind?

Maybe I'll keep and tweek this chassis and leave it under the sedan, who knows? But I will be building a new chassis either for this car or another one soon, so lets work under that premise. If I was to start over with some brand new 2"X4" material and could build any chassis I want, what are the key ingredients to making a chassis handle the bumps?

 

I just re-read Andy's post about deflection. When I built the chassis I had it set up for a 6.5" ride height with no rake, pool table flat. When it was all said and done, the rear frame at the kickup is 5.5" off the pavement, and the front is 5" at the Z. So it settled 1"-1.5" I'm not sure how much it settles after being completely unloaded but I will look into it.