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REAL crawl ratio?

76zimmer

76zimmer

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After perusing through some meanless banter on another site, crawl ratio comes up too often.
It is described as something like 60:1 or whatever it works out to be from multiplying lowest trans gear X lowest low range gear X axle ratio.

Well what about tire diameter?

if the trans is 2.48 X 4.1 X 4.56 that equals 46.3:1

So everytime your axle moves 1 revolution the engine turns over 46.3 times right?
So thats your crawl ratio right?
Well I disagree:
I think it should be measured in feet per revolution of the tire.

So what if you have a 29" tire. How many feet are you going per revolution?
7.6 feet per revolution of the axle shaft.

So what if you have a 44" tire. How many feet are you going per revolution?
11.5 feet per revolution of the axleshaft.

So a crawl ratio being the same for both axleshafts, would truly have a lower crawl ratio for the 29" tire, than the 44" tire.

So how does that relate to feet traveled per engine revolution?

Any mathemeticians out there?
 
((rpm / (First gear * Transfer case * axle)) * (diameter x PI)) / 12 = feet per minute

So, for 1000 engine RPM with a 29" tire:
((1000 / (2.48 * 4.1 * 4.56)) * (29 * 3.14159)) / 12 = 163.75 feet per minute

And for 1000 engine RPM with a 44" tire:
((1000 / (2.48 * 4.1 * 4.56)) * (44 * 3.14159)) / 12 = 248.47 feet per minute

Actually, once you know the feet per minute for one tire size, you can quickly solve for other tire sizes by just using the ratio of the tire diameters like this:

(New tire / old tire) * old feet per minute = new feet per minute
(44 / 29) * 163.75 = 248.4 feet per minute

I hope that is correct and that it helps...
 
that does make more sense, I don't do math so well.

But overall shouldn't tire size be considered in a "crawl ratio"?
 
From what i've heard and read your crawl ratio should equal the tire roll out. If you run an auto you have to remember to double for the convertor.
 
Well sure if you want to get really technical about it but the thing that I have always thought is you change tire size alot more than gear ratios. even beyond that you would need to measure static loaded radius of the tire to get a true fpm measurement, as the tires wears your crawl ratio will be different, static loaded radius changes with air pressure, air pressure can change with temperature. The tire is a a wild card in the figuring out a crawl ratio and probably the reason not very many consider it.

Some poeple think that there is no way my truck would work with only 4.56 gears but with my other gearing it actually works great. With the auto I never really want for control and I can spin the tires anytime I want too. Sure it might be pepier on the road or the dunes but for me 4.56s work well, I run 44s
 
4X4HIGH said:
If you run an auto you have to remember to double for the convertor.
Click to expand...

I just don't buy this idea. I understand why people say it and believe it to be true, but when the TC is not locked up - I just don't see it as being the same as having an extra 2:1 gear set somewhere in the system.

I don't believe that an automatic would idle over obstacles like a manual of similar gearing. I'm not trying to say one is better than the other - that's not the debate I'm interested in discussing. however, when i drive an automatic (in example, I rented a 2010 dodge half ton this weekend) - even in 4low I have to press the gas pedal to move it over small stones (like on the edge of a lake). In a manual, you put it in low gear and it just pulls you over everything - no throttle required. to me, that's the standard of low gearing - if it can idle over something.

on topic though, whether or not there's a real standard to how to include tire size in crawl ratio, the tire size should at least be stated. Clearly, a truck with a typical 465/205 trans/tcase and 4.56 in the axles is going to crawl much more impressively with 30's than it would with 44's (this obviously negates the value of clearance, though).
 
Static loaded radius has nothing to do with feet per minute calcs. A loaded tire's circumferential shape may be deformed by the loading but it does not get any smaller. Think of it like a tank track.

That being said, the static loaded radius would matter for things like torque calculations since the effective lever arm is shorter.

Tire size should be considered since comparing crawl ratios is meaningless without knowing the tire circumference.

Just because magazines or other people do something a certain way doesn't make their method correct.
 
What about the engine? Why can't a 1000 HP race car pull a 80,000 trailer? A tractor truck with only 250 HP can. Transmission ratio is important but so is torque output low RPM.
 
Yes, the engine should be considered too. It is a system, to look at just one aspect of it never made much sense to me.
 
sloth said:
Static loaded radius has nothing to do with feet per minute calcs. A loaded tire's circumferential shape may be deformed by the loading but it does not get any smaller. Think of it like a tank track.

That being said, the static loaded radius would matter for things like torque calculations since the effective lever arm is shorter.

Tire size should be considered since comparing crawl ratios is meaningless without knowing the tire circumference.

Just because magazines or other people do something a certain way doesn't make their method correct.
Click to expand...

Static loaded radius definitely does effect the feet per minute calculations. You are contradicting yourself by saying it does not, but then saying it effects the torque calculations.

Take a truck with 44" tires fully inflated (lets assume 22" radius) and roll it for one complete revolution of the tire and measure that distance (apprx. 138" per the circumference calcuation). Now pull the valve cores so the truck is basically sitting on the rims, lets say they are 15's. Assuming you have about 2" of tire between the edge of the rim and the ground, the rolling radius is about 9.5". Roll the vehicle forward for one revolution of the tire only equals about 60"
 
I don't believe that an automatic would idle over obstacles like a manual of similar gearing
Click to expand...
That's because at low (idle) RPM, the pump is at relatively low pressure and relatively little fluid (and torque) is being passed through the torque converter - this is by design as it allows you to apply the brakes and park without stalling the engine. How much pump pressure and how much torque transmittal at idle occurs is specific to the individual setup. The pressure on my 700R4 ramps up very quickly once the accelerator comes off its idle position.

What 4x4HIGH is referring to is the torque multiplication that takes place in the torque converter. Passenger automotive converters typically deliver between 1.75-2.5 times the input torque at the output. Crawl ratios are all about final torque multiplication ratios, via gearsets or other means, so you must factor this in to be accurate.

Yes, you should also definitely include the tire size to determine torque delivered to the ground.
 
v30mongo said:
What about the engine? Why can't a 1000 HP race car pull a 80,000 trailer? A tractor truck with only 250 HP can. Transmission ratio is important but so is torque output low RPM.
Click to expand...

you are right that it is important, but not for crawl ratio. obviously, however, a 1lb*ft engine is not going to crawl with the same success as a 300 lb*ft engine. however, i'd argue that a vehicle with a 454 and a vehicle with a 350 with otherwise exactly equal drive trains are going to complete the same obstacles at idle in equal ways if they have equally awesome crawl ratios.
 
longbedder said:
That's because at low (idle) RPM, the pump is at relatively low pressure and relatively little fluid (and torque) is being passed through the torque converter - this is by design as it allows you to apply the brakes and park without stalling the engine. How much pump pressure and how much torque transmittal at idle occurs is specific to the individual setup. The pressure on my 700R4 ramps up very quickly once the accelerator comes off its idle position.

What 4x4HIGH is referring to is the torque multiplication that takes place in the torque converter. Passenger automotive converters typically deliver between 1.75-2.5 times the input torque at the output. Crawl ratios are all about final torque multiplication ratios, via gearsets or other means, so you must factor this in to be accurate.

Yes, you should also definitely include the tire size to determine torque delivered to the ground.
Click to expand...

i understand the principle of it, but i still don't think it has much real-world application.
 
colbystephens said:
i understand the principle of it, but i still don't think it has much real-world application.
Click to expand...
Hmm. I guess I don't understand your understanding of it.

An SM465 has a low-gear ratio of 6.55:1. This is absolute if the clutch is applied and not slipping. Neglecting internal loss (bearing/gear friction) you are multiplying input torque 6.55 times at the output shaft.

A 700R4 has a first gear in the planetary set of 3.06:1. This part is absolute if the trans is in good shape and the clutch packs are not slipping. At part throttle when crawling, the torque is multiplied by about 2x in the converter, so the torque at the output is multiplied around 6 times.

Based on these examples, you get roughly the same amount of torque multiplication for both trannies - you're accomplishing the same essential thing (the SM465 has a slight numerical advantage). It's not like the 700R4 has an "extra" gear, it's just that alot of people when doing desktop math don't figure it correctly in the first place. The difference is that the auto is designed not to start multiplying very much until it comes off idle - maybe think of it as having the clutch feathered at idle to allow the engine to spin while stopped?

Also keep in mind that diesels and gassers behave much differently with a manual, especially at the low end. The torque curve difference as well as being fuel-metered instead of air-metered means that it can be more difficult to stall a diesel engine when crawling.
 
longbedder said:
Hmm. I guess I don't understand your understanding of it.

An SM465 has a low-gear ratio of 6.55:1. This is absolute if the clutch is applied and not slipping. Neglecting internal loss (bearing/gear friction) you are multiplying input torque 6.55 times at the output shaft.

A 700R4 has a first gear in the planetary set of 3.06:1. This part is absolute if the trans is in good shape and the clutch packs are not slipping. At part throttle when crawling, the torque is multiplied by about 2x in the converter, so the torque at the output is multiplied around 6 times.

Based on these examples, you get roughly the same amount of torque multiplication for both trannies - you're accomplishing the same essential thing (the SM465 has a slight numerical advantage). It's not like the 700R4 has an "extra" gear, it's just that alot of people when doing desktop math don't figure it correctly in the first place. The difference is that the auto is designed not to start multiplying very much until it comes off idle - maybe think of it as having the clutch feathered at idle to allow the engine to spin while stopped?

Also keep in mind that diesels and gassers behave much differently with a manual, especially at the low end. The torque curve difference as well as being fuel-metered instead of air-metered means that it can be more difficult to stall a diesel engine when crawling.
Click to expand...
what I'm saying is that I'd bet that a manual trans truck put into it's first gear - 3.(something):1 would crawl better at idle than an auto trans with a 3.06:1 - all other things being equal. thus, i don't think the 2x multiplication for the torque converter has much real-world significance.
 
this is a very interesting topic, i know what u mean by manual vs auto at idle and it depends on idle speed and all too but a manual yes will go faster at idle or more power at idle cause its all there, but saying that i have a 87 k5 with a crate 350 700r4 stock tcase (not sure what) and 4.56 gears and at idle in 4low without hubs locked i can crawl my curb and it even pushed itself and my dead silverado up my drive way at idle, and no lie i have like a 30 degree incline for a drive way lol, so imo everything kinda has a slight depends factor too but
 
I actually agree with you Colby at idle an manual truck will idle over stuff better than an auto, given the same gearing. But if the load becomes too great for the manuals gearing to overcome it stalls the motor. With the auto it just stops. If the gearing is the same on both trucks I guarantee a small blip on the throttle with the auto will get it moving again.

Whether this really enters into your idea that the torque converter really doesn't give a 2 to 1 gearing advantage is somewhat besides the point. The auto truck will crawl easier over stuff if the objects being crawled are enough to over come the manuals gearing and stall the truck.

This is of course assuming the driver of the manual truck cannot operate 3 pedals I know I was fairly good at it when driving a manual and I bet most who wheel them are too.

Back to the discussion of crawl ratio. I think it is expressed in the simplest of terms. To figure a true crawl ratio you would have to enter in a number of variables that change so often it is a moot point. It really could not be expressed in a simple terms.

The terms it is expressed in now are very easy to understand. Its more of a bench racing number than anything. Like I said everyone says oh 4.56 gears you must hate that. But my truck has no problem crawling right along. I do not hate it at all.
 
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