Crawl ratio

[UKTJ]

In the US many Jeepers get hung up about the crawl ratio of their Jeep, but it is not really something I see mentioned much in the UK.  Do people care about crawl ratio?  Indeed do people even know what the crawl ratio of their Jeep is?  Jeep seems to think it is at the very least worthy of mention, as I see they quote it on the specs for MY23 JLs (the Rubicon's is far better than other models at c.77 vs c.44).

 

My current set up has a dismal crawl ratio of c.33, meaning with 33" tyres at 1500rpm I am running at nearly 4.4mph.  A stock JL Rubicon with a crawl ratio of c.77 and 32" tyres would be doing less than 1.6mph at the same revs.  This may explain why I end up being rather 'enthusiastic' off road, as I have little alternative, and why I am constantly catching newer Jeeps up and having to brake when on lanes.  The problem is going deeper with axle gearing only helps a little, to really improve the crawl ratio means looking at the transfer case.

 

[Caroline12]

Never really thought about that before, never had to factor that it, but I completely get your point. 4.4mph does sound rather fast for a slow crawl over rough terrain. 
 

I’m sure Vince, Phil and others will probably explain more with options.

[UKTJ]

1 hour ago, Caroline12 said:

Never really thought about that before, never had to factor that it, but I completely get your point. 4.4mph does sound rather fast for a slow crawl over rough terrain. 
 

I’m sure Vince, Phil and others will probably explain more with options.

I get the options, it is more how crucial it is to get the ratio up for UK conditions.

 

The maths is very simple 1st gear ratio x transfer case ratio x axle gear ratio = crawl ratio.  So for my regear set up it would be 4.04 x 2.72 x 4.56 = 50.1.  Even if I geared as deep as possible for a Dana 30 (5.13 gears) the crawl ratio would only go up to 56.4, and the Jeep would be close to undriveable on road.  However, stay at 4.56 but swap my 231 transfer case for a Rubicon 241 and the crawl ratio jumps up to 73.6.  Put an aftermarket transfer case in and you can be way over 100.

[digger]

I've often thought how good it would be if we could just change a cog and maybe alter chain length in order to reduce the lower ratio overall ie the crawl ratio. I've not done much research but it is something I would consider because to me it solves all problems especially the enthusiastic downhill performance as UKTJ suggests. When on 31s mine was acceptable but on 33s it is quite fast now which is concerning especially when there are rocks sticking up. It also means that one is able to retain mpg and reduce noise in high range whereas going lower overall does not.

In racing in the past, myself and others clubbed together to get gears made. I seem to recall it was a minimum of six at a time but it worked out surprisingly cheap for each of us. I think it was about £100 a cog so maybe £200+ today. If others are interested   then I will try and find out if the company still operates(motor sport is a fickle game!)

Edited September 7 by digger

[UKTJ]

1 hour ago, digger said:

I've often thought how good it would be if we could just change a cog and maybe alter chain length in order to reduce the lower ratio overall ie the crawl ratio. I've not done much research but it is something I would consider because to me it solves all problems especially the enthusiastic downhill performance as UKTJ suggests. When on 31s mine was acceptable but on 33s it is quite fast now which is concerning especially when there are rocks sticking up. It also means that one is able to retain mpg and reduce noise in high range whereas going lower overall does not.

In racing in the past, myself and others clubbed together to get gears made. I seem to recall it was a minimum of six at a time but it worked out surprisingly cheap for each of us. I think it was about £100 a cog so maybe £200+ today. If others are interested   then I will try and find out if the company still operates(motor sport is a fickle game!)

I’m not sure I 100% understand how that would work.  If I understand correctly, are you suggesting reconfiguring a 2.31 transfer case to increase the ratio from 2.72:1 to something more like the 4:1 in a 241 case.  But however it works I’d certainly interested.

 

The approaches I have seen online are to replace the original transfer case with a 241 case from a TJ/LJ Rubicon (there were none over here) or with something like an Atlas (which is many thousands of pounds).  Or, alternatively to add a “doubler”, it can be done reasonably cost effectively with an adaptor kit and means running two transfer cases in series.  This allows you to have the option of ‘low low’, so with two 2.72:1 cases you would achieve 7.4:1 in ‘low low’.  As far as I can tell it also gives the option of having 2wd low, with the original transfer case in 2h and the doubler in low.  The problem with this in a YJ or TJ is the length, we use SYEs to shorten the case, lengthening it is a bit of a problem.

[digger]

That all seems a bit complex to me , downright hard work and the cost of another box. For my YJ , I don't think it would need that big a ratio change. I have not investigated the ratios in the different boxes or whatever. My experience with gearbox ratio changing is that often just one tooth was enough! Rear chain drive ratio changing  was similar.

If you remove the rear case of the transfer box you will see that the drive is by multi link chain via 2 cogs. Bear in mind I have not studied this idea ! Yes , my suggestion is to remove the two cogs and chain (easy) and simply add or subtract a tooth from one. Which one(or both) , exactly,  would be decided by the resulting gear ratio change and the practicality re size change etc within the space parameters of the casing. I wouldn't have thought changing the chain length would be an issue . It may be possible to DIY but a company like 'Reynold' work miracles.

 

I came up with the idea when I fitted my SYE. I fitted the SYE in the car (lot of nonsense when the professionals say you have to remove the box, just work creation!) I just thoroughly cleaned everything before I  started and 'Bobs your Uncle''.  It looked to me as if there would be no problem with a 'regear' of the drive. There may of course be hidden issues of which I am not aware ( Hi Vince!)  Its now about 15years ago that I last had gears made. However I would have thought that its even cheaper and easier now with computer technology. They probably just scan it, press a button and change the number of teeth!  I can soon find that out.

Edited September 7 by digger

[UKTJ]

It appears there used to be a commercially available kit to regear the NP231 to 4:1, the TeraLow I believe.  So it would appear it is completely possible.

 

Using the calculator on Crawlpedia I have plugged in a few alternatives:

1. My current set up with 3.07 gears and 33” tyres gives me 1513rpm in 1st in 4l at 4.4mph and 1463rpm in 5th in 2l at 60mph (you can see the on road need for a regear is pretty clear).

2. By regearing the axle to 4.56 I should get 1532rpm in 1st in 4l at 3.0mph and 2173rpm in 5th in 2l at 60mph

3. Going to 4.88 results in 1530rpm in 1st in 4l at 2.8mph and 2325rpm in 5th in 2l  at 60mph

4. With 4.56 but going up to 35” tyres I get 1541 in 1st in 4l at 3.2mph and 2049 in 5th in 2l at 60mph

5. with 4.88 and 35s it would be 1546rpm at 3mph in 1st in 4l and 2192rpm in 5th in 2l at 60mph

 

Taking scenarios 3 and 5 the effect of regearing the transfer case to 3.5:1 would be to lower the speed in 1st in 4l to 2.3mph with 1538rpm.

 

From my reading over the last hour or so, the key question appears to be the trade off between that lower crawl speed and the loss of wheel speed when getting out of mud.

 

EDIT: @digger I think from what I can see online you will have the AX15 5 speed transmission.  The ratios are a touch different to mine, so with 33s and 4.1 gearing I think you will be at about 3.5mph at 1500rpm.

 

 

 

 

 

 

 

Edited September 7 by UKTJ

[TimC]

Going back to your first post and question (and please bear in mind I'm no expert, just thinking out loud ) why is crawl ratio a bigger topic in the US and why is it not more widely discussed in the UK? I think it's likely to be because they actually do a lot of rock crawling in the US where there is virtually none over here. For Rock crawling traction is not so much of an issue, particularly when massively aired down. You need articulation, clearance and torque. What you don't want is a lot of speed because when you hit rocks at speed you break your Jeep, so high reduction ratio's in the transfer case is what you are looking for in a crawler. In places where you don't have Utah and Colorado relatively nearby the off road obstacles you may encounter will more likely be Mud (UK), Sand (Dubai) or to a lesser extent snow. For Mud and Sand traction is poor unlike rock , if you try crawling in mud and sand you will likely just dig in and bog down but mud and sand is soft ( unlike rock) so momentum is useful to get you through therefore you have less need of a really good crawl ratios. 

 

I hate to say it, because I already did on your previous topic about re-gearing,  but the logical option would be, again, to just buy a JK Rubicon because it already has all the things you want. It has 4.10 diff gearing, it has 4:1 reduction transfer case, it has D44s, it has lockers in both axles, everything you are looking at doing to your TJ and spending huge amounts of time and effort on is right there factory fitted in a JKR.

 

Still, as we know Jeep ownership is anything but logical  and as we've said it's a matter of personal choice. 

[digger]

Yep , UKTJ, I have AX15. That's probably about right. Without going out and checking I think I  am  below 2000 rpm in 5th at 60mph. With 33s she still pulls well throughout the gears although not quite as good as on 31s of course. However my MPG has improved again with the 33s.  I should say that I mainly drive at 50/55 ish because I avoid Motorways etc whenever possible. My gearing seems gr8 to me now except that the low 1st is a bit high for really steep hills. It was fine on 31s.  I haven't had the 33s  long enough to give accurate mpg but my guess is that it is around 25 average based upon my experience across France and last weekend. It may be that my modified throttle body is contributing to my mpg as well but I can't be bothered to swap it on and off to find out but everyone seems surprised at my mpg. My 1st Jeep was a brand new XJ 4.0L which was monitored for mpg by my company. This came out at 23 over 2 years which included a lot of motorways. I suspect that your mpg is down because you are forced to give it beans due to the high gearing. On 31s before my regear i rarely used 5th. Even now on 33s if the road is slow or hilly I stay in 4th. I just listen to how the motor sounds and if its labouring at all I change down.

Perhaps one could change the 1st ratio rather than the transfer drive but that is more effort than changing the chain drive

I personally wouldn't worry about wheels speed for getting out of mud. You can always change up! I generally off road stay in low box and use 2nd and 3rd any way ,although I'm manual .

Edited September 7 by digger

[TimC]

Oh, and I know what it's like not matching the speed of the rest of the group laning. If I'm laning in my CJ I'm constantly on the brake as I have no low range at all just a 3 speed auto box ( I did get a low range reduction unit for it some years ago but somehow haven't got round to fitting it yet ) and a rather enthusiastic carburetta means even on tick over I'm doing 7 or 8 miles per hour.

[digger]

Hi Tim, now this I understand. Sounds like your carb needs looking at. Wanna chat?

[UKTJ]

4 hours ago, TimC said:

Going back to your first post and question (and please bear in mind I'm no expert, just thinking out loud ) why is crawl ratio a bigger topic in the US and why is it not more widely discussed in the UK? I think it's likely to be because they actually do a lot of rock crawling in the US where there is virtually none over here. For Rock crawling traction is not so much of an issue, particularly when massively aired down. You need articulation, clearance and torque. What you don't want is a lot of speed because when you hit rocks at speed you break your Jeep, so high reduction ratio's in the transfer case is what you are looking for in a crawler. In places where you don't have Utah and Colorado relatively nearby the off road obstacles you may encounter will more likely be Mud (UK), Sand (Dubai) or to a lesser extent snow. For Mud and Sand traction is poor unlike rock , if you try crawling in mud and sand you will likely just dig in and bog down but mud and sand is soft ( unlike rock) so momentum is useful to get you through therefore you have less need of a really good crawl ratios. 

I completely agree with all you say, nothing like Moab around here.  At Walters at the weekend there were areas where there were climbs / decents that were rocky (smaller rocks, but some to big to easily get over) with gravel / shale between.  That sort of terrain is something I have only really come across in Wales (but it might be out there in England and I just haven't found it).  It is these type of slopes that made me start wondering about my crawl ratio.  But as you say mud is more common, hence my comment about not getting a crawl ratio so low it means lossing too much wheel speed.

 

5 hours ago, TimC said:

I hate to say it, because I already did on your previous topic about re-gearing,  but the logical option would be, again, to just buy a JK Rubicon because it already has all the things you want. It has 4.10 diff gearing, it has 4:1 reduction transfer case, it has D44s, it has lockers in both axles, everything you are looking at doing to your TJ and spending huge amounts of time and effort on is right there factory fitted in a JKR.

 

Still, as we know Jeep ownership is anything but logical  and as we've said it's a matter of personal choice. 

I agree the JK Rubicon is well specified.  I just had a quick look on Autotrader and the cheapest RHD 3.6 JK Rubicon is a shade under £30k for a 2016.  There is actually a 2019 JL Rubicon for only a few thousand more.  Even if I am super optimistic about what I would get for my TJ there would still be a chunky amount of extra cash needed.  But then I would still need to spend more money, suspension lift, bigger tyres, replace the stock front bumper (sorry, but I really dislike the 'tea tray' look) and get a winch - maybe another £4k.  I reckon that would total at least £25k in addition to selling my TJ.  For that I could buy brand new front and rear axles (stronger than the stock JKs 44s) with appropriate gears and switchable lockers, add a cab operated SwayLOK and give it highline fenders.

[TimC]

Hmmmm, got a point there Terry, I had no idea second hand JK Rubicons were so expensive! My JKUR 3.6 is a 2014 and I paid under 30K for it new. Strewth! Must be the scarcity and the desirability of the V6 that makes it hold its value.. I know that by the last of the JKs in 2018 prices new had reached about £40k, I think the Recon was £42k, but for a 2016 at around £35k or so that’s hardly depreciated at all. How is anybody supposed to get into Jeeping these days at those kind of prices!

[UKTJ]

1 hour ago, TimC said:

Hmmmm, got a point there Terry, I had no idea second hand JK Rubicons were so expensive! My JKUR 3.6 is a 2014 and I paid under 30K for it new. Strewth! Must be the scarcity and the desirability of the V6 that makes it hold its value.. I know that by the last of the JKs in 2018 prices new had reached about £40k, I think the Recon was £42k, but for a 2016 at around £35k or so that’s hardly depreciated at all.

Those are the short wheelbase of course.  There is a decent looking JKUR 2016 with an AEV 2.5” lift and the front bumper replaced.  3.6 with only 31k miles.  Price, as good as £40k.

 

Alternatively a Stock looking 2017 swb Recon with only 24k miles could be had for £35,600.  If they were £42k new that would have been a very sound purchase with only about £6.5k depreciation over six years.

 

1 hour ago, TimC said:

How is anybody supposed to get into Jeeping these days at those kind of prices!

If you want to use it as a ‘fun’ vehicle and not a dual purpose main vehicle then for most people the only option will be something older like a TJ, YJ or XJ.  Or if you are happy with IFS then KJs can be had at very reasonable prices.

[TimC]

Going off topic here but there is a '98 XJ one owner 18K miles on ebay asking £23k! Blimey! YJ's are pretty scarce, KJ is a a great value for money entry level option though I do agree. If you want a Jeep as a fun vehicle and a daily driver I still think the JKU is hard to beat. I ran mine as fun and work vehicle for 5 years and about £75k miles, I can't believe the second hand prices though.  

[UKTJ]

1 hour ago, TimC said:

Going off topic here but there is a '98 XJ one owner 18K miles on ebay asking £23k! Blimey! YJ's are pretty scarce, KJ is a a great value for money entry level option though I do agree. If you want a Jeep as a fun vehicle and a daily driver I still think the JKU is hard to beat. I ran mine as fun and work vehicle for 5 years and about £75k miles, I can't believe the second hand prices though.  

All about personal preference, circumstances, etc.  If I could have only one vehicle for on and off road a JKU or a JLU would be a very good option.  But as even the second hand cost is so high that does present other options, as long as you have parking for more than one vehicle.  For me a dedicated on road vehicle and a separate fun / off roader is a great combination.  As it happens we are very lucky and we have my wife’s Golf and my L200 pickup as well as the TJ.  But then the cost of all three is probably no more than a three year old JLUR!

[Stew the Jeep]

Bring back the rocks at Tong!

[V]

On 06/09/2023 at 16:02, UKTJ said:

In the US many Jeepers get hung up about the crawl ratio of their Jeep, but it is not really something I see mentioned much in the UK.  Do people care about crawl ratio?  Indeed do people even know what the crawl ratio of their Jeep is?  Jeep seems to think it is at the very least worthy of mention, as I see they quote it on the specs for MY23 JLs (the Rubicon's is far better than other models at c.77 vs c.44).

The crawl ratio is useful but not as much as knowing what R&P ratio is installed. Tyre size prevents crawl ratio from being a useful comparative unless the tyre diameters are the same. If the R&P is selected to offset the effect on gearing from running larger diameter tyres, after that the transfer case reduction (2.72:1 or 4:1 in most Jeeps) becomes important.

 

The difference in behaviour between a manual and automatic transmission, petrol and diesel engines will make neither perfect for all situations. A hill descent in a diesel manual 2.72 may need the driver to accelerate a little to stop tyre slip whereas a petrol auto with a 4:1 will just burble down hill without tyre slip. Having both 2.72:1 and 4:1 in the same transfer case and the ability to compound them to get 10.88:1 gives a much wider range of ratios available to the driver to choose for the terrain conditions.

 

On 06/09/2023 at 19:38, UKTJ said:

I get the options, it is more how crucial it is to get the ratio up for UK conditions.

An extreme under drive would get used a lot rock crawling, but it wouldn't be redundant in the UK. When driving off road and the available grip of the tyres is overcome by the slowest speed that can be driven, that's when an extreme underdrive becomes useful.

 

E.g. You are trying to escape from deeply rutted ground where the surface is firm clay, the tyres gain some grip climbing the sides of the rut but persistently fall back into the rut with forward motion. An underdrive will cut your forward motion down to 1/3rd or 1/4 of the speed. This may be enough for the tyres to gain sufficient traction to pull themselves out of the rut.

 

E.g. You are trying to recover a stuck vehicle attached to yours with a tow rope but your tyres are losing grip and neither vehicle is moving forward. This is another time when reducing the wheel speed and multiplying the torque available could make a difference.

 

In both examples, nothing is guaranteed but hopefully it shows why the UK doesn't necessarily mean an underdrive is pointless.

 

On 06/09/2023 at 19:38, UKTJ said:

Even if I geared as deep as possible for a Dana 30 (5.13 gears) the crawl ratio would only go up to 56.4, and the Jeep would be close to undriveable on road.

If your Jeep is primarily a vehicle to be used on public roads, always select your R&P ratio for optimum performance and fuel economy for road use. Unless you are running 1 ton axles in a Jeep with 200 bhp at the engine going over 4.88:1 significantly reduces your pinion strength. I don't have the facts to hand but I would not be surprised if a Dana 44 with 5.38 gears on 37" or larger tyres would be weaker than a Dana 30 with 4.56 gears on 33" tyres. By weaker I really mean mechanical longevity during it's intended use.

 

On 07/09/2023 at 11:58, UKTJ said:

The approaches I have seen online are to replace the original transfer case with a 241 case from a TJ/LJ Rubicon (there were none over here) or with something like an Atlas (which is many thousands of pounds).  Or, alternatively to add a “doubler”, it can be done reasonably cost effectively with an adaptor kit and means running two transfer cases in series.  This allows you to have the option of ‘low low’, so with two 2.72:1 cases you would achieve 7.4:1 in ‘low low’.  As far as I can tell it also gives the option of having 2wd low, with the original transfer case in 2h and the doubler in low.  The problem with this in a YJ or TJ is the length, we use SYEs to shorten the case, lengthening it is a bit of a problem.

I have wanted to do this for years. I recently revisited the 231 doubler mod but I eventually rejected it in favour of an 8-speed auto with a 4:1 transfer case. One 2 minute drive of Ignacio's Rubicon convinced me this was the right choice for a conversion.

 

A doubler would be very difficult to do on a 4.0L TJ but may be possible on a 2.5L with the engine moved further forward. The doubler adds around 9" to the drivetrain length.

 

On 07/09/2023 at 13:01, digger said:

If you remove the rear case of the transfer box you will see that the drive is by multi link chain via 2 cogs. Bear in mind I have not studied this idea ! Yes , my suggestion is to remove the two cogs and chain (easy) and simply add or subtract a tooth from one. Which one(or both) , exactly,  would be decided by the resulting gear ratio change and the practicality re size change etc within the space parameters of the casing. I wouldn't have thought changing the chain length would be an issue . It may be possible to DIY but a company like 'Reynold' work miracles.

The chain's function is to transmit power to the front axle with the front output shaft rotating at the same speed as the rear output. Changing the tooth count on one would cause transmission wind-up. The low ratio gear reduction is done in the planetary gear set. On a gear driven case reduction is done on the layshaft gear.

 

There is another option that leaves your R&P at 3.07:1, bolt-on portal axles! There was a company years ago that made them for 5x4.5" Jeeps but I can only find 5x5" JK/JL/JT kits now. They are very expensive and can be noisy over 40mph but they are not as daft as they seem because you don't need to upgrade your axle shafts. They are the best way of safely upgrading to much bigger tyres on a Jeep that will be used primarily off road.

Edited September 8 by V

[digger]

Ah, ok. Its obvious, having just looked underneath.  From what you have said V, am I right to conclude that the idea is ok but the method wrong. Would it be possible then to make a new cog either in the planetary set or in the layshaft gear and thus achieve the desired result of lowering all the ratios in low range? 

 

[V]

1 hour ago, digger said:

Would it be possible then to make a new cog either in the planetary set or in the layshaft gear and thus achieve the desired result of lowering all the ratios in low range?

The difference between the 2.72:1 and 4:1 transfer cases is the planetary gear teeth ratio. You would likely have to change the tooth profile of the sun, planet and annular ring gears.

 

Teraflex used to make a 4:1 conversion kit for the NP231 but it was dropped some time ago.

 

This is how epicyclic gearing works...

 

This is how the now discontinued Teraflex 4Low conversion was done...

 

 

[UKTJ]

3 hours ago, V said:

I have wanted to do this for years. I recently revisited the 231 doubler mod but I eventually rejected it in favour of an 8-speed auto with a 4:1 transfer case. One 2 minute drive of Ignacio's Rubicon convinced me this was the right choice for a conversion.

 

A doubler would be very difficult to do on a 4.0L TJ but may be possible on a 2.5L with the engine moved further forward. The doubler adds around 9" to the drivetrain length.

 

Have you ever thought about an Atlas @V?  Very popilar across the pond it seems.

[V]

Yes, I have but they are big. My XJ would need sheet metal modification to the floor that affects the front passenger seat and seat belt anchorage.

 

I really liked the Midnight Metalworks D300 because it was small, very strong and lighter than the Atlas.

[digger]

Hi V, thanks for that info.

I've been a numpty. The only 4x4 gearbox I've ever worked on was the early Landy one and from your info I can see they are quite different to my Jeep!  The only planetary gears I've come across were in machinery, overdrives and autos and that was a very long time ago! I should have looked in my Chrysler manual first! Never mind it was just a thought!

I like the 'Tera low' kit . Pity its no longer available.

[V]

5 hours ago, digger said:

I like the 'Tera low' kit . Pity its no longer available.

I guess there's no point selling them in the USA if there are sufficient quantities of NV241OR in US junk yards now. However, they missed a market opportunity in other parts of the world where there are none or very few NV241OR available as used parts.

Edited September 9 by V

[UKTJ]

7 hours ago, digger said:

Hi V, thanks for that info.

I've been a numpty. The only 4x4 gearbox I've ever worked on was the early Landy one and from your info I can see they are quite different to my Jeep!  The only planetary gears I've come across were in machinery, overdrives and autos and that was a very long time ago! I should have looked in my Chrysler manual first! Never mind it was just a thought!

I like the 'Tera low' kit . Pity its no longer available.

These guys in Spain claim to have them (though you never know for sure until you try to order.  Not cheap though.

Tera Low