True crawl ratio

[blowedupmotor87]

Dont sell yourself short, I got an RPI of 1.03 for that setup. 6.55:1 / 4:1 / 4.56 gears. MPH @1100rpm would be 1.01 mph.

interesting. same numbers, but flipped. its late, maybe i screwd something up lol.

I think the low range of a 205 is actually 1.96 though, isn't it? That may be the difference in our numbers. Crazy that they stayed the same but in opposite positions though!

Also, my numbers may be a bit of since I used 6.56 as the first gear for the sm465 though, I believe you are correct in stating it as a 6.55 first gear. At any rate, nice and low and slow!

 

[Hogback Fabrication]

Why use 1100 as in gear idle?

I had my 465/205 setup down to ~400 RPM all the time, there's a difference there between manuals and autos and the RPM they can run.

 

[K5dreamer]

Why use 1100 as in gear idle?

I had my 465/205 setup down to ~400 RPM all the time, there's a difference there between manuals and autos and the RPM they can run.

yeah, i noticed that when Rene stated his idle at 600 rpm, thats the beauty of the formula, just plug your rpm into it, I just arbitrarily picked 1100 out of thin air.

 

[Greg72]

Fun stuff......

Yep, a lot of this has been discussed before but it's worth repeating that a crawl ratio is meaningless without considering the tire size (circumference) that it's being applied to.

Your travel speed across the ground at a given engine RPM is the key.


I think you are onto a cool topic here, but think the use of an "index" number is unnatural as it feels somewhat arbitrary and doesn't really connect to anything in the physical world.

As I think about it, referencing a MPH value doesn't seem right since the numeric values are so small and fractional anyway. An interesting concept might be to express things in terms of "feet traveled per minute"...

This would be interesting because it would isolate a few variables like tire size, idle RPM, etc. and really boil everything down to how fast the vehicle is actually moving across the earth at it's own particular idle speed.

I've got a crawl ratio of 144:1, a tire circumference of 119" and an idle travel speed of 59ft/min. (assuming 850RPM)

It seems that ultimately, throwing numbers around like 59ft/min (or 70ft/min, or whatever) would be a lot more descriptive and useful....as it more clearly expresses something that people can visualize.

Just a few random thoughts while I waste a few minutes at work!

 

[Greg72]

Of course the other "x factor" is the amount of torque being developed at the wheels at idle. This is largely variable depending on the engine type, but I'll bet a lot of the arguements about "good crawl" are between people with VERY different engine types.

A diesel or BBC with loads of torque at idle won't need to blip the throttle to actually move the weight of a truck over an obstacle, whereas a tired old 350SBC would need more throttle to move it's own weight..... at that moment, the crawl speed isn't 59ft/minute (or whatever)....it's at least twice that much (when you double the RPM from 1000 to 2000 to also double your wheelspeed....ALWAYS). So that guy is now trying to crawl at almost 120ft/min. No wonder he is complaining that his crawl ratio is lousy....in his particular situation, he STILL has too much wheelspeed.

 

[K5dreamer]

greg72 - very good points. I really wasnt sure what number format people would be most agreeable to. but a crawl ratio in terms of feet per minute could work.... just gotta move the variables around....

C
-- = Inches per revolution (IPR)
CR

(IPR)(I) = inches per minute (IPM)

IPM
---- = feet per minute (FPM)
12

so...

((C)/(CR))(I)
------------- = FPM
12

 

[K5dreamer]

HA!... just realized my crawl ratio is the same year my truck was made

so using the new FPM measure, BIGGRIZ has....

CR - 19.86:1
C - 103.62 in
(@850rpm) - 370 FPM

although, it would seem to make these numbers less arbitrary, we would need to agree upon some sort of standard measure for "I", unless we want to consider the idle speed of a rig critical to its crawling ability. It would seem that manual transmission rigs would have a distinct advantage here as two otherwise identical setups, one manual and one automatic, would have different idle speeds, and thus feet per minute measures.

hopefully that wont spark a manual vs auto for crawling debate

 

[Greg72]

Ultimately, the truck needs to climb an obstacle....that's the whole purpose of the exercise, so maybe the value for idle speed should be the value that it takes for a given truck to climb a particular grade??

Pick a number, say a 30% grade and put the truck in low range and let it idle.... if it won't move forward give it some gas until it finally does. Then take a look at the RPMs necessary to get it to move and use that value for the calculation.

Maybe?


I'm sure this has been solved before, one of the mechanical engineers needs to chime in and tell us what this topic is actually called in engineering circles. It must already have a name and a cool forumula associated with it.

 

[K5dreamer]

So i guess it boils down to what is easier to visualize, a MPH figure, or a FPM figure? Personally i have a better idea of what 0-10 mph is. My 370 FPM figure had me all confused, i was thinking that was the speed of a paintball, until i realized paintballs are in FPS. i really dont have much of a frame of reference for how fast exactly 370 FPM is.

 

[Greg72]

370FPM is fast.....divide it by 60 and you're traveling 6 feet per second!

That's not exactly "Crawling" by anyones standards.

 

[munepit]

Wow! My brain hurts now!

 

[K5dreamer]

370FPM is fast.....divide it by 60 and you're traveling 6 feet per second!

That's not exactly "Crawling" by anyones standards.

why dont you give me a paper cut and rub salt and lemon juice in it while youre at it.

although it kind of puts things in perspective, im even more impressed that ive never gotten my truck stuck anywhere while wheeling. although there is a particular trail i couldnt get up because of rocks. some gearing would definatly be helpful there.

 

[Greg72]

sorry.....I meant to poke you in the eye, kick you in the nutz and give you a wedgie.

 

[K5dreamer]

oh, and i dropped out of college, but i was an engineering major, many aspects apply to this, but cant say as there is a feild of engineering that focuses specifically on wheeling science.

 

[Hogback Fabrication]

Of course the other "x factor" is the amount of torque being developed at the wheels at idle. This is largely variable depending on the engine type, but I'll bet a lot of the arguements about "good crawl" are between people with VERY different engine types.

A diesel or BBC with loads of torque at idle won't need to blip the throttle to actually move the weight of a truck over an obstacle, whereas a tired old 350SBC would need more throttle to move it's own weight..... at that moment, the crawl speed isn't 59ft/minute (or whatever)....it's at least twice that much (when you double the RPM from 1000 to 2000 to also double your wheelspeed....ALWAYS). So that guy is now trying to crawl at almost 120ft/min. No wonder he is complaining that his crawl ratio is lousy....in his particular situation, he STILL has too much wheelspeed.

I can't say that I've ever felt the need for more torque than my old 200K stock 350 had, this was with a 465/205. To be honest I've never really seen an instance where "not enough torque" was an issue rock crawling with any V8.

Also, with an auto, doubling RPM won't double wheel speed. Because they slip you could not be moving at both 1000 and 2000 RPM, or you could be moving 1 mph at 1000 RPM and 1.2 mph at 2000 RPM, autos aren't linear like manuals are.


In my opinion, low gears are 95% for control and 5% for more torque multiplication, with a V8 anyway.

 

[K5dreamer]

sorry.....I meant to poke you in the eye, kick you in the nutz and give you a wedgie.

its in the sig...

 

[Greg72]

Chris,

Good point about the converter....that is clearly a consideration.

I might not have described it well, but my feeling is that there could be situations where a heavy truck with a weak engine might be trying to make a climb (at idle) and there simply isn't enough power at the tire to make anything happen. The engine is just spinning and pouring heat into the torque converter but there is no forward motion.

My perception is that an engine with more torque in that same situation (again at idle RPMs) could actually produce forward motion...but maybe that's not really true?

 

[Hogback Fabrication]

Chris,

Good point about the converter....that is clearly a consideration.

I might not have described it well, but my feeling is that there could be situations where a heavy truck with a weak engine might be trying to make a climb (at idle) and there simply isn't enough power at the tire to make anything happen. The engine is just spinning and pouring heat into the torque converter but there is no forward motion.

My perception is that an engine with more torque in that same situation (again at idle RPMs) could actually produce forward motion...but maybe that's not really true?

The big problem I see with undergeared autos is they get up to an obstacle and the truck stops, so you hit the gas a little more, a little more (nothing happening) until all of a sudden the tires spin and things hop around, no control. You lose the line, you don't have traction when the tires are spinning, can't react quick enough to where the truck wants to go.

With a manual, you can slip the clutch to help out gears that aren't deep enough (not good for the clutch obviously). Basically with a manual you go or the motor dies. If it's not geared deep enough, you have the same control problems.

As far as motor goes, IMO that just makes it easier to not kill the motor with a manual or easier to spin the tires with an auto, nothing but gearing will give you that control with either.

 

[blazinzuk]

The other monkey wrench you can throw in here with an auto at least, is when I am crawling I usually use the brake alot. I bring the rpms up a bit with the throttle and leave it there and use the brake to regulate my speed.

I have a good freind with a 465 in his truck and he actually uses his brake alot too. Hand throttle and he will use the brake and slipping of the clutch to control his ground speed while keeping the engine at a higher rpm.

Although I do idle going across mild rocks but if I get into a technical section my brake lights will be on alot

 

[Hogback Fabrication]

I hit the brake and gas at the same time all the time when I'm wheeling. It's a good way to keep control of things, the power from the motor king of "loads" everything while holding the brake keeps your speed down (to make up for the lower gearing that you don't have ). Good speed control that keeps you from killing the motor (with a manual).