Which rear end (and tires?) for my JW powered 65 Special Wagon?

Re: Rear-ends and . . . other matters (Re: Rear end for 65 Special Wagon?)

LOL!


I figure my camp trailer (I'm sure you've seen the pictures of it) weighs I figure in the neighborhood of 600-800lbs empty. I can lift each side of the trailer off the ground with a lot of effort, so figuring that it's about half the weight or so.

 

Eduard,

I hate to say it, but you are venturing into uncharted waters, so to speak.

I think you may have to experiment with your final drive ratio quite a bit to find an optimum solution for a given cruising speed, and I don't think we can help much because (1) we've not seen an engine like this before and (2) we've not applied the idea of fuel economy with an engine like this to an old wagon like yours.

Welcome to uncharted seas! We all hope to learn from your experience, if that's of any comfort.

The last thing we want to see is excessive throttle opening for maintaining a given speed, but the trade-up is your car's wind resistance as speeds increase. I hate the word "wind", it's just air, but "wind" is fine if you understand my point.

Put more plainly, even though the new engine makes a significant amount of torque at a certain low rpm, that does not necessarily mean your best fuel economy will happen at that point. As rpm rises, so may your new engine's part-throttle volumetric efficiency. You will have to be willing to experiment to find this out for yourself. Then you have to decide at what rpm you would like for a given mph.

I know this is no help to you as far as answering your question at the moment...but I wanted to give you my thoughts.

Devon

 

Rotation radius, trailers, and uncharted waters (Re: Rear end for 65 Special Wagon?)

Howdy Larry, Devon, Silver Buick, and V-8 Buick problem-solvers,

Huh? . . . . That just doesn't seem right to me . . . .

Okay, I'll take your word for it, but you take all the fun out of trying to apply my rusty Physics!! :grin:

Gulp! That's rather what I'm afraid of! :shock:

Since that hardly constitutes a comfort zone . . . I've decided to see if I can find some other examples of Buicks with big-block engines and decent highway gas mileage. I started this thread: Classic Buick's with Overdrive: what's your cruising RPM and highway MPG? I'm hoping the owner of one of Jim's creations will chime in with that relationship. Right now I'm trying to find a posting Jim made about 18 months ago in which reported on one of his creations that was getting 24 (actually 20) MPG on the freeway with a 200-4R. Unfortunately finding things like this - isn't easy!!

However, as often happens, I found something else that bears on this overall question.

While looking for Jim's posting, instead I found a post by Silver Buick in response to an earlier query about towing. In it is a photo of the camping trailer (I think). Is this your critter? (splicing in the same image link from that earlier posting)



If so, do you really believe it weighs less than 1000 pounds?

I didn't remember it to be as tall as it turns out to be. I thought it was more of the traditional "teardrop" style.

If you find that you need to tow a trailer like this in 3rd gear, perhaps that's all I can hope for. Certainly the aerodynamic drag of a travel-trailer is considerable!!

Okay, that's some more food for thought and questions for you'all to ponder. Now back to trying to find Jim's posting!!

Thanks for tuning in!!

Cheers, Edouard :beer

P.S. Well, I shouldn't have given up so quickly. A little more searching and I found Jim's post in the Switching to EFI thread. It reads as follows:

So that's one combination to emulate for getting reasonably good gas mileage.

P.P.S. Wonder how I got managed to "inflate" 20 MPG into 24? :laugh:

 

Edouard,

Using loaded rear tire radius in the formula is absolutely the correct variable to be using. Unloaded tire diameter is irrelevant. The distance from axle centerline to the ground is what should be used in the calculation.

How much loaded radius changes as tire rpm increases is another matter, however...I haven't considered it too much.

Devon

 

would the circumference of the tire be the same loaded or unloaded? in other words won't the tire have to travel the same distance to achieve one full rotation?

 

In that regard you're right, if the distance calculation is all you're interested in, then it's probably moot since the tread length remains constant. I think my habit of always using loaded radius comes from making torque calculations.

Devon

 

Thanks for the input, my understanding was that rpm was a factor in this dilemma and loaded or unloaded wouldn't affect that number. am i thinking correctly?

 

Yes, I stand corrected. If we assume the tread length of the tire is constant even when loaded, then using circumference based on diameter should be fine. Keep in mind that radial tire diameter can increase as much as 7% comparing zero mph to 80 mph.

Fun experiment for someone...with the tire in the air, measure circumference in the center of the tread with a piece of string.

Then, with the tire on the ground, mark the sidewall at 6:00 and also the ground at the 6:00 spot. Roll the car forward one tire revolution and mark the ground again where the tire mark hits 6:00 again.

Compare string length with the distance between the two marks on the ground.

Then report back.

Devon

 

Thanks!! NOW I get it!! (Re: Which rear end for my JW powered 65 Special Wagon?)

Dear Devon, Bob, and V-8 Buick problem-solvers,

Hey, thanks for helping me understand this puzzle about the radius!! :TU:

Like Devon, I was thinking about torque and that of course depends on the lever arm.

However, even if the tire is sagging under the weight of the car, the rubber isn't stretching (since the belt underneath won't stretch.) So the actual perimeter is the same. What is different is the shape of the tire that is now distorted.

Okay, so NOW I understand what Larry is saying!!

Thanks!!

Cheers, Edouard :beer

 

Found this answer to someone else who asked the same questions:

Devon

 

Yup that's my car and $250 POS camp trailer, an aero dynamic brick.. I haven't actually weighed it, but as I said, I can grab the wheel well and lift with all my strength and get the tire a hair off the ground, on each side, and figure that has to be in the neighborhood of 400lbs. The tongue weight unloaded is around 75lbs I figure, and when I load it up with camp gear I try for around 100-125lbs tongue weight. Towing in overdrive with the 3.08's wasn't terrible, but it it did require quite a bit of extra throttle, especially on any kind up uphill grade, and because it's a manual transmission in my car the downshift was up to me. In the 1:1 ratio gear (3rd for you, 4th for me), it pulls like the trailer isn't even there.

 

Lets say a 25 diameter tire has a circumference of 78.5 inches so it would be logical to conclude that for each revolution the same point on the outside of the tire would have to travel 78.5 inches whether the tire is round or distorted.
So you measure the rollout with the tire unloaded and round then take a second measurement with the tire loaded and a flat area on the pavement and discover the rollout is less. Why?
It is because the distance from one side of the flat to the other side of the flat (Being a straight line) is less than the distance between those two points when the tire was round meaning there is indeed compression of the outer tire surface between those two points.
Since that compression occurs over the complete tire circumference as you roll the tire, it effectively decreases the tire circumference by the difference in the radius from the loaded tire versus the unloaded tire.
If you want to look at it from another prospective try velocity of a point on the outside of the tire.
The velocity of that point is a function of the tire radius and the tire RPM. When that point reaches the place where the tire flattens on the road surface, the radius begins to decrease which decreases the velocity of that point and is the slowest at the road surface directly below the center of the axle. Then the surface velocity increases as the point returns to where the tire is round. However the cars speed is only the relationship of the tire to the road surface which is the reduced outer tire velocity due to the reduced radius directly below the axle center.
<o></o>

 

Sold.

Devon

 

with my build i have a 455 / th400 with a 2.75 first gear in the tranny, a 26" tall tire on a 17" rim and a 2.78 open rear diffin my 65 skylark conv. i am not too concerned w/ milage but the engine dynoed at 475 hp and 525 ft lb and this this launches really nice and cruises around 3000 around 70 - 75 mph.

 

I find the same thing! My supercharged Buick park Ave gets 38.5 MPG at 65 MPH, 35 MPG at 75 MPH and 28 MPG at 80 MPH. I prefer to drive at 75 since it still gets good mileage... When I started watching the tach at these speeds I noticed that it is at 2000 RPM at 75 MPH and anything above that the MPG goes way down.

I noticed the same thing with my 3/4 ton 4x4 chevy truck, I can get 18 MPG at 65 MPH as it is at 2000 RPM... Anything above 65 MPH and it goes down to about 14 MPG and that is with a fuel injected 5.7 engine with 2000 miles, new trans, fully rebuilt truck.

So yes, if you can keep the cruise RPM under 2000 you will get great mileage!

I also agree that the trick for towing is to use a rear gear that allows you to tow in 3rd gear without revving to high. With the 3.73 gear in my truck I have to go a little slower than I want to when going up hills so I can keep the RPMs under 3500. My truck needs the gearing though being so heavy, tall, and having about 150 foot pounds less torque than your car.

 

Part of the difference in gas mileage is the fact that the amount of power it takes to maintain a certain speed increases exponentially with the increase in velocity.


Paul

 

Quickie check in and why exponent? (Re: rear end for 65 Special Wagon?)

Howdy Paul and trusty wagon fans,

Today has been really crazy so I haven't been able to give V-8 Buick its proper attention - never mind I think I've got reasonable strategy to figure out which rear end ratio for the car, if I could just sit down and crunch a few numbers . . . . so hopefully later today maybe?

Mostly an excuse to poke in, but I thought the power required to increase speed was only increasing as the square of the velocity? Drag increases as the square of velocity. Increasing kinetic energy goes as the square. I suppose rotational inertia could also be increasing as a square, but that would only make the power increase to the power of 4. I can't think of any way to imagine how mechanical friction responds to increasing speeds. What's increasing to the exponent and what's the base to compute that exponent?

Given the sort of machines you build, I'm sure you are right - but I am curious . . . . perhaps I should be frightened!! :eek2:

Cheers, Edouard :beer

 

This is true. I always get better mileage at 60 MPH in any vehicle I have had compared to when I drive faster. My life is too short to drive that slow so I find the sweet spot and drive as fast as I can without burning too much fuel. In every vehicle I have had so far (32) they all get a good fuel mileage at 2000 or less RPM.

Then there is the fact that Ed's engine will defy the normal rules and get good mileage due to the massive low RPM torque... Compared to a lower powered engine that would have to rev higher to get the job done.

 

Rough graph of power vs speed? (Re: Rear end for 65 Special Wagon?)

Howdy Sean and V-8 Buick armchair engine designers,

As soon as I wrote my last posting on this thread, I had to go water a long hedge and . . . . . . considering how boring that was - I tried to imagine what a formula to compute the power at a given velocity would look like . . . . .

and ya' know . . . .

I think it would be one ugly beast!!

So after that thought, I realized that what I really was after is simply some sort of approximate graph of engine power first speed for a car in the same gear. Does anybody know of such a thing? I'm really just curious to know how much more power does it take roughly to go from 50 to 60, 60 to 70 and so on. I could imagine government agency would have come up with such an illustration to back-up claims that we really should - slow down!!

Thanks everyone for all your help on this great wagon caper and have a great week!!

Cheers, Edouard :beer