Need help from all of you that are smarter than me (gearing question).

Ok, so as the title says I'm looking for help from every man, woman and child on earth (since you are all smarter than me ).
I am thinking about an engine swap but before I take the plunge I want to figure out some numbers, namely torques. How do I figure out how much torque is actually acting on my contact patches? Do I take the torques at the shaft, multiply that by each gear ratio in the trans, then by the rear end? I'm sure it isn't that simple. Also, I'm looking to figure out the torques at the contact patch is possible, not at the axle shafts.
Thanks.

I don't know if it's possible to figure out, exactly, how much torque is acting on your contact patches without actually measuring it on a rolling chassis dyno--even then..

There are the parasitic losses, and they'll vary by temp and load. You can probably get a very close estimate at the axle shafts, but, even then, it'll only be a mathematical estimate. IF you could do this, accurately, you'd be working in F1 or for a top-flight engine team, me thinks.

I mean the coefficients of friction of all the greases, oils, bearing surfaces, parasitic drag involved. I think the best you can do is take the torque output from the engine, subtract 15% or so for parasitic drag, and go from there.

What are you designing where you need to be so precise?

Yep, take the torque and multiply it by the gear ratio. Each ratio in the transmission will be different, so you would have to do each one seperately. The axle ratio stays the same.

No, no need to be that precise. I'm just wondering if a engine in a given application will be far worse or better than the existing engine. It's a real apples to oranges comparison so if I can say, for instance the stock engine is putting XXX amount of torque on the patches or axle shafts and the swapped in one is way less I'd know not to even try it. Or I could compare the difference between the two commonly available gear ratios in this axle.

Hey, is this traction contol? By knowing the torque at the contact patches, and the coefficient of friction for the tires, you're in essence figuring out maximum traction that can be applied before the tire breaks free.

EDIT: Was thinking about this while you posted the above. Guilty of over-thinking this one. Although it IS an interesting question, and was fun pondering an answer.

I'm thinking about putting the grease-burning diesel from my Mercedes in a Wrangler. The HP numbers are below those of a 4 banger, but the torque numbers are equal to the carb'd 4.2L and not too far off the FI 4.0L. I don't want to do all that work just to find it's a dog off road (I know, there is 4-low).

Like they said, a good estimate will be:

Engine tq x gear in trans (different for each gear) x final drive ratio.

So an engine with 100lb of torques x a 3:1 first gear x 3.55:1 final drive = 1065 torques

i was going to help you until i read z31's post above. now i'm not. torque is singular, and the pound is not a unit of measure of torque. nor is torque a measure of torque.

According to Clarkston, torque is plural. See, my lawnmower has one torque, an S2000 has two torques, well 3 torques if it's the 2.2L.

as long as i'm bitter, i will also add that there's no such thing as torque at the contact patch.

AngryCorvair wrote: as long as i'm bitter, i will also add that there's no such thing as torque at the contact patch.

Yeah I know. There's no "twist" at the contact patch. If you don't leave me alone I'll tell Nader what you did to his favorite car...
On that note, I was at the Dream Cruise (for those not in Michigan, you are really missing out) and saw a 'vair with a personalized plate that read "FNADER". Love it!

AngryCorvair wrote: i was going to help you until i read z31's post above. now i'm not. torque is singular, and the pound is not a unit of measure of torque. nor is torque a measure of torque.

So educate us oh great one.

It's really unfortunate that crappiness of other forums is starting to permeate this one.

if I'm wrong, explain it to me, I never said I was Einstein, this is just how I understood multiplication through gears.

Okay, I'm going to do it, and out myself as a pedant and a fool all at the same time.

I'm 49% sure (and hence still plunging doggedly ahead) that everybody here already knows this, but the commonly used U.S. automotive unit for torque is the pound-foot, as in one pound-foot is the twisting force generated by one pound of force applied to the end of a one-foot lever attached to a shaft. As in (torque in pound-feet times rpm) divided by 5252 = horsepower.

There, I've said it. I strongly suspect between Dr Boost's understanding of torque not being applicable to contact patches (and his tongue-in-cheek Clarkson references) and z31maniac's correct calculations (barring units!) that I haven't said anything anybody didn't already know.

But I'm willing to undergo (a little) flaming in the name of using proper units! C'mon, there might be impressionable youths reading this! You don't want them to grow up thinking 'torque' is a unit, do you?

Besides, keeping the pound-feet part in is a handy reminder about how easy the calculation is to get from torque to force at the contact patch, if you like that sort of thing...

Ransom, you are right. But it was so much more fun to ask how many torques a car has and I really thought anyone here would have got it.
And I was pretty sure you take the shaft torque and multiply it by the transmission ratios, then by the axle ratio but I wanted to make sure. I was also wondering if there was a way to compensate for tire size since the donor car is turning 25" tires or so, and the recipient will have 33" tires.

AngryCorvair wrote: as long as i'm bitter, i will also add that there's no such thing as torque at the contact patch.

There is if I'm sitting still turning the steering wheel.

DrBoost wrote: Ransom, you are right. But it was so much more fun to ask how many torques a car has and I really thought anyone here would have got it. And I was pretty sure you take the shaft torque and multiply it by the transmission ratios, then by the axle ratio but I wanted to make sure. I was also wondering if there was a way to compensate for tire size since the donor car is turning 25" tires or so, and the recipient will have 33" tires.

Since a moment (torque) is force x distance, all you need to do is ratio the distance.

So if you have 1000 lbf-ft at the center of the tire, a 24" tire (diameter) will be 1000lbf (1000 lbf-ft/ 1ft) at the tire patch, whereas a 36" tire (dia) will be 667lbf (1000lbf-ft/1.5ft). I'm too lazy to use your actual tires, since simple math can be done without a calculator...

I tend to prefer Nm, N, and kW, since I don't have to worry about lbf and lbm, but I see kg used as a force all the time- no idea how that started, since kg is a mass and not a force....

DrBoost wrote: Ransom, you are right. But it was so much more fun to ask how many torques a car has and I really thought anyone here would have got it.

Well, I apologize for the pedantic outburst. It just feels wrong to me to not use the units, and I think that using incorrect/no units can contribute to the sort of cluelessness you see all over so many lists.

I should have known that on a GRM list it wasn't an issue. Moreover, I never want to be the person who says you can't have a laugh playing with words. Now that's just wrong...

Of course, as alfadriver pointed out, the answer to calculating force at the contact patches across tire sizes is right there in the unit description...

DILYSI Dave wrote:

AngryCorvair wrote: as long as i'm bitter, i will also add that there's no such thing as torque at the contact patch.

There is if I'm sitting still turning the steering wheel.

[Cartman] I hate you guys. [/Cartman]

apologies to z31 and DrBoost:

z31, your calculations are correct. no disrespect to your math brain. but it kills me when people get the units wrong or use nonsensical units like "1065 torques".

DrBoost: i didn't get the joke. i think it's a top gear reference, but i've never seen that show so that one is lost on me.

AngryCorvair wrote: apologies to z31 and DrBoost: z31, your calculations are correct. no disrespect to your math brain. but it kills me when people get the units wrong or use nonsensical units like "1065 torques". DrBoost: i didn't get the joke. i think it's a top gear reference, but i've never seen that show so that one is lost on me.

No apologies needed as far as I'm concerned. But how does a GRM'er such as yourself (with more than 3 posts) not have seen top gear? Actually the first time I heard Clarkston say "this car has 350 torques" I was cracking up. I stil snicker every time I hear it.

Thanks Eric. Now I just have to hunt down the transmission ratios....

AngryCorvair wrote: apologies to z31 and DrBoost: z31, your calculations are correct. no disrespect to your math brain. but it kills me when people get the units wrong or use nonsensical units like "1065 torques". DrBoost: i didn't get the joke. i think it's a top gear reference, but i've never seen that show so that one is lost on me.

Hehe, no harm no foul.

Yup, a Top Gear reference, that always cracks me up for it's ludicrousness.