Technical My Model T Thinks It Is a Porpoise!

There is one section of road in town where my T really starts to porpoise. The road is concrete and is apparently not poured very level! My T is 4" dropped front axle with spring over axle and 9" ford rear with coilovers. My OT Corvette displays the same tendency on the same section of road, but just barely noticeable. My wife's car shows no signs of it. I have driven up and down this same section of road since they poured it many years ago, and never noticed any other car or truck porpoising.
The T rides really nice and drives straight down the road. Shock travel and ride comfort shows the spring rates to be about correct. This after a lot of experimenting and changing spring rates in the rear several times and playing with the front spring pack.
I'm not sure if wheelbase is a contributing factor here or not. The T wheelbase is 105.5". The 'vette WB is 108". Not sure what wifey's car WB is, but it's longer.
I don't know. Is it the difference between front and rear spring rates, or a factor of wheelbase? Or something else entirely?
It's not a big thing. That is the only section of road I have ever run across that does it over almost 30K miles. Just curious.
Any of you guys run into this or have an explanation?

 

yep, short wheelbase makes a difference

 

I would suggest that it's a result of the wheelbase and the spacing of the "waves" in the pavement. I don't know if it's the correct term but I would suggest it's going into an ocellation (spelling?) tied to the compression and rebound of the suspension and the spacing of the high and low spots of the pavement.
Several years ago I was on a stretch of highway in Iowa in my Tahoe that at 60-65 MPH made it feel like I was riding an enclosed bucking bronco.

Lynn

 

What’s porpoising?
Never heard of that description as a ride symptom. Must be a new buzz word.

 

You know how a Porpoise jumps out of the water nose up then nose down then nose up and nose down and nose up and nose down and nose up and nose down...the tail end doing the same thing in inverse order....
It's a description of a ride symptom been around a long time

 

Does it seem the entire vehicle is rocking / pitching, without much compression/rebound in the suspension?
Or is there a lot motion between the axle and the frame ?

The front springs need to be quite a bit softer than the rears, generally.

Have you checked or estimated what % of the weight is on the front and rear ends ?

 

An oscillation is correct...the car's natural suspension frequency is being excited by the road surface into bouncy bounce.... back and forth like a teeter totter in extreme cases
Dual acting shock absorbers will help dampen the effect...most shock absorbers are single action...dampen only in one direction
Probably driving speed has an affect on it too

 

Good explanation.

 

Yeah, the speed limit on this section of road is only 40 MPH. The porpoising gets bad enough at 40, I can't imagine 65. It's a good thing the section is only about 2 miles long. I think any more than that it would make me sick. I mostly try to avoid that section of road.
When I was pulling the race trailer all over the country there was one place on I69 in Indiana where that combo (crew cab dually with 32-foot gooseneck) would porpoise like crazy. Had to slow down.

 

Several good definitions posted already. New buzz word?, no. Often used in boating and aviation as well. As for the origin of the term, simple no words needed. Just a simple connection of said mammal is all that’s required.

 

My truck does it more then I would like it to, its on a 112" wheel base. I have custom made Eaton Detroit springs, tube shocks, and a 50/50 weight split front to back. Mine gets so bad I have to slow down, it's always on concrete paved, short sections, one after another. Softer springs helped, but didn't cure it. I just figured the leaf springs designed suspension was causing it.

 

Young whippersnapper!

Ben

 

Maybe “Dog Tracking” is going by way of the “Doo Doo Bird” as well?

 

Hobby-horsing is another term I've heard to describe it. Common on short vehicles like old Jeeps. Freeway expansion joints will get it rocking on vehicles with 80"-90" wheelbases.

 

I would be of the opinion that if it is an ocillation it would be dependent on the combination of vehicle speed, "wavelength" of the pavement high and low spots, and the wheelbase of the car. Get out of the "perfect storm" speed (the only variable in a particular car) and the porpoising will probably lessen. My T roadster has a front end shake around 30-ish MPH, but I can drive through that speed range and it goes away.

Lynn

 

My 55 bird does that on one spot in one road I’ve ever found. A knowledgeable friend told me the easiest way to tune around it is the shocks, it’s some kind of oscillation based on springs, wheel base, shocks, and road. My bird has adjustable shocks, but since it’s only one spot I never bothered to try to tune around it. Pretty much the same experience and advice as given above.

 

Never heard that one before, but yeah

 

Remember Hoppity Horse? Here’s a stationary 30’s version.

 

Short wheel base plus weak shocks was the cause of my 64 Chevy pu problem. Got so bad wife said she wouldn’t ride with me anymore. I’ve never told her about the new shocks I put on. Went a long ways to curing it.

 

So smooth ride and no head ache now?

 

I've never seen one quite like that. Neat! But it doesn't quite describe the porpoising motion. My son had one mounted with 4 springs mounted 2 front and 2 rear. That one would porpoise/hobby horse. It is still in the attic of the shop (35 years later). I remember I cut a slot in its forehead and made a piggy bank out of it. It would rattle like hell!

 

Some years back, some of my coool hot rod friends and I re-hashed our old pastime of Control Line Model aircraft building and flying. Our planes were 'C' class, (38"-52" wingspans) so they flew reasonably smooth.
Until my newest one. Porpoising was its habit, no way could I get it to fly in a constant height.
Stunts were great, then it would return to its bad habits.
The type plane? It was a DOLPHIN!

 

Lol, I had a U-Line once that I didn’t check CG on. What a time that was.

 

Ever been in a boat that raises then falls & slaps the water ?

 

When I read the****le, I was thinking WTF? Then I read the 1st post and went OHHHH!

Years ago, the State of WI was hiring independent companies as the lowest bidder to build a new Interstate Highway between Beloit WI and Milwaukee WI (I-43) . When the road was finished and opened up, the section at least 20 or so miles long, closest to Beloit was really bad. The new road about bounced you out of your car.
The highway was poured concrete. At every joint, the road surface was 2" higher then it was in the center of each slab. At the 65 mph posted speed, if the wheel base on your car was short, your car would porpoise over each joint and about make you seasick, but it the wheel base was longer, it would bounce front and rear together over each high or each low point in the cement.

It took about a year before someone at the State determined they could cut/grind the high joints off to level the road surface. At first they thought they could just set up a machine and cruise along slowly and just remove the high spots. They did about a 2 mile stretch before they figured out that wasn't going to work, the machine more or less just followed the contour of the cement and only removed the very highest points. They shaved that 2 mile section a few times taking a deeper cut each time until they came to the conclusion they had to come up with a better plan.

The better plan was to set the machine up and grind about 2" of height off each side of the joint, then move the machine to the next joint and repeat the process. They had to set the machine up, grind 2" of of about 2'-3' of the hump, then tear the set up down and move the machine about 7' and set it up again and grind the hump off, tear it back down, repeat. They ground the 2" "hump" (about a 2' to 3' long grinding process) off the concrete road surface about every 10' for 20 miles, then they changed lanes and do the entire deal again on the other lane of the highway. I-43 is a 4 lane road, The process was done 4 times, for just the worst 20 miles of about a 100 mile long road.

A few years ago IL hired a low bidder to do a section of highway in cement, I think they hired the people from WI. At least IL discovered the problem before the entire section of road was screwed up.

The point I'm making is, its not always the car, and sometimes there is nothing you can do with the car to solve the problem. If you are only getting this feeling from one section of road, its probably not your car's fault.

 

There's a " porpoise section" of road south of kangley Illinois , one along side of lake shelbyville in Illinois & a 20 mile section near Williamsburg , Pennsylvania . My crew dually with an**** heavy 11 foot slide in camper could really get rompin' on those roads a few years ago ...

 

" Go for help Flipper".

 

Pitching occures when the spring rates are missmatched. In its simplest terms the higher the corner weight the stiffer the spring needed. If there is significantly more weight at one end if the car it makes balancing of spring rates more difficult.

A crude test is to disconnect the shocks and let the wheels droop the amount of movement from this position to normal ride hight should be the same front and rear for leaf sprung cars

 

Are you talking about the 20 bypass in Southbend? That is a horrible stretch of road! They even tried shaving it to smooth it out. Didn’t work.

 

PA roads made of concrete are as @gene-koning described. Wife's late model loudly announces each joint. Bet the OP can fix it by going 55 MPH ..