Ackermann for offroaders

[Orgasmic Farmer]

[dirtydiesel]

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.

Very interesting, leads me to wonder whether scrub radius has more (or as much) to do with the feel of the steering as the Ackermann, as both the g wagon and 404mog run almost fully backspaced wheels.

It would be really good to have your builds and thoughts behind them all in one thread.

I believe Michele has archived a lot your pics, and I save an article I found on your 6x6

It would make for a hugely educational read.

[Daan]

There is more, the mog has positive camber build in, 1.75 degrees, according to the data in this thread:

http://www.pirate4x4.com/forum/mercedes/973022-why-does-404-mog-camber-front.html

Will have an influence on the scrub for sure.

The last time I saw that was on a coach pulled along by horses.....

Daan

[bill van snorkle]

There is more, the mog has positive camber build in, 1.75 degrees, according to the data in this thread:

http://www.pirate4x4.com/forum/mercedes/973022-why-does-404-mog-camber-front.html

Will have an influence on the scrub for sure.

The last time I saw that was on a coach pulled along by horses.....

Daan

That is also a factor that could contribute to my experiece, because my portals,scrub radius and reverse ackerman geometry are all built around Series Swivel assemblies with their 1.5 degrees of positive camber. I have also retained the standard 3 degrees castor angle as this is a road registered vehicle and I need to be able to still drive it in the event of a power steering pump/belt failure.

Dan, As far as compiling all my thoughts and experiences on one thread is concerned, I'm only theorising as to the reasons my vehicle works the way it does.Probably more to do with good luck than good management. I don't really know what geometric factors contribute to a particular characteristic. The only positive thing about Series Landrover front axle componentry is that everything bolts together and stuff like castor,Ackerman angles, even camber can readily be rearranged to experiment and hopefully find the optimum setup.

Regards 6x6 tech, I think I have shared most of the lessons I learned, or at least the ones I remember on that subject on Teunico's 6x6 Disco thread.

Bill.

[wanger]

so the camber dials out on full lock and allows the tyre to meet the ground square on and bite in?

was trialing today and with the 3 diffs locked you have little control apart from forward.

[missingsid]

Hi,

I have seen lots of comments about Ackerman effect etc. so this thread is a good learning point for me. However I have visited the websites at the beging of this thread and can see the calculations for wheelbase alignment etc. What i don't get is that I have a Series axle with a front mounted drag link does this give me Reverse Ackerman and how does that work with the calculations (drawing would be great)?

Ther has been comments that you cannot fit a RRC swivle set on to a Series axle case with the swivles the wrong way round as the Ackerman angle will be wrong?

Cheers,

[bill van snorkle]

Hi,

I have seen lots of comments about Ackerman effect etc. so this thread is a good learning point for me. However I have visited the websites at the beging of this thread and can see the calculations for wheelbase alignment etc. What i don't get is that I have a Series axle with a front mounted drag link does this give me Reverse Ackerman and how does that work with the calculations (drawing would be great)?

Ther has been comments that you cannot fit a RRC swivle set on to a Series axle case with the swivles the wrong way round as the Ackerman angle will be wrong?

Cheers,

you have written 'draglink' whereas the steering rod that is relevant to Ackerman is the trackrod or tierod.

But no the series axles still have roughly true Ackerman linkage, because the tie rod ends are further outboard from the swivel pin axis, so a projected line from tie rod end through swivel pin axis and beyond will,as Dan has calculated eventually converge somewhere behind the rear axle centre, whereas for Reverse Ackerman the front mounted tierod ends would be inboard of the swivel axis and the projected lines through both would converge somewhere ahead of the vehicle.

To have absolute true Ackerman angles, LandRover would have to make different swivel housings for each wheelbase of vehicle they sold. In the coil sprung range that would mean at least 4 different swivel housing sets to cover 90's, RRC,s, 110's and 130's. They obviously haven't done that, and the original Ackerman angle that was designed for the 100'' RangeRover was carried over to all the other wheelbase variants.

Fitting coil sprung swivels to series axles the wrong way round would give reverse Ackerman, a low hanging trackrod that would clash with the leaf springs and no place to attach the draglink unless you made an earlier Toyota LandCruiser style trackrod and plugged the draglink on to that.

[missingsid]

Ah I get it the arm from the swivle is curved so it is close to the wheel rim and as you say outside of the swivle pin so it is still normal ackerman. Cheers now I can what it is about. I love learning stuff on this forum.

[Snagger]

As Bill said, to have accurate Ackerman angles you'd have to have special arms or swivel housings for each wheel base of vehicle type (ie, Series vehicles would have different arms for 88 or 109, and coilers would have different housings for 92.7, 100,110 and 127"). The original spread sheet calculations at the beginning of the thread appear to overlook wheel track, which is just as important as wheel base. This would explain why some axle conversions seem to work better in this regard than others - while the vehicle's wheel base has remained largely standard, the track has been greatly changed.

I've never tried it, but I can't see reversed Ackerman angles being a good thing. You're forcing the front tyres to scrub, which will lose the traction that the diff lock was fitted to retain. You're also reducing the sideways force of the front wheels that are trying to steer around a corner or out of a rut, so tight corners or changes of direction on slippery surfaces will be compromised. From an engineering and logic stance, I'd suspect that those running reverse angles and finding improved performance are actually benefiting from whatever other mods precipitated the reversed angle in the first place, and that restoring the angles while retaining the other mods would further enhance the vehicle handling and performance. If that's not the case, I'd be quite interested to learn how reversed Ackerman angles help, as they seem at face value to be a bad thing.

[bill van snorkle]

Snagger, aside from my portal axled rig, I tried reverse Ackerman on a standard axled Series 2a LWB on 7.50 x16 bar grips some years ago, but also retained the original steering arms at the bottom of the swivels in addition to an identical set of arms fitted at the top in the reverse Ackerman position.I also carried the original length trackrod which is longer than the one for reverse Ackerman.

I tested the vehicle in a variety of on and offroad conditions by swapping the trackrods over from reverse to true and back again several times, before throwing the original trackrod back in the shed after being satisfied that steering feel and precision,in addition to a measurably tighter turning radius on loose surfaces was significantly improved with reverse ackerman.

On a series 2 or 3 landrover there is relatively little investment in parts or labor to experiment with both designs, and had I decided that reverse Ackerman was a mistake it would have taken 5 minutes to swap trackrods and revert back to standard.

[Snagger]

I'm happy to accept your experience - as I said, I've never tried it. I'm curious as to why simply reversing the angle would decrease a turning circle. Regardless of the scrub, the inboard wheel is not pivoting as much as it would otherwise do, so the outboard wheel must be pivoting more to compensate. It's usually the inboard wheel that its a stop lock, so having the outboard wheel be the restricted one should increase lock considerably, so having the outboard wheel pivot to full swivel motion and the inboard wheel a little less must be giving more average movement than having the inboard wheel limited by the stops. This would mean that it's not exactly the angle reversal that is the cause for increased turning but the position of the stop locks and the side effect of angle reversal on them. I wonder if winding the locks all the way in so that the inboard swivel's seal is just covering the chrome ball with standard Ackerman angles would provide a better turning radius than the work you have already done.

Ican see why feel is enhanced - the only time the steering and transmission forces will be balanced is when the steering is straight; it'd be the same effect as increasing the castor angles.

[bill van snorkle]

Aside from when the vehicle is stationary on relatively level ground,I have often found that on a series landy on 7.50x16 with standard offset rims, the actual steering lock is limited by the inner tyre fouling the spring shackle before the outer tyres swivel housing contacts the steering stops. Reverse Ackerman causes the outside tyre to turn sharper than the inner one so that full lock can be had on the tyre with most bite whilst the inner tyre still has clearance from the shackle. The noted improvement in manouverability was really only apparent on unsealed surfaces, so possibly the outer wheel having a sharper turning angle and more grip due to weight transfer to the outer side when making a turn had less slip than a tighter turning inner wheel would have .I don't really know.

Your point earlier about the difference a wider wheel track is a good one, although unless the banjo housings have been widened and custom longer halfshafts fitted, or coiler axle assemblies fitted to leafers, then most increases to track width are gained through different rim offsets, so the steering arm spacings remain unaltered.

Whatever the science behind it all, the fact that Dans G wagon axles have more than double the Ackerman angle of a RangeRover suggests that there may not be any hard and fast rules on this aspect of steering geometry.

[Snagger]

I'm just speculating, Bill - I'm no engineer and like I said, have never even tried it. I think that offset wheels or spacers would have a significant effect on the angles - the swivel point may be unaltered, but the path of the wheel and the radius around which it is turning is affected. I suspect that it would have a similar but lesser effect than wider axles, probably related to the ratio between overall track and the distance between swivel point and tyre centre. For example, a Defender axle is 6" longer than a Series axle - if you fit wheels with 3" greater offsets than standard to a Series axle, I'd imagine the ideal Ackerman increase would be half that of the difference between the Series and Defender angles. Gets complicated, doesn't it!

Fair point about the tyres contacting the suspension before the swivel hits the stops - I have seen that before too. It shouldn't happen, and shows that the stops have not been adjusted correctly when swivels have been worked on or different tyres/wheels have been fitted. I can see it happening on 88s being fitted with bigger tyres.

I would imagine with how the Ackerman angles are controlled on Land Rovers, that fitting offset wheels or spacers to allow greater lock would be accompanied by a greater Ackerman effect - the more you move the steering, the greater the angle difference becomes. And while fitting wider wheels should require a stronger baseline Ackerman setup, a short wheel base vehicle like an 88 or 90 should already have an over-done angle because of the standard arms and short wheel base, giving the best lock and also the best Ackerman angles.

I think you're right about weight transfer having a lot to do with the practical results. I also think you're right that the exactness of it all is unimportant, otherwise the different wheel base models would have dedicated swivels/arms.

[bill van snorkle]

Well, after bull****ting to you all about my reverse Ackerman geometry, i did a wheel alignment this afternoon after noticing that I had a bit too much toe in. Afterwards, I put the truck in low range 2wd,tied the steering wheel at full lock and watched it do a few 360 degree turns on a flat gravel surface to check for tyre scrub. Both front wheels left very little impression on the road surface and there was no sound of the tyres fighting each other. Checked the wipe of the swivel ball seals and they were exactly the same. So instead of the outer wheel turning tighter than the inner, it looks as though I actually have parrallel steering, at least at full lock.

I spent some time scratching my head trying to work that one out but gave up after getting too many splinters under my fingernails.

[discomark10]

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

you are right, but If the front wheels are not working in unison (100% ackermann), they fight each other and lose grip. It would help to fight the rear axle's desire to go straight having 100% ackermann.

looking at a few scale drawings i have done, the axle width also comes into play, the wider the more ackermann needed. But offset doesn't as it extends the wheel at the steering angle.

[Snagger]

Well, after bull****ting to you all about my reverse Ackerman geometry, i did a wheel alignment this afternoon after noticing that I had a bit too much toe in. Afterwards, I put the truck in low range 2wd,tied the steering wheel at full lock and watched it do a few 360 degree turns on a flat gravel surface to check for tyre scrub. Both front wheels left very little impression on the road surface and there was no sound of the tyres fighting each other. Checked the wipe of the swivel ball seals and they were exactly the same. So instead of the outer wheel turning tighter than the inner, it looks as though I actually have parrallel steering, at least at full lock.

I spent some time scratching my head trying to work that one out but gave up after getting too many splinters under my fingernails.

What controls the Ackerman angle is the difference in length between the track rod and the swivel pin separation, and how far behind or in front of the axle line the rod is. In essence, it depends on the angle between the effective steering arms. For track rods in front of the axle, having the arms "toe in" will increase Ackerman and for trailing track rods, having them widen as you move away from the axle will do the same. If the arms are parallel, then you should get a neutral Ackerman angle. Presumably, the front arms on coiler axles, meant for the drag link, are 90 degrees to the stub axle axis, which would make sense to get even responses for left and right inputs, otherwise the steering would be more sensitive in one direction than the other unless the PAS box drop arm was set at a similar angle, so you get parallel effective arms and a neutral Ackerman angle..

That effective arm angle is not the angle between the swivel housing and centreline of the arm - the arm can be attached anywhere on the swivel and work. It is the angle between a line drawn from the track rod end eye to the swivel pin axis at its mid height, horizontally level with the centre line of the stub axle after considering swivel camber angle and setting the axle up for castor angle. Messing about with camber (can't with Def/RR axles) or castor angle (can be altered) will affect the effective arm angle, and this Ackerman.

[bill van snorkle]

Whilst this discussion has been interesting, I wonder how applicable or relevant any of the findings/theories are to the current Landrover scene? If the UK is anything like Australia, very few serious builders are messing with Series based axles, on which Ackerman angles can be experimented with, and coiler axles can't readily be altered due to the steering arms being cast integral with the swivel housings.

Further to my findings on my previous post. Series LandRover swivel assemblies have 1.5 degrees of positive camber. Super Swamper TSL and Simex ET tyres have very aggressive outer lugs. That would seem to be the worse combination for breaking through the surface crust in boggy ground, Yet my reversed steering arms, which should in theory give a much tighter steering angle to the outer wheel, actually give near enough to parrallel steering and the tyres didn't scrub or leave a heavier marking when making those 360 degree turns on the gravel over clay surface. Further more, when the vehicle was my daily driver, used extensively on sealed roads, I noticed that the tyres wore evenly and never developed heal and toe as they tended to with normal Ackerman angles.

[o_teunico]

Some pics I took that may be of interest in this thread. It´s a Santana IIIA front axle with discovery rims ans 7.50-16s. Sorry for the sun/shade.

[discomark10]

so it has reverse Ackerman to match the steering arm on the front rather then the back.

[dirtydiesel]

so it has reverse Ackerman to match the steering arm on the front rather then the back.

No it still has conventional Ackermann, just with the track bar at the front.

If you look closely at the pics you can just see that the track bar ball joints are just outboard of the swivel pins.

[discomark10]

yea, got it wrong in my head, still normal Ackerman, just on the front as you said they go out rather than like on the rear

[bill van snorkle]

so it has reverse Ackerman to match the steering arm on the front rather then the back.

No, that is the normal Ackerman arrangement with splayed out steering arms for forward mounted trackrod.