Let's Nerd Out On Some Calculations

I like to run various calculations that may or may not have much meaning....I am totally fine if it doesn't have any meaning, I enjoy looking at the mathematical relationship and then trying to tie that to something more tangible or feel-able when driving. Much like my idea of start off ratio calculations a couple years back, this is in a similar vein.....been trying to get more out of crawl ratio calculations. The classic CR calc of Trans low gear x TC low range x axle gears is a great start, but I wanted to compare CR across tire size changes as well as highlight engine factors. Basically, the question started out from trying to understand how a tire size change (say from 33 to 35) would affect the 'feel' of the crawl in a rig with no other changes. Obviously, a larger tire acts against the crawl of a rig, so how much increase in CR is needed to offset the increase in tire. These calculations can be done as proportions, but sometimes that is hard to feel, especially when working with very small or very large ends of the measured quantities.

So I got sidetracked down a separate calculation that was more tangible to me.....I wanted to know what the crawl speed of my rig was....that means put the jeep at idle in crawl, and what speed results. For my CJ, with CR of 48, 33s and a v6 that doesn't pull much below 850rpm, the crawl speed is 1.74 mph. Obviously, in real life usage, a lot of additional stuff takes place to go slower, such as clutch usage, hand throttle to hold the low rpm, elastic action of the tires and whatnot. But that is the starting point. For reference, before I put low gears in the D18, the crawl speed was 2.25mph. It seems that for the common ecj5 jeep build of 33s, a CR of 60 is starting to hit the sweet spot (or maybe minimum end) for a hard trail use jeep. For my CJ, if it had a CR of 60, the crawl speed would be 1.39mph. one of these days, I might finally swap out the T14 for something else, and the 4:1 T18 sitting on the floor would yield a CR of 61 in my current CJ....ok....more to come, cause I need to get into tire size, and then get to the real question of engine design and rpm....

 

So the next step is looking at tire size....I initially had 31s on the CJ with 2.46 D18. With the OEM CR of 37, that yielded a crawl speed of 2.12. When I upped the tire size to 33, that speed went even higher to 2.26mph. in either case, I needed to increase the CR and went with low gears in the TC for lots of reasons that don't need to be rehashed now. So, let's say I now want to upsize tires from 33 to 35 - I don't actually have any interest in doing this on my ecj5, but it's a starting point for bigger ideas. As mentioned above, with my current set up if 33s and CR of 48, the crawl speed is 1.74mph. If we jump to 35s, speed increases to 1.84 - in other words, not a big jump. In terms of CR, to keep the same speed as with 33s, the 35s would need an increase of CR to 51.

If we go to CR at 60, as that seems to be the low end of CR for hard trail use rigs in the ecj5 world, with 33s we see a speed of 1.39, with 35s it jumps slightly to 1.48. to keep the same speed as the 33s, the 35s would need a CR of between 63-64. In other words, jumping up in tire size has an effect, but it's not huge. I think the 3rd variable is the often overlooked one....

 

Agreed, but there is yet another variable that has more impact on crawl than just tire size

 

3 things are needed for speed calculations: gearing, tire size and engine rpm

No 35s on the CJ....this is for the theoretical long wheelbase, bigger trail build project....

 

Let's look at engine rpm variable....for all my previous calculations, I used an engine rpm of 850. That's based on my personal CJ....whether the 252 cam, 2g carb, or general use at higher elevation that limits manifold vacuum, it doesn't seem to like running much below that rpm. Actually, I often set the hand throttle just above1000rpm if working a tough obstacle so that I can work the clutch with one foot and the brake with the other foot (and both hands on the wheel of course given my steering). Anyway, I've seen some other OF v6s that seem to pull down to a lower rpm.....on my jeep, with it's current setup (crawl speed 1.74mph), if I could drop the rpm by just 50, to 800, that yields a reduced speed of 1.64. Another way to look at it....with 35s and reduced rpm, the speed would be 1.74. in other words, up sizing the tires from 33 to 35 can be offset by either an increase in CR from 48 to 51, or by a reduction of 50 in rpm. Obviously changing the low end rpm zone to optimize this effect is something that happens during engine selection and build design....but that is kind of the point. Duffer has mentioned a few times the B had a lot of functionality despite a relatively low CR (due mostly to transmission selection), but the sbc383 offset that through low rpm performance. I feel like the standard stock sbc350 in my 58 can idle down really low and still pull....if my CJ could idle down effectively at 650 rpm, I could get the crawl speed down to 1.34, which would be equivalent of a CR of 62. Heck, I wouldn't have to swap to the 4 speed if that was the case!

 

So I guess the question is how low of an rpm can be functionally designed and used? If nothing else, seems like going with one of the classic, longer stroke v8s is a good start, be it sbc305/350/383 (or AMC 304?)...heck, could even throw in a 4.7 stroker! The basic sbc350 in the 58 seems to pull down to 650 with no problem - granted, it's never tried turning bigger tires and has more CR than the CJ. Anyway, this all ties into my theoretical long wheelbase build.....there is the perfect world of 6:1 granny gears, low TC gears and reasonable axle gears that allow for street and trail usage (and no warnOD).....but there is also the reality of what one has, can build, and can afford etc....

in other words, a 4:1 T18, combined with a 4:1 TC and 4.27 gears yields a CR of 68...that was worrying me that it might not be enough crawl for 35s....using a moderate engine rpm of 850, yields crawl speed of 1.30 - that is just a bit better than my CJ with 33s and the target CR of 60...but if the engine is functional at 650 rpm, it would be a crawl speed of 1.0mph! Small downside is the 4.27s with 35s and no OD yields a 60 mph rpm of 2500. Considering this would be with 35s on a rig built more for high end trails, I'd probably be fine with it....but would 3.73 axles work (CR of 60)? That is right at our theoretical minimum for CR, and , that might not be enough. At 850rpm, it would be 1.48 mph, but at 650rpm, it would be 1.12mph.....and the 3.73s would yield better highway cruise....so in this case, I am not worried about having enough CR at 68, and would consider a bit less CR, but would need to match axle ratio to engine design....now to learn about amc304....

 

yes, but that is not really the goal, and while a viable solution, doesn't change the mathematical relationship we are looking for....you wanted a relationship for CR including info on tire size and other factors...the key would be seeing if there is real significance in the crawl speed numbers....if something around a CR of 60 is kind of a 'minimum'....then maybe something like a crawl speed of 1.3mph would be a better representation?

its not so much about under vs oversquare, but rather as a comparison to say the the 283 and 327 which are even more oversquare....

exactly why its useful to have calculations to help guide decisions instead of what I 'felt' the one time i drove a rig with this or that CR in different locations at different times....plus its easy and free and entertaining too.

 

this was the whole point....an engine that is functional at 650 rpm vs 850 rpm has an equal impact on crawl speed as upping the CR from say 48 to 60 in my CJ. or in another example, it allows for use of 3.73 gears on a 35 in tire instead of 4.27 gears.