Technical Unsprung Weight Issues In Hot Rods: School Me

No matter what I do I can't seem to get my '29 highboy coupester to have a decent ride. The suspension just won't absorb the small road imperfections like I think it should. I'm beginning to think that the weight of the big cast iron GM disc brakes adapted to both front and rear axles may have something to do with it - too much unsprung weight for a light Model A highboy.

Does anybody have any hands-on experience with excess unsprung weight on a straight axle hot rod?

 

It's a big issue!

Post up some pics and something may pop out at us.

 

I doubt it's caused by the brakes. Probably need the right shock and spring combo. Possibly you have unrealistic expectations for ride quality.

 

How smooth a vehicle rides is often determined by what other vehicle you drive all the time you are comparing it to. That OT rig of mine with the 17 inch wheels and 235-40 tires rides rougher than a cob on the roads around here but someone who was used to it would think it rode smooth. My 77 Chevy dualie actually rides smoother than it does.
Do you have the correct shock valving for the car? I've seen too many guys think they need a real stiff shock to "make the car handle" and end up with a bone jarring ride. I've also seen it the other way and especially on rods where the shocks really didn't do anything because they were chosen for their looks rather than function.
Add in tire pressure for good measure, Some of us are guilty of running more air than we should because we don't like the look of the sidewall with the correct air pressure. When radials first started being common on new cars off the showroom I had an old far come into the Firestone I worked in complaining that "Those damned Firestone radials ride too F#$^&*G rough. Checking the pressure it pegged my gauge that I carried in my pocket and checked with the truck tire gauge he averaged 75 lbs in each tire because he had aired them up to where he thought they "looked' right. A few days he came back with the pressure too high again and we finally had to tell him that he either had to deal with not liking the looks or trade them in on bias tires.

 

I have just been through this myself in my roadster.

Reverse eye, less leaves and incorrect arch if not done right produce a horrible ride.

I was after the lower look inside the wheel wells w/o a z'd frame but paid the price.

So, I have returned to a new rear spring pack with reverse eye and the top three leaves placed under the pack but now have the correct arch.
So long to the rear no gap from tire to fender well but hello to a much improved ride.

Now, it will never, ever ride like a modern vehicle as long as modified buggy springs are involved so, that is part of this type of Hot Rod.

With my current build, 28 Tudor, I have eliminated buggy springs and look forward to a much more pleasant ride.......

 

No hands on experience yet but I’m going from ‘72 Chevelle calipers and rotors to Volvo P1800 rotors ( I know, odd, but I had them) and Wilwood four piston calipers. All told with brackets and hardware factored in I’m saving just under 20 pounds per side.
The Volvo’s give me a 4 1/2” bolt circle to match the 8” rear end in the car. The 8” was in the car when I bought it and I think it is a plus with it’s lighter weight compared to the typical 9”. Everything I’ve read says lower unsprung always helps.

 

Oh Boy! The Model A Pick Up I'm building with the Race Car quick change rear is going to spank my butt every time I run over an earthworm.

 

The problem isn't the unsprung weight but What is between the sprung and unsprung weight [the spring stiffness]
When you lighten a vehicle the spring stiffness needs to be lowered, and spring load needs to be changed to restore ride height.

On a leaf spring: Spring stiffness = Number of leafs x length x width
Spring Load = The shape of the arch

On the street you generally want a car to settle 5-7" for normal ride height [7" being softer]

Too much unsprung weight on a correctly sprung car will cause the wheels to skip over bumps at high speed [because the frequency is too low]

 

The most objectionable is the feeling that the wheels are out of balance or out of round on the front end...but not all the time. If they were the front end would "bounce" or oscillate on smooth pavement as well as on slightly rough pavement, but it doesn't. On smooth pavement and at higher speeds the front end is dead steady. But on normal roads that have a little character the front end just isn't right. It feels like the suspension just isn't controlling the wheels. It's weird.

Here's a photo of what I've got for a front end. The spring is from Posies and seems plenty flexible when I step on the spreader bar. Shocks are Pete & Jake's. There's nothing unusual about the front suspension at all other than the big clunky brakes. The calipers are the big, single piston late '60s to late '70s jobs along with the matching hubs and rotors.

 

Just to add to what has already been stated, on my roadster I have a Posies front spring with the teflon buttons and in the rear I have coil overs with hairpins. The short test drive I have been on it seems to have a decent ride. Of course I got the shocks and coils suggested by Speedways website for the weight of my car and I am sure that helps. Front shocks are just standard Monroes and the spring is a dropped spring with reverse eyes on it.

 

This frequency being too low may be what the problem is. The motion of the car and the motion of the wheels seems to be out of sync as the car traverses mildly uneven pavement. The springs (coil) in the rear may be too stiff as well but the strange feedback seems to be coming from the front, through the steering as well as through the seat of my pants. All and all just very weird.

 

@missysdad1 is your frame notched for the spring?

My 29 has no notch and my spring may be contacting the inner frame possibly contributing to my bumpy ride on less than stellar pavement.

The Hotrod has been driven on this setup I have for since about 2010 but I have been meaning to add a spacer above the front spring to raise the nicely raked profile a tinge perhaps 1 to 2 spring thickness's to add some clearance at that location and possibly improve the ride.

You hightlight a dilemma many of us face...which is not having driven another similarly configured vehicle to compare or even have friends close by with similar setups.

One other thing is some highways and roads are very vintage car unfriendly. We have freshly paved roads that are what I call budget roads or pathetic excuses for paved.

 

The way we decided if it was spring or shock when dirt racing.

Stand along the road and watch the car go over bumps from the side.

If the body goes up and down to exaggerate the bumps, springs are wrong.
If the body stays smooth but the axles seem to move excessively nd never calm down, shocks are wrong.

SPark

 

Yes, the bigger the "bump" the more violent the "experience" from the driver's point of view. In my case it's extreme. I avoid major pavement disruptions like the plague. But it's the minor disruptions that really seem to upset my suspension with an oscillation that works its way up through the steering wheel even when it's not visible (I can see the left front wheel from the driver's position).

Here'e a little insight from a Formula One page:

"Reducing unsprung weight is the key to improving handling. The lower the unsprung weight, the less work the shocks and springs have to do to keep the tires in contact with the road over bumpy surfaces. Lot of problems, if not all of them is caused by inertia. Bigger weight means higher inertia. Higher inertia means more workload for shocks and springs to keep tiers on the ground. If unsprung components have a high mass they are harder to accelerate/decelerate and thus it is more difficult for the suspension to maintain a consistent tire load."

There is also some very interesting information in Wikipedia of all places, including some about solid-axle vehicles, a group to which hot rods like mine belong...

https://en.wikipedia.org/wiki/Unsprung_mass

 

Resonant frequency varies with spring rate and sprung mass. For ride quality you're aiming in the region around 0.9-1.4Hz. Lower is more comfortable, but go too low and you're into the sorts of frequencies that cause seasickness.

The more likely situation when you've got too much unsprung mass is too high a resonant frequency and, consequently, a harsh ride. Apart from the aspect Kerry mentions (which I think has more to do with the inertia of the unsprung mass than with the resonant frequency) it's not so much a case of too much unsprung mass as too little sprung mass. The real point of reducing unsprung mass is that it gives you the opportunity to increase sprung mass without increasing the total mass of the car.

You can set up a suspension system to give a cushy 0.95Hz ride, but as soon as you stick a few suitcases in the back – increasing the sprung mass – you're into seasickness territory. It's a perennial problem in designing light cars, because the sprung mass can vary far more wildly with passengers and payload than with heavy cars. That's where variable rate springs, modal rate differentiation approaches, and other fun stuff comes in.

The other reason for wanting more sprung mass is that the roll stiffness bias manipulations we do for chassis tuning only work on the sprung mass. The more of it you've got, the more scope there is for tuning with spring rates, anti-roll bars, etc.

 

In 1954 Mercedes-Benz went so far as to run inboard front brakes on their rear-wheel-driven W196 racing car:

A whole front-wheel-drive system, with drive shafts, CV joints, etc., with no drive from the engine, just to be able to hang the front brakes on the frame.

 

The same principals apply
The difference is the rebound hit in your hot rod is contained and absorbed into your ass.

 

I have been that little ball.... I do have a safety net however...the roof......its a fair bit better now with the new shocks but still exists...being so many of us have jalopies running some form of original spring with heavily modded bodies and mechanical configs it easily could require leaf adjusting which has been the topic of many tech discussions.

While the ride could very well be improved through several changes which I will needle away at...some being mentioned already the vintage harshness is something I paid for...

​

 

Unsprung weight is a very big issue. Anything you can do to get it down helps. Wildwood brakes and disc, aluminum hubs, aluminum wheels, tubeless, radial tires, etc. Another problem for us is the gas filled shocks. They just ride harsh. Google oil filled shocks- I think there is still one or two manufactures out there. Good luck!

 

Your Chevelle disc setup weighs considerably less than the original drum brake setup.

Reducing unsprung weight will always improve the ride and handling. You will get to the point that the results do not justify the expense.

SPark

 

I agree with LM14 .... your front end unsprung weight is much less than the original.

One of the smartest chassis guys ever, taught me that our hot rods should have close to the softest spring rate possible to maintain your desired stance and provide you the softest ride. I think that a Posey super glide spring is probably the best ..... when installed properly.... that means the right length so that the shackles are at the correct angle ..... and then the axle will swing side to side on the chassis (which makes handling unpredictable) .... so I always install a pan hard bar. Sometimes I have removed the 3rd and 5th leaves.

So we have a soft spring and the stance is right ....... now the ride of this hot rod depends on the control of the bump and rebound performance characteristics of the spring which is the job of the shock absorber. Available today are many double adjustable shocks that, when matched and adjusted properly, will change ride performance greatly.

I have seen it where really soft front spring may need a lot of "bump" to minimize compression and less "rebound" control. Although my current car likes to move the spring over a bump (little compression control) and a ton of "rebound" to keep the front down and allow the rise back to ride height to take several seconds. Which seems to be incredibly smooth. Say 1/3 of a football field at 70 mph.

I am no expert and don't claim to be ...... but I do believe that there is huge area of learning to matching spring rates to the available shock technology ..... and I haven't observed much of it in the past when looking really at good cars.

 

I just weighed a ‘40 backing plate, hub and drum, shoes and all hardware- held by me on an old bathroom scale it was 34 lbs.
A GM intermediate rotor and ‘70-‘77 style caliper with bearings and hardware including bracket but no pads comes in at 36 lbs.
YMMV if the ‘78^ metric calipers are used.

 

There's no "Rule" really, as to the ratio of sprung to unsprung weight.
However when the unsprung weight remains the same but the sprung weight gets lighter the negative effects get amplified. On that same note keeping the sprung weight the same or lightening it but increasing the unsprung weight also increases the negative effects.
I've really not had to much trouble with the front ends, there's just a lot of weight from the engine but the rear is another story. Get a light in the ass roadster and that rebound can be fierce.

there's a bunch of GOOD ways and GOOD reasons to lighten sprung weight, it's easy & it's almost hot rod 101. Hot rod 201 is swaping in better stopping powers and beefier rear ends so the lightened car doesn't have problems stopping or hooking up. Hot rod 301 is wondering why the stuff works the way it does and why driving it is not more fun than building it. Hot rod 401 is shaving pounds off the unsprung weight and the reducing the negative effects of it. Half of 401 and the next one 501 is OFF TOPIC!!!!


I've had an idea in my head that's been rattling around for a while. Maybe I should build it and see if it works? If I used air bags maybe I could get it to fly.

Zooming around in a car that weighs about as much as a fart box Chevy AVEO with brakes big enough to stop full size truck that's fully loaded with an extra ton of payload shouldn't be on the GOOD list. Monster strong differentials from trucks and 300 -400 HP engines in light cars with skinny tires shouldn't be on the GOOD list either. For example, a ford 8" vs a ford 9" has about 30 lbs or so depending,, maybe a few more maybe a few less,, weight difference. That 9" was under full size trucks and vans. Will swaping out an 8" adding 30 lbs of the 9" in unsprung weight to your already lighter and further lightened car increase the negative aspects of unsprung weight? HELL YEAH IT WILL!!!

 

There are some basic things,that seem over looked by new guys{I'm 76}, How strong a spring{read also as stiff} has a lot to do with how its placed. Take a look at how early Ford had a spring on top of axle,later on it was moved out front of axle/also out behind rear axle{ Every wounder why???
By put the spring near the outer ends of car,the spring dose not need to be as strong=softer spring and softer ride. Tire tech maybe needed for some as well,"big an littles" after bigger wider an there for heavyer tires came into being used,the ability to control bounce of a front axle set up,is about shocks an how many lbs. your trying to control. For the most part "little" refers to a front tire that is about the same lbs an wide as a stock tire the car came with<< that size lbs. can be controled pertty well,but bigger and wider* can't. And that's also were rim off set comes in=needs to also be like car came with,not sticking out off the brake drum. Why is,it adds bad scrub,plus as you go more an more away from center of car,the number of lbs. is harder and harder to control{ lot of deathwabble happen from not knowing an fixing this.

 

Which bracket ?

 

Dana, while I agree that spring placement, or how it is 'loaded' by the suspension can certainly affect spring 'rate' needed, a good example being a 'direct acting' spring, say a coil on top on the axle, compared to a coil operated by an "A" arm suspension.

However, I think Ford changed the spring position from on top of the axles to in front or behind, to facilitate lowering the chassis and in the rear, lowering the floor height as well. Other than those potential benefits, I don't see the Ford leaf springs being affected by any sort of 'leverage' than would direct affect their rate for a given weight vehicle. On second thought, maybe the increased length of the spring was beneficial to ride comfort.

Ray

 

Thanks for your post. I had serious doubts about the claim of the particular disc setup being lighter. Didn't know what the weight would be, only closer to the same than not was my guess.

Ray

 

true.

piss on engineering charts and graphs. If you want a great ride you need to experiment "trail and error", many times, with each car build type. It is total nonsense to order some "John Doe Hotrod catalog shop 'suggested' rear shock" and expect that it will do some miracle no matter if it has a soft spring or not, and a very heavy rear axle , compared to using a wimpy rear axle. How can you pick one shock as a "fits all"? You can't!

Mine rides wicked NICE.. but it took a lot of effort, and swapping leaves, re-arc several times, and getting the correct shock only "After" the spring felt good on a static bounce. Also, the angle of the rear shocks MUST be considered when tuning the ride( on the rear especially).

The biggest problem is the rear of the car on a typical hotrod body style like the A or 32 "two seater", (not a heavier 4dr sedan). I run a 55 Olds very HEAVY rear axle, a later 40s Ford longer(wider) rear spring which is better than a narrower 40 front spring some use in the rear.

One thing that likely cannot be perfectly dealt with, is when you finally get the right setup, then you add a big 20 gallon tank like I have, and then add passengers. That is just too many variables, as having two people, and only a 1/4 full tank, to a full tank with two in the car....to 1/4 tank with just one.

I run a 55 Olds engine, on a reverse eye stock spring with leaves removed and spacer leaves above the spring pack. My car runs on dirt roads, cornfields, and New England poorly maintained potholed roads.

As said in this thread above, a buggy sprung solid axle car will never ride like a new medium size Toyota sedan....but you can get it really nice with a bunch of effort. Toss the catalog builds suggestions and then custom build for your ride tastes.

Tire pressures on the bias tires I use, must also fine tuned.

You simply must "create" the ride you seek.

.

 

That video is a good basic explanation of the energy.
All we need to do is imagine a spring between the 2 balls. If the spring is too stiff the small ball will move as usual.
But if it is soft it will absorb a large proportion of energy. [but it will require some distance to do so]

The problem with soft springs with high unsprung weight . If the wheel hits a bump [above the surface] the wheel momentum keeps moving upward and will bottom out . And when going over a pothole the spring "load" isn't enough for the wheels to follow the hollows.
Which is................... High frequency oscillations with low frequency springs [if you drive slower to lower the frequency, the problem disappears]
The first thing the OP should do is add some ballast to the front and try it . This will tell him if the springs are to stiff [too high frequency]

If the car is rough riding at low speed and smoother at high speed the spring frequency is too high.
The opposite occurs if the spring frequency is to low
Another phenomenon of too low spring frequency is the car pitching over bumps at high speed
This pitching [or porpoise-ing] is caused by the time delay between the front and rear wheels hitting the same bump.
Manufacturers use a slightly lower frequency in the front suspension so both ends will absorb the bump at the same time at "Normal" speeds but when you speed up the pithing will return [70's movie car chases show this]

I've quoted this before "the biggest mistake made in building/ handling/ or racing is the wrong choice of springs"

 

Flame cut, 3/8” plate from an old Butch’s Rod Shop kit.