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standard crawl ratio for rocks

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Stephen, Jan 15, 2002.

  1. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    I'm trying to figure something out to get in the ballpark with overall low range for rocks. So here's what I'm thinking:
    I'm at 62:1 with the auto, so figure about 124:1 with the converter. With a 42" tire, circumference is 130" or so. So I go about 1" for every revolution of the motor. I do have a 468 so I have some good torque available but this works for me.
    Previously I was running 55:1 (110:1 with t/c) with a 37" tire, circumference = 116". so the 1" per engine revolution theory holds up for that configuration also (it worked for me too).
    With a manual, I know a buddy (butch) was at 6.55x2x4.10 = 53.7 with 35's and it didn't work so well. So he did the Doubler and went to about 107:1 and with 35's it seems like it worked nicely. So once again we have 110:1 gears with a 110" tire circumference and a v-8.
    So this seems to hold up so far. I know displacement, vehicle weight, personal preference, etc all figure in but this seems good so far. I bet it doesn't hold up real well with small displacement motors and heavy rigs. A really heavy 4cyl toyota might have problems with the same gearing. And a really light rig with major beans could probably stand "shallower" gears.
    What do you think? Where does everyone stand as far as inches traveled per motor revolution? Is 1" of travel per revolution of the motor a good guideline?


    Making the world better, one truck at a time.
    SW-ORD
    <a target="_blank" href=http://www.offroaddesign.com>www.offroaddesign.com</a>
     
  2. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Steve,

    When I read this post originally (in the thread about the 3.6 Gears for a 203) I was already excited about the idea of making it into some kind of worksheet to compare various combinations.

    The part about the gearing and tire size is pretty straightforward....and I practially have that completed already. The NEW part of the equation is the weight.

    The Gears/Reduction Ratios/Tire Sizes will let you calculate: Drive Wheel Torque &amp; Wheel Thrust

    The Weight of the Vehicle will let you get additional factors like: G-Force &amp; Acceleration (ft/sec)

    I think the nature of your question now is to identify the "rule of thumb" value for G-Force or Acceleration....since that will determine the driveability of the vehicle....

    Here are some numbers for the ORD Jimmy that are interesting:

    Assumptions:

    Engine has a peak torque of 500 Lb-ft
    Vehicle Weighs 6600 Lbs
    Transmission is 700R4 w/ standard 3.06 1st Gear
    203/205 Doubler is used in "compound low" for this example
    42" Tires

    The Numbers:

    Total Effective Crawl Ratio: 123.70:1
    Drive Wheel Torque at Engines Torque Peak: 26286 Lb-Ft
    Wheel Thrust (factors in the tire/wheel radius): 15021 Lb-Ft
    G-Force @ Torque Peak: 2.276 g's
    Acceleration: 73.22 ft/sec

    Some of this is kinda bogus because it is assuming that you can instantaneously achieve peak torque on the engine.....the vehicle will already be moving when it hits peak-torque, so the G-Force would probably never really be that high!!!

    I have a feeling that if you get enough data points for comparison....the vehicles who perform well offroad will probably have similar numbers.

    Anyone want to contribute their data to help out? You'll need to provide:

    1. Engines peak torque value (Lb-Ft)
    2. Vehicle Weight
    3. Tire Diameter
    4. Transmission used (TH350/400, 465, 700R4)
    5. Transfer Case used (203, 205, 208)
    6. Doubler used (if any)
    7. Axle Gear Ratios (3.73, 4.10, 4.56, etc) &lt;- forgot this one originally, thanks Rene!

    I would be willing to build a table of examples and provide the data....I'm not sure if this is really going to answer Steve's original question, so we should probably wait for him to respond before everyone starts throwing numbers at me.

    DISCLAIMER: My enthusiasm for this topic may be FAR greater than my actual KNOWLEDGE of these subjects. If I've completely "blown it" in trying to describe a solution here....at least now you'll realize that it wasn't intentional! [​IMG]


    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a><P ID="edit"><FONT class="small">Edited by Greg72 on 01/15/02 06:32 PM.</FONT></P>
     
  3. CK5

    CK5 In my underwear Administrator Premium Member GMOTM Winner Author

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    With my 37's 3.06(700r4)2.42(NP241) and 4.88 diff gears I could stand to go a little lower especially with taller tires. I would say some where in the 80-100 range would be great for a full-size crawler. Let's face it, at near 6000 pounds I don't think an insane crawl ratio of let's say 200.1 would pull us up a large ledge, you would be looking for a little wheel spin to bump your way up. I don't know why you keep calling me "Throttle Fox" [​IMG]

    Steve

    Director of Global Expansion
     
  4. BLUESMAN

    BLUESMAN 1/2 ton status

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    Let's not forget that we are airing down these huge tires,so we will lose alot of tire circumfuse.The 241 case has an 2.72:1 lo gear.Gotta love that Rock-Crawlin'!!!!
     
  5. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Until your rear shaft broke, your transfer case spewed all its fluid on the ground, and you were really screwed then, well, unless you have the SYE.

    I'll supply my info for the the little test.


    1. Engines peak torque value (Lb-Ft) : 330 lb-ft.
    2. Vehicle Weight: 5000 lbs.
    3. Tire Diameter: 35"
    4. Transmission used (TH350/400, 465, 700R4): SM 465
    5. Transfer Case used (203, 205, 208) : 205
    6. Doubler used (if any) : N/A

    4.10 gears if it matters

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Hey Greg, try these ones out.

    1. Engines peak torque value (Lb-Ft) : 375 lb-ft.
    2. Vehicle Weight: 5700 lbs.
    3. Tire Diameter: 36"
    4. Transmission used (TH350/400, 465, 700R4): SM 465
    5. Transfer Case used (203, 205, 208) : 205
    6. Doubler used (if any) : none yet
    7. Gear ratio : 3.73 &lt;&lt;&lt;&lt;kinda hard to do this without gear ratio.[​IMG]

    Rene



    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
    <a target="_blank" href=http://coloradok5.com/gallery/project_T2> tRusty pics...</a>
     
  7. 84silverK5

    84silverK5 Registered Member

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    Damn you guys sound like College math professors or scientists, i wish i could understand what you guys were talking about heh
     
  8. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Tim,

    Here are your specs:

    Crawl Ratio: 52.88
    Drive Wheel TQ: 14832
    Wheel Thrust: 10170
    g-Force: 2.034
    Acceleration: 65.45 ft/sec
    Inches per RPM: 2.079"

    Looking at those numbers, I'd say you have the right to say "lifted, fast &amp; loud" [​IMG]



    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a>
     
  9. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Rene,

    Here are your specs:

    Crawl Ratio: 48.11
    Drive Wheel TQ: 15333
    Wheel Thrust: 10222
    g-Force: 1.793
    Acceleration: 57.70 ft/sec
    Inches per RPM: 2.351"

    Fun stuff......! [​IMG]



    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a>
     
  10. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    OK come on, we all have gears and we all have opinions, let's hear them.
    Figure it like this:
    final drive ratio = trans gear x t-case gearing x diff gears

    distance traveled with each tire revolution = tire diameter x 3.14 (pi x d) (this is tire circumference)

    Distance travelled per revolution of the motor = tire circumference divided by final gearing ratio.

    example: 6.55 transmission, 4:1 transfer case, 4.56 diff gears = 122.5:1 final gear ratio

    37" tire circumference = 37 x 3.14 = 116"

    Distance traveled/revolution = 116/122.5 = .95 inches

    For 33's and 86:1, you would have 1.2" per motor rev, Going to 44's you would have 138"/86=1.6"/rev.
    You have to do the distance divided by ratio.


    I've been on the trail with trucks running the 6.55 x 2 x 4.56 = 60:1 with 35's giving a dist/rev of 1.83" and it's not enough for big rocks. The truck survived but there was some clutch missing.

    What we're really after offroad is 2 things, torque at the contact patch to propel our collective butts up stuff AND doing it at a slow enough speed to be in control. So we really have to figure out the motor's capacity to move our vehicle weight and the operating range of the motor. This is where big motors help by putting out lots of torque down low, compared to a toy 4cyl that has no torque and what it does have is when it's wrapped up. That's why I added that this 1:1 ratio works out pretty well for v-8 trucks.
    Greg, I've been looking for a simple way to add the motor displacement/torque capacity and vehicle weight into this and it's not obvious. I think that the "dist/motor rev" ratio will just have to be adjusted somehow to accomodate different motor's operating range and toque to weight ratio. I don't know that we can distill all this into one number. I have a feeling we might be able to come close, something is telling me we might have all the info we need to really break this down. I'll keep mulling it over.


    Making the world better, one truck at a time.
    SW-ORD
    <a target="_blank" href=http://www.offroaddesign.com>www.offroaddesign.com</a>
     
  11. Butch

    Butch 1/2 ton status

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    I think you have had a little too much coffee today Stephen. J/K I think the 1 to 1 theory may be close to spot on. Now with 38's the 107 seems a little too high and I think going to 513's will err on the side of a little low, but then 2nd will put me in the 68 to 1 range and I believe it will work well for all situations (and who knows what tire may be on this thing in six months anyway). Throw a big block in the mix and 107 with 38's would probably be fine. One thing I do think is true is if you have a manual you need a very tame cam or it will get to lunging on you so that makes torque at really low rpm's a little more difficult where with an auto you can adjust your torque convertor stall speed to better hit your torque sweet spot and that would let you slide on gearing just a bit.

    I thought I was wrong once,
    but I was mistaken
     
  12. CK5

    CK5 In my underwear Administrator Premium Member GMOTM Winner Author

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    <font color=red>Current setup:</font color=red>
    -------------------------------
    3.06(700R)*2.92(NP241)*4.88(diff) = 43.60
    With converter 43.60*2 = 87.2

    37(td)*3.14(pie) = 116 116/87.2 = 1.33 inches

    <font color=red>This summers setup (I hope)</font color=red>
    --------------------------------
    2.48(TH400)*7.2(doubler/203 low set)*4.88(diff) = 87.13
    With converter 87.13*2 = 174.26

    39.5[​IMG](td)*3.14(pie) = 124.03 124.03/174.26 = .711 inches

    Side note: With the Wagoneer 203 low gears I would also have 3.6:1 and that works out to 1.42 inches

    Steve

    Director of Global Expansion
     
  13. Donovan

    Donovan 1/2 ton status

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    Very near future setup. (TH350) 2.52 and the Transfer case 2.72 and 2.72 and the rearend of 3.73 = 69.54 and with the converter it is 139 to 1. Will see how that works. How about 37" tires and a 500cid Caddy engine that should make around 500ft+ torque in a 5500lbs blazer.

    Donovan
    &lt;a target="_blank" href=http://www.rustbucket.rockcrawler.com&gt;www.rustbucket.rockcrawler.com&lt;/a&gt;
    Got Leafs??<P ID="edit"><FONT class="small">Edited by Donovan on 01/15/02 09:39 PM.</FONT></P>
     
  14. eds77k5

    eds77k5 1/2 ton status

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    if you are using tire diameter wouldnt you have to use actual diameter, i.e.- if a 35" tire is actually 34" or 33.5" wouldnt you have to use that figure, or if you air down then the diameter would change and the actual rubber on the ground would be different too or would the numbers just be neglegible

    "growin old is not for sissies"
    77k5 400/350/np203/stock for now
     
  15. Thumper

    Thumper 1/2 ton status

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    Is there somewhere a guy can find out the gear ratios of trannys and t cases?

    Mike

    Never mind... found it [​IMG]
    <font color=blue>Thumper
    85 Fullsize Jimmy
    &lt;img src="http://coloradok5.com/forums/images/icons/cool.gif"&gt;
    <font color=red>Aint Skeered!! </font color=red>


    <P ID="edit"><FONT class="small">Edited by Thumper on 01/15/02 11:41 PM.</FONT></P>
     
  16. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    One thing that might make this easier for everyone.....

    Let's assume that the "1 Inch Per RPM" is a good target to shoot for. (...and it seems like it IS)

    Since the formula is: tire circumference / final gearing ratio = 1.00....just think of your tire circumference as your "crawl ratio target value"!!! [​IMG]

    If you have a 35" tire.....the circumference is 110". Therefore you should have a crawl ratio of 110:1 to meet the "1-inch/RPM" criteria

    Here is a simple table of tire diameters (and therefore CRAWL RATIOS too!):

    32".........100.53
    33".........103.67
    35".........109.96
    36".........113.10
    37".........116.24
    38".........119.38
    39".........122.52
    40".........125.66
    42".........131.95
    44".........138.23

    You can use these to quickly compare your current crawl ratio to your tire size and see how far off you are. Based on the tire sizes that we all like to use, it's no wonder that things like the ORD-Doubler products exist!!! It's practically impossible to achieve these crawl ratios with a stock driveline, unless you are willing to use extremely small tires. (and we aren't willing to do THAT, are we? [​IMG])

    Anyway, I hope this helps get more people involved in this discussion thread. I'm still scratching my head a little bit over this topic, but I think we will get to some interesting answers by pursuing it further.



    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a>
     
  17. shaggyk5

    shaggyk5 1/2 ton status

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    my brain hurts....

    -matt
    <font color=purple> "Stuff's only good for 2 things, degreasing engines and killing brain cells"</font color=purple>
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  18. Thumper

    Thumper 1/2 ton status

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    Sheesh... guess I suck. Mine is like 2.5". I got some work to do before I try to step off the porch!

    Mike


    <font color=blue>Thumper
    85 Fullsize Jimmy
    [​IMG]
    <font color=red>Aint Skeered!! </font color=red>
     
  19. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Considering I haven't driven it with an SM 465/NP 205 combo yet, I hope that's the case.

    Right now it has a 700R4/NP 208 but it's in my garage and won't be driven again with that setup.

    Thanks anyway.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  20. Burt4x4

    Burt4x4 3/4 ton status

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    Gawwwlly Wilber I just can't keep up with thease hear Fellers!!! Man, someone already said it MY BRAIN HURTS!! I'm to old to worry about this stuff! here is my 'stuff'
    350 built for lowend(no idea what kinda HP or Torque I have)
    SM465 w/Centerforce cluch
    NP205 stock
    4.11 rear 4.09front
    35x12.50-15 SSRs with 2000 miles on um
    24 pack O beer
    18oz beefjerky
    and a few other unmentinables[​IMG]
    I figure with this combo I should be crawling at about 11:30PM[​IMG] hehehehe
    Sorry had to brake the school room tension that was building......



    72K5[​IMG]Led Zeppelin[​IMG]Rock ON![​IMG]
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