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4wd vs 2wd steering radius and popping noise


Madkaw

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Can someone explain to me why the turning radius changes so radically when going form 2wd to 4wd ? I loose quite a bit of turning radius when using 4wd drive . 
I also usually get a pop noise coming from the front during an initial turn out of my driveway ( usually in reverse) . My driveway is pretty steep incline and it’s a sharp turn left backing out . Sometimes it pretty damn loud . The pop seems to happen whether I’m in 2wd or 4wd . 
Any input ?

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Well for one the 4x4 is limited by a constant velocity joint that can transmit power through a bend. Like a U joint on a driveshaft. You never see much of a bend on those. To a lesser degree I imagine the 4x4 runs a larger diameter tire that will rub the frame or suspension at extreme turning angles.

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On 4/27/2021 at 4:10 AM, Madkaw said:

 


I also usually get a pop noise coming from the front during an initial turn out of my driveway ( usually in reverse) . My driveway is pretty steep incline and it’s a sharp turn left backing out . Sometimes it pretty damn loud . The pop seems to happen whether I’m in 2wd or 4wd . 
Any input ?

 

 

Again the front axle must articulate when turning weather 2 or 4wd. Probably the CV joint in the hub.

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16 hours ago, butter fingers said:

i know Madkaw knows his way around a 4x4 . what ball joints do i buy for my 86 4x4 ? i see some cheap ones out there but i want quality . thanks , butter fingers 

I know nothing - really . I’m a newb here . Datzenmike is the man 

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On 4/27/2021 at 9:10 AM, datzenmike said:

Well for one the 4x4 is limited by a constant velocity joint that can transmit power through a bend. Like a U joint on a driveshaft. You never see much of a bend on those. To a lesser degree I imagine the 4x4 runs a larger diameter tire that will rub the frame or suspension at extreme turning angles.

What I’m saying is the turning radius changes on my truck based on whether it’s in 2wd or 4wd.  The axles are there either way right ? 

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The 4x4 will have axles through a hollow hub, turned by the engine in 4wd. A 2wd will just have a hub on a spindle. A 4x4 becomes difficult to turn as the turn becomes tighter and tends to want to bind. A 2wd spindle is more like a shopping cart wheel.

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Am I right in understanding, Madkaw, that you’re saying that there is a difference in the turning radius of your truck, depending on whether it’s in 2wd or 4wd?

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Its because in 4wd your tcase is locked.  All 4 wheels are trying to travel the same distance even though their arcs are different.  That causes the truck to crow hop reducing radius.  All 4 tires try to travel the long radius, so you end up pushed into a longer radius turn.

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I guess I read that wrong. So not a difference between two trucks but a difference when shifted into 4wd from 2wd.

 

26 minutes ago, Lockleaf said:

Its because in 4wd your tcase is locked.  All 4 wheels are trying to travel the same distance even though their arcs are different.  That causes the truck to crow hop reducing radius.  All 4 tires try to travel the long radius, so you end up pushed into a longer radius turn.

 

If you had an LSD or the differential was welded. The 720 didn't have LSD so both front and rear differentials are open and the L&R wheels are free to turn separately.

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Are you driving in 4x4 on pavement? Don't, its hard as hell on your driveline. The tires need to slip since there isn't a differential between the front and rear. AWD vehicles have either a viscous coupling or some type of differential arrangement that allows the front and rear to turn at different speeds. When turning each tire's traveling a different speed and distance. Dirt, snow, sand ect allows everything to equal out gently.

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21 hours ago, Lockleaf said:

Its because in 4wd your tcase is locked.  All 4 wheels are trying to travel the same distance even though their arcs are different.  That causes the truck to crow hop reducing radius.  All 4 tires try to travel the long radius, so you end up pushed into a longer radius turn.

All great responses- thank you . That makes sense Lockleaf

 

2 hours ago, bottomwatcher said:

Are you driving in 4x4 on pavement? Don't, its hard as hell on your driveline. The tires need to slip since there isn't a differential between the front and rear. AWD vehicles have either a viscous coupling or some type of differential arrangement that allows the front and rear to turn at different speeds. When turning each tire's traveling a different speed and distance. Dirt, snow, sand ect allows everything to equal out gently.

i don’t make a practice of driving 4WD on the road . I have a steep driveway and I like to put it in 4WD to go up it . I’m usually in 4 low , so I would know if I’m in 4WD . 
 

Maybe this “ binding “ issue is causing a pop when maneuvering around from driveway to street . 

23 hours ago, NC85ST said:

Am I right in understanding, Madkaw, that you’re saying that there is a difference in the turning radius of your truck, depending on whether it’s in 2wd or 4wd?

Yes

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You won't need 4wd going down the driveway, try without it.

 

I had a '79 Bronco and at full turn the U joints would gently bind and let go, bind and let go as the wheel turned. I think the 720 uses a constant velocity joint so it shouldn't be as bad. Tight turns are hard to do in 4wd.

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Lockleaf's post is likely the reason.  4wd shouldn't be used on solid surfaces due to the lack of center differential, even though you may have open front & rear diffs.

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I have a Chevy PU in 4X4. What you describe is common for any non-full time 4X4. I also can tell a difference between 2X4 and 4X4 when turning. What Lockleaf said is the most plausible way to describe things. Also, I NEVER use 4X4 on a hard, dry surface, EVER. Doing so repeatedly is asking for future problems with your transfer case and/or axles due to wear/breakage. 

 

Don 

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5 hours ago, datzenmike said:

You won't need 4wd going down the driveway, try without it.

 

I had a '79 Bronco and at full turn the U joints would gently bind and let go, bind and let go as the wheel turned. I think the 720 uses a constant velocity joint so it shouldn't be as bad. Tight turns are hard to do in 4wd.

Usually I disengage before I back down my driveway to disengage the hubs . I have forgotten before to drive in reverse a bit and just take off and I usually figure it out by the sound that the hubs are still engaged . 
Thanks again for the warnings. I just slip the clutch more in the driveway. My truck had the unusually high 3.88 gear set for a 4wd . Low drive is nice for poking around slow in my yard or up my driveway . Since I don’t off-road , I’d like to keep things working and use them every once in a while . Never owned a truck with a transfer case before , so maybe just like playing with all the shifters - lol. Always happy and fascinated that everything works on this truck as it should , as old as it is . Miss my 2017 Colorado - but mostly the power and luxury of it . 

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5 hours ago, thisismatt said:

Lockleaf's post is likely the reason.  4wd shouldn't be used on solid surfaces due to the lack of center differential, even though you may have open front & rear diffs.

When I first started driving it I had the hubs stuck engaged and didn’t know it . I couldn’t believe what a crappy turn radius truck I had . It’s like a Mack truck compared to driving it in 2wd drive . I wanted this truck because it was small enough to drive in my back yard , but at first I was disappointed with the turn radius . Happy now though !

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5 hours ago, thisismatt said:

Lockleaf's post is likely the reason.  4wd shouldn't be used on solid surfaces due to the lack of center differential, even though you may have open front & rear diffs.

When I first started driving it I had the hubs stuck engaged and didn’t know it . I couldn’t believe what a crappy turn radius truck I had . It’s like a Mack truck compared to driving it in 2wd drive . I wanted this truck because it was small enough to drive in my back yard , but at first I was disappointed with the turn radius . Happy now though !

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