Overdriving the Front Axle?

['84 Bronco II]

Lately I've been seeing more and more discussion about overdriving the front axle of your crawler. This is something that is very common in the RC crawling world, but has generally been considered bad in full size rigs due to binding up the transfer case and generally being hard on parts. The benefits being better climbing, tighter turning, and "sucking" down the suspension. The cons are obviously binding up the drivetrain, but also unsettling the suspension when going in reverse. I think I remember woody mentioning 6% overdrive being a good number to get the performance advantage without excessive bind, but I was just reading some comments Jesse Haines made about running an 8% overdrive in his comp car (4.88s in front, 5.29s in the rear). Jesse made it sound like 8% wasn't an issue and he could drive around on high-traction surfaces locked in 4x4 no problem. Of course his comp car is significantly lighter than most trail rigs which could be a factor.

All this discussion I have been reading has me considering running 5.13s in the front of my buggy and 5.38s in the rear (about 5% overdrive). Is anyone else on here running overdrive in the front of their rig? Is it an awesome performance advantage, or are you constantly having to shift in and out of four wheel drive? Leave it to the cone dodgers, or is it worthwhile on a recreational buggy?

 

[Byro]

I’ve always thought about it, IIRC Ford did it from the factory with the Broncos and F150s. I could be imagining it though.

 

['84 Bronco II]

I’ve always thought about it, IIRC Ford did it from the factory with the Broncos and F150s. I could be imagining it though.

Not really though, Ford was mostly a result of Dana front axles and 9" rears. 4.09/4.11, and 3.50/3.54 are some ratios that come to mind. Pretty much in the 1% or less range which is too subtle to make a difference one way or the other.

 

[YotaAtieToo]

The binding is not an issue. Think about how hard those guy bind thier rigs every time they hit an obstacle. I'd wager that just turning hard on high traction surfaces binds more than a slight over drive.

5% will be unnoticeable. I helped a buddy build a rig years ago with 4.37s and 4.56s (5%) and we could push it back and forth on concrete in 4 hi.

I did something like 15% in my rc and couldn't tell a difference. I think you need to be much more aggressive to really get an advantage.

I'd like to see a moon buggy type rig try something extreme like 4.10s and 5.29s just to see what would happen.

 

['84 Bronco II]

The binding is not an issue. Think about how hard those guy bind thier rigs every time they hit an obstacle. I'd wager that just turning hard on high traction surfaces binds more than a slight over drive.

5% will be unnoticeable. I helped a buddy build a rig years ago with 4.37s and 4.56s (5%) and we could push it back and forth on concrete in 4 hi.

I did something like 15% in my rc and couldn't tell a difference. I think you need to be much more aggressive to really get an advantage.

I'd like to see a moon buggy type rig try something extreme like 4.10s and 5.29s just to see what would happen.

The thing about RCs though is that the strength to weight ratio is much higher. Some of those dudes are overdriving around 30%

So for shits and grins, 15% overdrive with 5.38s in the rear would work out to roughly 4.56s in the front. Seems kind of nuts

 

[YotaAtieToo]

The thing about RCs though is that the strength to weight ratio is much higher. Some of those dudes are overdriving around 30%

Yes, plus if you try something and it goes boom, you're out 10s of dollars vs 1000s

Those little portal buggies are pretty tough, but I've seen fairly mild rc bind of one tire and just flip the whole rig over

I still can see oberdriving being near as a bind as some of the shit I saw at trail breaker.

 

[Etyler2]

Traction is the issue. Blasting around spinning tires OK.

Moab and a 10% difference probably would break parts.

Making most of this up, but makes sense to me.

 

[bdkw1]

The 7S ranger I raced went to its first race with 4.88 in front and 6.0 in the rear. 4x was only for emergencies. There were a lot of emergencies that race, fortunately it took out the front diff and not the T-case. After that we had Culhane swap the sprockets in the T-case to make it under drive the front so the ratios would be closer. No issues after that.

 

[Hooligan]

I thought that was a mud racer thing to assist in launch and keeping the truck tracking straight.

 

[white-rhino]

If you're trying to get every advantage for comp, you wouldn't care but for a rec wheeler that you want to last, seems like it would be hard on parts.

 

[[memphis]]

This didn’t work out very well on Dirt Every Day. Dave didn’t do it but remember stock trucks across the rubicon and later the Chevy on Backdoor?

PO had 4.10s out back and 3.73 out front or vice versa. It destroyed the 10 spline SM465 output and the input on whatever the chain drive transfer case was.

Sounds like an expensive kaboom

 

['84 Bronco II]

This didn’t work out very well on Dirt Every Day. Dave didn’t do it but remember stock trucks across the rubicon and later the Chevy on Backdoor?

PO had 4.10s out back and 3.73 out front or vice versa. It destroyed the 10 spline SM465 output and the input on whatever the chain drive transfer case was.

Sounds like an expensive kaboom

I haven't seen that episode, but it doesn't make sense that breakage at the transmission output/transfer case input would be due to mismatched gear ratios I would expect the breakage to be internal to the transfer case where the power is being split or at one of the ends of the drive line.

I remember wheeling with a guy in a CJ many years ago who I believe was running 3.73s in the front and 4.10s in the rear. I remember him having issues with binding and having to constantly be shifting in and out of 4x4. However, that is a 9.1% overdrive, which is more than Woody or Jesse Haines were talking about.

My only other experience with mismatched gear ratios was briefly after I originally geared the 8.8" in my Bronco II to 5.13, but I hadn't swapped the Dana 35 in and still had the Dana 28 with 3.73s. I was out screwing around in 2wd and ended up getting stuck in a washout. I was so afraid of blowing something up I put it in four wheel drive just long enough to get unstuck (maybe 3') and immediately took it back out

Here are Jesse's comments on the topic for those that aren't on the Facebooks:

4.88/5.29 is a noticeable improvement. T-case shifts great. I’ve run it this way for 3 years. At times I’ve run it in high range 4wd for long distances and forgotten about it.

If you’re thinking about trying something like 5.38/5.40 or even 5.29/5.38, it’s not enough of a difference to notice any performance benefits.

RC stuff is usually around 25% overdrive. I’m sure there are places where that would be awesome, but others (like reverse) where it would suck. At 8% I don’t notice the car getting weird in reverse.

 

[Sterlingfire]

Couldn't you just do it with tire size to test? Of course you would be dealing with circumference and not height. Might be the easiest way to test before committing to the work and cost of gears. Borrowing a tire the right size to do this would be much more pocket friendly.

 

[WaterH]

I thought that was a mud racer thing to assist in launch and keeping the truck tracking straight.

That’s what I’ve heard, also it tends to keep the heavy end up on top of the mud.

 

[bdkw1]

All this is a bandaid for not having a proper center diff. On the McPherson 7 trucks with the cyclone T-cases, they had different gear sets to manage F/R torque split.

 

[YotaAtieToo]

Traction is the issue. Blasting around spinning tires OK.

Moab and a 10% difference probably would break parts.

Making most of this up, but makes sense to me.

Yes, but have you ever just turned sharp with a locked rig on aired down stickies while on sand paper rock?

I promise, 5% is nothing and no one would ever know.

This didn’t work out very well on Dirt Every Day. Dave didn’t do it but remember stock trucks across the rubicon and later the Chevy on Backdoor?

PO had 4.10s out back and 3.73 out front or vice versa. It destroyed the 10 spline SM465 output and the input on whatever the chain drive transfer case was.

Sounds like an expensive kaboom

Were talking about well built buggies with the best of the best parts, not an old beater chevy with parts that are already know to break

Couldn't you just do it with tire size to test? Of course you would be dealing with circumference and not height. Might be the easiest way to test before committing to the work and cost of gears. Borrowing a tire the right size to do this would be much more pocket friendly.

Thats actually a pretty interesting idea. 42 to 39s is ~7%.

I'd think these portal guys are running Toyota 3rds, so swapping 3rds wouldn't be a big deal. Not sure what op is running.

I hate to admit it, but I've also ran 14% and 23% difference in a few rigs because of "temporary" situations. Both were opposite though, faster rear, and both were very noticeable. I'd think the opposite would be much less noticeable, since the ass end isn't trying to pass its self.

 

['84 Bronco II]

Yes, but have you ever just turned sharp with a locked rig on aired down stickies while on sand paper rock?

I promise, 5% is nothing and no one would ever know.

I saw someone claimed Thomas Meiser (rock lizard dude) is running a 5.13/5.38 combo in one of his buggies. I might reach out to him on his thoughts about it.

Thats actually a pretty interesting idea. 42 to 39s is ~7%.

I'd think these portal guys are running Toyota 3rds, so swapping 3rds wouldn't be a big deal. Not sure what op is running

I am poor, so superduty axles for now Maybe when I decide to go rear steer in the future, I'll start over with some fabricated 9"-based axles. Seems a little funny running big tires up front, but that doesn't really help me since The axles aren't geared yet. I guess it is only a $300 gamble if I don't like it.

 

[YotaAtieToo]

I saw someone claimed Thomas Meiser (rock lizard dude) is running a 5.13/5.38 combo in one of his buggies. I might reach out to him on his thoughts about it.



I am poor, so superduty axles for now Maybe when I decide to go rear steer in the future, I'll start over with some fabricated 9"-based axles. Seems a little funny running big tires up front, but that doesn't really help me since The axles aren't geared yet. I guess it is only a $300 gamble if I don't like it.

5.38s in the rear only at first to really test the theory

 

[Treefrog]

I have no experience wheeling out west, but it seems that the longer stretches where you forget that you are in 4x4 would be lower traction sand and dirt not high grip rock.

 

[blakes]

I had a YJ years ago that I built with tons. Had 5.38s in the rear and 4.10s in the front. Took it on a trip to the dunes prior to regearing the front. It pulled the sand hills nicely, but I took it out of front drive when not actually climbing.

 

[woody]

8% is pretty common, quite a few guys I know are set up that way. Sub 4000# buggies, and weight biased correctly (~60f/40r). Most of us run between trails in front or rear only (I'm usually front only) so there is no bind issue at speed. Selectable front/rear lockers make it even less of a concern, most comp guys are set up that way. I'm spool front/ARB rear which is also common.

Traction at Sand Hollow is about as grippy as it gets.

4.88/5.29 is pretty standard. Going much outside that 8% different can cause some nasty handling issues. Kalif tried 4.10/5.29 and it didn't work well....lol

The idea is for the front to pull more than the front, helping squat the front suspension and pull the front of the car down on steep climbs. Big advantage there. Disadvantage on sidehills where the front spins quicker and wants to slide downhill, but that's more controllable when you unlock the rear and use the high-side rear cutting brake (I do that constantly)

I'm currently 4.88 f/r but have a set of 5.29's available that I may toss in at some point this spring. Just got a spare diff case, so why not :)

Toyota makes it easy with a dropout diff and their ratio options:
4.11 > 4.56 = 11%
4.56 > 4.88 = 7%
4.88 > 5.29 = 8%

 

[YotaAtieToo]

I had forgot about the side hill issue with OD front. I'd think a front selectable would help even more there?

Sometimes it's easy to get caught up in climbing ability and forget that you're only doing that occasionally.

 

[woody]

I had forgot about the side hill issue with OD front. I'd think a front selectable would help even more there?

Front selectable can help mitigate the oddities, but they are still present. Just a matter of getting used to it I suspect, like everything. I'll likely be finding out soon enough ;)

 

[Adamwende]

Tracy Jordan’s last portal rig was overdriven. It never made anything the rest of us didn’t make.

Doesn’t Clibon have on the fly overdrive?

 

['84 Bronco II]

8% is pretty common, quite a few guys I know are set up that way. Sub 4000# buggies, and weight biased correctly (~60f/40r). Most of us run between trails in front or rear only (I'm usually front only) so there is no bind issue at speed. Selectable front/rear lockers make it even less of a concern, most comp guys are set up that way. I'm spool front/ARB rear which is also common.

Traction at Sand Hollow is about as grippy as it gets.

4.88/5.29 is pretty standard. Going much outside that 8% different can cause some nasty handling issues. Kalif tried 4.10/5.29 and it didn't work well....lol

The idea is for the front to pull more than the front, helping squat the front suspension and pull the front of the car down on steep climbs. Big advantage there. Disadvantage on sidehills where the front spins quicker and wants to slide downhill, but that's more controllable when you unlock the rear and use the high-side rear cutting brake (I do that constantly)

I'm currently 4.88 f/r but have a set of 5.29's available that I may toss in at some point this spring. Just got a spare diff case, so why not :)

Toyota makes it easy with a dropout diff and their ratio options:
4.11 > 4.56 = 11%
4.56 > 4.88 = 7%
4.88 > 5.29 = 8%

Nice to read your take on it. My buggy should end up in the mid-upper 3,000Lb range, so maybe a little over 4,000Lbs depending on how much water I end up running in the front.

I am planning on selectables front and rear so I can do some fun stuff with cutting brakes

So my question to you Woody, if you were in my situation and building with Superduty axles where you can't just swap thirds out to experiment easily, would you set it up with front overdrive, or stick with matched gearing?

If I build it with matched gearing, odds are I would never change the front gears out. I think a 5% overdrive wouldn't be too crazy, but I am worried about hating it and having to redo things right after I finish. Farmington and southern NM (very similar terrain to southern AZ) are what I wheel the most. Sand Hollow will probably be a once or twice a year thing.

 

[woody]

Tracy Jordan’s last portal rig was overdriven. It never made anything the rest of us didn’t make.

Doesn’t Clibon have on the fly overdrive?

Clibon has on-the-fly rear disconnect, fully variable....effectively slipping a rear output "clutch" so it varies between 100% and 0%. $15k option (ish)...he has 5 on order and is 3 months late on the first one. His next car has significantly more tricks...

So my question to you Woody, if you were in my situation and building with Superduty axles where you can't just swap thirds out to experiment easily, would you set it up with front overdrive, or stick with matched gearing?

IMO, drive one before you decide. It's not for everyone, and since you don't have dropouts it's not a "3 hour" swap if you hate it.

 

[Adamwende]

Damn… 15k for that option is a lot of quid…. But it’s really cool. Good to see in innovation happening.

 

[Thatg9659]

Middle of my first way back when build I ended up with 3.55s in the front and 3.45s in the rear, fucked around and found out by shifting into four-wheel-drive on bare pavement. took about ten ft forward to bind up, back up then shift back to 2hi. Underdriven front,

Mud racers like to run od front, says it helps the truck go straight.

 

[arse_sidewards]

I’ve always thought about it, IIRC Ford did it from the factory with the Broncos and F150s. I could be imagining it though.

The biggest ratio difference offered was 3.05 front and 3.08 rear. That's really not enough to matter for any vehicles that doesn't run on a cog railway.

 

[Bebop]

The Campbells used to do that when competing in rockcrawling.

All this is a bandaid for not having a proper center diff. On the McPherson 7 trucks with the cyclone T-cases, they had different gear sets to manage F/R torque split.

How having a center diff would help the situation in a rockcrawling application ? You don't want any differential action.