Rolling resistance and driveline efficiency?

[asp]

i'm making another post for the purpose of making a post to up my count (like travis has been) and to get this thread locked.

 

[5spd_xj]

sounds good to me. no need for people to be talking about this junk anyhow.

PURPLE xj. although, i'm not gonna lie, they don't look half bad.




...not really. i don't like em. but i'm just trying to be nice here.

 

[igotanxj]

mechanical engineering is so much more than cars, but cars happen to incorporate all aspects of mechanical engineering. i've learned metallurgy, thermodynamics, manufacturing processes, automatic controls and system dynamics, component design, assembly desgin... i left stuff out from that list too... and we get machine shop time for a few of the classes. it's pretty cool, but it's a lot of work. what school are you going te be doing this at?

I haven't decided yet. Im finishing my 2 year at JC and I'd like to transfer into Carnegie, but that might be tough so maybe U of Pittsburgh. I'm an auto tech right now, so i'm obviously very mechanicly inclined and it seems very interesting.

 

[asp]

yeah there's a lot more to it than cars... like right now i work at a place that designs and manufactures periscopes and other neat toys for the navy... there's really a lot of material that you have to learn to be a good ME... lotta cool jobs out there that pay pretty well too though.

 

[Ty2003K]

yeah better materials and tighter selection tolerances will make a bearing smoother and all that jazz, but all the bearings like unit bearings or carrier bearings or stuff like that isn't really available in "smooth rotating" options. if anything better fluids (gear lube, motor oil, etc) will help but even that won't do much. the thing is that the xj produces too much power for low speed operation. because it's not using all of the power it produces, it becomes inefficient...

moral of the story:
if you want good gas mileage go buy a honda.

How exactly do better materials and tighter selection tolerances make a bearing smoother? And better lubrications will do nothing for your gas mileage.

 

[the_weirdo]

How exactly do better materials and tighter selection tolerances make a bearing smoother? And better lubrications will do nothing for your gas mileage.

Tighter tolerances = less slop = less friction. The downside is that tighter tolerances cost more to produce.

Better lubricant will help your gas mileage. Try water in your motor, then try oil - which is better for mileage? The problem is that oils are very good and tend to be within a pretty tight range of each other in terms of friction. Some are better than others, but it is not a difference on the order of magnitude (10x).

 

[asp]

How exactly do better materials and tighter selection tolerances make a bearing smoother? And better lubrications will do nothing for your gas mileage.

a bearings smoothness directly relates to how close in size the bearings are (be it needle, ball... whatever). it's only cost efficient to manufacture bearings accurate to .0005" or so (not exactly sure what it is, but somewhere around there) but there can be large variations in that. bearings are selected for their closeness in size with respect to the others in the unit for the one particular bearing assembly (for really good bearings). obviously a bearing or a few bearings in a cartridge that were not the same size (+/- .0002ish) would not feel smooth.

in practice i agree that better lubes do nothing for gas mileage, but in principle, better lubes decrease friction and friction decreases gas mileage. the thig is that the friction recovered by using better lubes is negligible compared to the rest of the system. it's all relative.

i was on the super mileage vehicle team at my school for a year and now i'm the teaching assistant for it... i kinda have some experience here....

damnit i was hoping to get this thread locked. here i am posting tech again...

 

[the_weirdo]

ASP, good luck with your ME degree. I got mine 7 years ago and love my career. Understanding how things works and figuring out how things work is awesome.

 

[seanyb505]

I'm not looking to get 30, or even 20 MPG!

So some people with XJs ferquently see 20mpg. Adjust your driving habits accordingly, probly put a cat back on, and call it a day.

 

[5spd_xj]

damnit i was hoping to get this thread locked. here i am posting tech again...

here, lemme help...pointless post... :anon:

 

[asp]

ASP, good luck with your ME degree. I got mine 7 years ago and love my career. Understanding how things works and figuring out how things work is awesome.

i absolutely agree, and thanks! :cheers:

 

[seanyb505]

will you not try a vacation in switzerland this year?

 

[seanyb505]

see the loveli lakes

 

[codyyy]

When I had my stock tires on I could see average city+hwy at about 19 miles per gallon per what the gauge says. Now I'm getting about 15 usually, it's dropped to around 18 average because of my 32s and no regearing.
There is a lot of friction in my Jeep compared to my mom's KJ, I can actually about slow down rolling down a hill of about 7% grade if it is in drive, both with stock tires and 32s. My mom's KJ picks up speed down the same hill, I'm guessing because hers is 2wd and newer, as well as probably lighter, more technology, etc.

 

[seanyb505]

the wunderful telefone system

 

[asp]

So some people with XJs ferquently see 20mpg. Adjust your driving habits accordingly, probly put a cat back on, and call it a day.

i'm on about 4.5" and 31s with 3.07s in my 98 with an ax15 (don't make fun of the little tires, i'm waiting to get my rear end geared to put the 33s on) and i currently don't have a front driveshaft in but for speeds around 40-60 mph i get 19-20 when driving conservatively. it's really not impossible...

i got 24 mpg on an all highway trip once when i was stock... something like 360+ miles on not quite a full tank...

 

[seanyb505]

Yeah thats what I was saying. Before I regeared I could get 20 easy in the city. But if hes not even trying to get that number, there are no "technologically advanced drivetrain mods" nessecary.


Mani fury interesting animals

 

[5spd_xj]

In Big Jeep, forget it, I don't want to know... But with the little guy, like others here it's not uncommon for me to see 20+ mpg...

 

[5spd_xj]

Mani fury interesting animals

yes indeed, yes indeed. :dunno:

 

[seanyb505]

Monty phython and the holy grail intro credits. I figured it would help the content of the thread.