Ackermann for offroaders

[dirtydiesel]

This is going to be a long rambling post so grab a beer

I have long searched for a decent answer to what the best ackerman setup is for offroad difflock equipped vehicles, and whether it even makes any differance, but there just doesn't seem to be much out there more than opinion, with little or no tech behind the opinion

This thread could/should/maybe be read first.

http://forums.lr4x4.com/index.php?showtopic=76199

Here's a really good explanation of ackermann (with pics)

http://www.rctek.com/technical/handling/ackerman_steering_principle.html

My thoughts and interest stem from an old observation.

Every noticed in your landy that with both difflocks engauged, turning becomes next to impossible? i have, and i've made a serious mess of some trials sections as a result.

I assumed that this was just what happened when you force all 4 wheels to turn at the same speed.

Then 9 years ago i bought a unimog 404, i took it off road and i was totally shocked and not just by its capability, also because the diff locks had little detriment to the steering, At first i even thought the diff locks just wern't working but they were working fine.

In my ignorance and youth i again just assumed that it was something to do with the portals??????

Fast forward a few years and i had fitted the unimog axles to my friends G wagon, and it's ability to maneuver with both lockers in, with power steering and actual engine power was quite phonomenal. and again i disregarded the reasons behind this chalking it up to the portals again.

Then later in it's life, after i rebuilt it, i moved the track bar to the front of the axle, and i lost a lot of "steerability" i did initially acount this to either the lock stops being the wrong sides now or my hydrosteer naffing things up.

But the tiny kernel of an idea formed about ackermann being responsible, i didn't however develop the idea beyond that kernel.

Fast forward to today and i'm at the point in my latest project (which i'll post in here soon ) where i want to place my track bar above and behind the front axle, this will afford it lots of protection, and allow me to explore ackermann.

So the first thing i did was to measure the ackermann on every axle i have to see what different manufacturers do.

The results are interesting

I'll go away now and make a little spread sheet and post it in my next post

[dirtydiesel]

To explain the table, i have physically measured the ackermann angle on the steering arms/ swivels, and i've measured and used trigonometry to confirm the angle i've averaged the few minor discrepancies

The actual wheelbase is obvious

The calculated wheel base, is where trigonometry (based on measured ackermann angle and kingpin to kingpin width) puts the "design" wheelbase

the percentage differance is nothing more than a comparision for how much the factory designers shortened or lengthened the hypothetical wheelbase when designing in ackermann. This is the most interesting figure to me.

[B reg 90]

Very interested in this. Please post your info!

My take is that you want 100% akerman so that at low speed muddy conditions your wheels track as true to the direction or travel as possible. Hence you have minium slip at each wheel and therefore maxium traction for forward movement.

Awaiting a counter arguement.

Adrian

[B reg 90]

mmmmm, by my theory the Patrol Y60 LWB will be best in low traction steering conditions. Speed will result in much sliping of wheels off road, so akerman is less important. If we drove on tarmac then this would not hold.

[Orgasmic Farmer]

We went through this issue when going full hydro on the challenge truck prior to KOVs. Can't remember the outcome though sorry. Long thread on Devon somewhere!

[dirtydiesel]

Now what i find very interesting is that the top 4 were all equiped as standard with front and rear diff locks. And they all have hugely exagerated Ackermann.

Look at the Gwagon the optimal wheel base for the designed in ackermann is 68 inches !!!!!!!!!!!!!!

Where as the patrol (optional rear diff lock only) is near perfect in lwb and under in swb

the venerable range rovers Ackermann is optimised for a 144 inch wheelbase.

I believe that those manufacurers built in this exagerated ackermann to account for the wheel slip induced by difflocks

I strongly belive that exagerated ackermann combined with the excellant scrub radius of the stock unimog setup is the cause of it's excellant steering.

And deep breath, fresh beer and discuss

[B reg 90]

To get 100% ackerman on a hydro steer axle is hard (assumes ram is on the front). You need to push the ram right back into the axle. If the ram is level with the axle tube you can't do this enough. You then need long arms at the axle ends at a fair angle that puts them some where inside the wheel rim/tyre.... You realy need large diameter rims (17"??) and /or the ram on the top of the axle D60 style. This reduces uptravel on the axle.

[B reg 90]

Now what i find very interesting is that the top 4 were all equiped as standard with front and rear diff locks. And they all have hugely exagerated Ackermann.

Look at the Gwagon the optimal wheel base for the designed in ackermann is 68 inches !!!!!!!!!!!!!!

Where as the patrol (optional rear diff lock only) is near perfect in lwb and under in swb

the venerable range rovers Ackermann is optimised for a 144 inch wheelbase.

I believe that those manufacurers built in this exagerated ackermann to account for the wheel slip induced by difflocks

I strongly belive that exagerated ackermann combined with the excellant scrub radius of the stock unimog setup is the cause of it's excellant steering.

And deep breath, fresh beer and discuss

Like your thinking on this. However work in the morning means I'll have to sleep on it!

[dirtydiesel]

To get 100% ackerman on a hydro steer axle is hard (assumes ram is on the front). You need to push the ram right back into the axle. If the ram is level with the axle tube you can't do this enough. You then need long arms at the axle ends at a fair angle that puts them some where inside the wheel rim/tyre.... You realy need large diameter rims (17"??) and /or the ram on the top of the axle D60 style. This reduces uptravel on the axle.

What about looking beyond hydrosteer as utilising a double ended ram and viewing it more as a hydraulic track bar??

the actual rams location on the axle is irrelivant (for ackermann) its the location of the the track rod ends and thier relationship to the swivel kingpins that govern geometry changes.

[steve b]

thats some interesting data on that spreadsheet Dan , have you looked at Ackermann angles on tractors and commercial grass cutting machinery ? pro-turf care is

particularly worth looking at with the No1 requirement being zero slip on tight lock

cheers

Steve

[LandyManLuke]

I've been looking at ackermann for a hypothetical hydraulic steering build. Putting the ram on the back of the axle created issues with diff/sump clearance. There is more room to put it in front of the axle, but as pointed out, ackermann considerations put the rod ends somewhere inside the tyre.

Using a track bar behind the axle and a single ended ram in front solves the ackermann geometry but leaves you with a single sided ram, which probably isn't much fun, and a track rod that needs to be pretty beefy.

Another consideration for ackermann, that I haven't thought about in detail, is fiddle brakes. We have had good success with fiddles, but the steering is not without issue. Moving the theoretical pivot point for the back of the vehicle must have an effect, but I haven't fully thought through what it means for ackermann.

Luke

[FridgeFreezer]

I'm too far from the beer cupboard.... but now you've got me interested in working out the Volvo portals to see what they're set up as.

[bluespanner]

thats some interesting data on that spreadsheet Dan , have you looked at Ackermann angles on tractors and commercial grass cutting machinery ? pro-turf care is

particularly worth looking at with the No1 requirement being zero slip on tight lock

cheers

Steve

My old Westwood mower, on full lock the inside front wheel is almost 90 degrees to the machine.

[Bowie69]

I'm sure this is related, but my mind is still a little sleepy this morning, but the distance between the hub swivel pins and the face of the hub (scrub radius?) will have a fair effect on the effective wheelbase once on full lock, and this will affect the Ackerman as well.

Is it possible that it is a combination of both the Ackerman and the scrub radius? Or are the two so intertwined I am talking ballcocks?

[dirtydiesel]

I'm too far from the beer cupboard.... but now you've got me interested in working out the Volvo portals to see what they're set up as.

I would say that the volvos have managed to use conventional ackermann with the track bar up front, as the track rod end is further out than the king pins.

I've been looking at ackermann for a hypothetical hydraulic steering build. Putting the ram on the back of the axle created issues with diff/sump clearance. There is more room to put it in front of the axle, but as pointed out, ackermann considerations put the rod ends somewhere inside the tyre.

Using a track bar behind the axle and a single ended ram in front solves the ackermann geometry but leaves you with a single sided ram, which probably isn't much fun, and a track rod that needs to be pretty beefy.

Another consideration for ackermann, that I haven't thought about in detail, is fiddle brakes. We have had good success with fiddles, but the steering is not without issue. Moving the theoretical pivot point for the back of the vehicle must have an effect, but I haven't fully thought through what it means for ackermann.

Luke

There is nothing wrong with a single ended ram system, they are just unfashionable

You should be able to package a track bar and a single ended ram above and behind the axle.

I much prefere a track bar and single ended ram set up, no need for the extra complication of double ended for the vehcle speeds and steering responce we see.

[landroversforever]

I can't add any tech to the thread, but a fascinating read

[Daan]

I would say that the volvos have managed to use conventional ackermann with the track bar up front, as the track rod end is further out than the king pins.

There is nothing wrong with a single ended ram system, they are just unfashionable

You should be able to package a track bar and a single ended ram above and behind the axle.

I much prefere a track bar and single ended ram set up, no need for the extra complication of double ended for the vehcle speeds and steering responce we see.

I would still try to fit a double ended ram, because of the difference in assistance between left and right. Also, if you go full hydro, there will be a difference between the amount of turns to lock going left and going right. Alternatively, you could go with 2 single ended rams, which could even be single acting. I hadnt realised the mog had exagerated Ackerman. My take has been so far that parallel steering is best, because you get the best turning circle, because usually the inner wheel is the limitation. Off course on a unimog, that is not the case.

The whole not being able to steer thing has never really occured to me, since I have the detroit setup, which contrary what most people tell me, is completely controllable when offroad.

Daan

[Three Blades]

An interesting thread here.
I am no expert but I would think that the existence or otherwise of a diff lock at birth should not be related to the designed Ackermann.

Any steering away from straight ahead will cause the outer wheels to travel further than the inner. If any of the diff locks are in then you will not be able to let this happen without some sort of slip/scrub. I would therfore suggest (but am totally open to be corrected) that the relative benefit or parrallel vs Ackerman geometry would be swamped by the straight-on desire of a rear axle with the diff lock in.

Go back to the comparison with pro turf care and imagine the mess on the lawn if the tractor had it diff lock in on the corner.

With the locks out, the perfect solution becomes far too complex for a Friday afternoon...
Limited slips sound like the best option (in theory)
TB

[dirtydiesel]

I'm not sure that any of the manufacturers added the Ackermann to counter act the extreme amount of wheel slip induced by diff locks but it seems strange the disparity between otherwise similar vehicles.

Take the 80 series cruiser and the y60 patrol, designed at a similar time, both were more or less scratch built to fill the same gap in the market, they have very similar dimensions, weights and engine size. Main difference being the patrol is part time and the cruiser is full time 4x4. Yet they have very different Ackermann

[dirtydiesel]

I would still try to fit a double ended ram, because of the difference in assistance between left and right. Also, if you go full hydro, there will be a difference between the amount of turns to lock going left and going right. Alternatively, you could go with 2 single ended rams, which could even be single acting. I hadnt realised the mog had exagerated Ackerman. My take has been so far that parallel steering is best, because you get the best turning circle, because usually the inner wheel is the limitation. Off course on a unimog, that is not the case.

The whole not being able to steer thing has never really occured to me, since I have the detroit setup, which contrary what most people tell me, is completely controllable when offroad.

Daan

I have used a 30x50x200 single ended ram on my g wagon for a few years, I can honestly say I have never noticed any difference in assistance, the steering lock seemed equal as well.

I think you would only notice the difference if you were under pumped or mismatched the ram and danfoss unit, with 38" boggers @ 5psi I could lock to lock with one finger.

I would have agreed with you on the parallel steering, from the stand point that in an offroad vehicle both tyres are unlikely to have the same grip and weight upon them.

But with the amount of steering lock available in say a mog hub and the fact that a good deal of time is spent at full lock both front wheels fighting each other to turn vastly different radius's cannot help traction and steerability.

On the 404 mog hubs they also have the lock stops set differently on the turn in and turn out, as stock they are 35° turn out and 30° turn out.

[bill van snorkle]

Might I make a suggestion for what may be a relatively easy way to prove or disprove the Ackerman principal as it applies to offroad vehicles? I'm assuming that just like over here Series LandRovers are a dime a dozen.

Series 2a and 3 Landrovers have bolt on forged steel steering arms that are bolted to the bottom of the swivel housings. By pressing out the swivel pins and pressing in Railko pins, these arms can be fitted to the top of the swivel housings to give reverse Ackerman geometry, or alternatively heated up and bent to give parrallel steering or any variation between.The left hand arm that also takes the drag link stays in place at bottom of swivel, but an identical arm is flipped and fitted over the top after having the drag link eye cut off.

I find that reverse Ackerman works well on my portalled SWB that is fitted with series swivel housings,and I believe the reason for that is, that even when making a turn at low speeds, there is sufficient weight transfer to the outside front wheel that it is beneficial if it turned through a tighter angle than the inner wheel, as is the case with reverse Ackerman angles.

How any of the data gathered by experimentation can be applied to later Landrovers with cast on steering arms is another question though.

bill.

[dirtydiesel]

Hi Bill nice to see you back,

How do you find the reverse Ackermann performs offroad? I appreciate your terrain is different to ours typically.

I think an exaggerated amount of Ackermann would be an advantage in greasy rutted areas, where there is little traction at all and the few degrees difference per wheel: could be enough to start a turn.

But in the same situation I cannot help thinking that with parallel steering, both wheels turned to they're maximum might be even more advantageous

But to counter that why would merc (actually steyr) design in so much exaggerated Ackermann in to the g wagon, a vehicle they built to win military contacts that they knew would be rigorously tested off road?

Maybe i'm just reading too much into my measurements?

[bill van snorkle]

Hi Dan. Thanks, just a brief visit, as I don't do much vehicle work these days.

I've been running reverse Ackerman with and without portals for around 15years.

I originally adopted it in pre portal days when running dual wheels front and rear in order to get the inner duals over the king pin axis, because reversed Ackerman steering arms angle inwards with track rod up front. Even so. the scrub radius of the outer duals was significant, but i could still drive the truck effectively both on and offroad without power steer. I've run the portals for quite some years now, and only powered up the steering box around 6 months ago. there is so little fight between the front wheels with the difflock engaged that I think I will have to fit a buzzer or warning light to the difflock valve, because I occasionally forget to disengage it and dont want to unnecessarily strain my cut and modified Toyota CV joints.

I'm happy with the performance both on and offroad of the setup,The scrub radius is around 1'' greater than a series 2A forward control and I find it handy for 'walking' the front wheels forward on low traction hillclimbs.

Bill.

[Orgasmic Farmer]

Dan

don't know if you ever read the Devon 4x4 forum but here is the discussion we had over there last year prior to mounting a full hydro set up on Steves' truck.

http://www.devon4x4.com/index.php?option=com_kunena&view=topic&catid=5&id=131555&Itemid=106#132026

Not sure it helped us much but a few valid points come out of it

[dirtydiesel]

I'd come across that thread whilst googleing, as much as its slighty interesting wills view points are not based on real world experiance and allthough rockwatt talks a good fight whenever i've met him his truck was either broken or stuck and he was still overly arrogant about its strength and capability.