Ackerman effect, steering ram infront of the axle?

[Steve 90]

Afternoon guys.

Just in the process of modifying the steering on my land rover. Really fed up of broken steering box’s and giving serious thought to a full hydro set up. That way I can have a strong setup that can easily be tuned for speed/force etc.

Does anyone have any experience in using a double ended (not the type Simon keeps under his bed) ram in front of the axle and its affect on the Ackerman effect?

My limited knowledge is that I need to have the steering arms as straight as possible and keep the rack (or ram in this case) behind the line of the track rod ends, This gives an angle between the steering arms and track rods of less than 90 degrees. The resulting increasing/decreasing angles when locking the steering around will give the difference in steering angles needed.

I have googled it quite a lot but had very few useful results, maybe I need to tweak the search terms.

Does anyone know of a calculator or can give me any pointers in calculating some dimensions to fit the ram and maintain (roughly) the correct Ackerman effect.

On my axles (Ashcroft force 9’s) with the existing (8” ctr to ctr) steering arms I can just about get the ram behind the line of the track rod ends and use an 8” stroke ram. I would like to check if this will give roughly the correct geometry. If not I will need to make some longer arms which of course will require a longer ram, different orbital etc etc.

Any help/pointers from anyone who understands the sytem or has any experience would be great.

Cheers.

Steve.

[Maverik]

Have you seen the hydro steering kit that Dave sells at http://www.llama4x4.co.uk/ ?

Mav

[Daan]

Well obviously, this:

http://en.wikipedia....eering_geometry

gives you the basic geometry. but with the trackrod in front of the axle, the ackerman principle is achived by angling the steering arms outwards, parallel to the angle as shown in the second schematic. moving the cylinder backwards will not nessecarily give you what you want, plus has problems like a diff in the way, and longer steeringarms will give you problems like your wheels in the way, especially when the steeringarms point outwards.

However, I think ackerman is a bit overrated, and I would try to achieve parallel steering geometry for a number of reasons: You can achieve a better turning circle on a landrover layout, and when going fast, it is more favourable because your backend always steps out a bit (ideally a 4 wheel drift I suppose).

Daan

[Orgasmic Farmer]

Mav, we have used that kit and various other hybrid kits together with 4 bolt boxes, D2 boxes, hybrid steering boxes and nothing is reliable enough to stand up to a multiday event like the Croatia challenge (we broke 3 boxes last year). At the end of the day the only more or less fit and forget solution is a full hydraulic steering system which is what Steve is looking into. However, he has real concerns about how the geometry will be affected when running on the faster sections (we got up to 80 mph at last years event and expect similar speeds will be involved at the King of the Valleys at times.

If anyone can suggest an indestructible PAS box then we would try and fit it but there appears to be nothing on the market that hasn't been tried and found to be wanting at the extreme end of the competition scene.

[ejparrott]

I guess you've tried 3 and 6 bolt boxes too? Only, through all my PAS planning i've had at least 1 guy adamant that the 6 bolt is way stronger than the 4, and another telling me its carp admitedly....

[Daan]

I guess you've tried 3 and 6 bolt boxes too? Only, through all my PAS planning i've had at least 1 guy adamant that the 6 bolt is way stronger than the 4, and another telling me its carp admitedly....

That is a good point, in the parts manual, the 6 bolt box is refered to as the adwest heavy duty box, sounds promising, but no experience myself.

Daan

[Maverik]

Mav, we have used that kit and various other hybrid kits together with 4 bolt boxes, D2 boxes, hybrid steering boxes and nothing is reliable enough to stand up to a multiday event like the Croatia challenge (we broke 3 boxes last year). At the end of the day the only more or less fit and forget solution is a full hydraulic steering system which is what Steve is looking into. However, he has real concerns about how the geometry will be affected when running on the faster sections (we got up to 80 mph at last years event and expect similar speeds will be involved at the King of the Valleys at times.

If anyone can suggest an indestructible PAS box then we would try and fit it but there appears to be nothing on the market that hasn't been tried and found to be wanting at the extreme end of the competition scene.

Intresting, have you looked at the broken boxes to see where they failed? - just a thought, then you know what to look for in a suitable box... I'd love to get involved with some of the engineering and fixing rigs on a challange like that.

[Orgasmic Farmer]

4 bolt boxes usually lose a tooth or 2, D2 boxes shear the shaft....

[Steve 90]

4 bolt boxes usually lose a tooth or 2, D2 boxes shear the shaft....

Other way round!

Steve

[Daan]

when you say loose a tooth or 2, does this mean from the hydraulic piston rack, or from the mechanical connection rack?

Daan

[simonr]

There is a very good book called Chassis Engineering by Herb Adams - which contains everything you need to know about steering geometry. Admittedly nothing about hydro steering - but the principles are the same using a double ended ram as a rack. This is the best source of info I know of. I would scan the pages, but I lent the book to Nick!

Bore size. The steering pump spits out about 1400psi - so it's easy to calculate the force developed by the ram. Accurate data on the breaking force of track rod ends is hard to come by, but lets assume it's about 1 Ton. Therefore, if you back calculate,

1 Ton = 2200 Lbs / 1400 = 1.5 sq in to deliver 1 Ton force

You need to take the area of the bore and subtract the area of the rod.

Pi(Bore / 2)^2 - Pi(rod dia / 2)^2 = 1.5 sq in

Assuming the rod is 1 1/4" ~ 32mm

Pi(Bore/2)^2 - 1.23 = 1.5

(Bore/2)^2 = 0.86

Bore = 1.86" or 47.3mm

2" Bore is close enough.

I know from experience a 60mm bore, 30mm rod will break Land Rover track rod ends. That delivers 4.6 Ton to the TRE's

The 60mm ram came from a Forklift. I replaced it with 50mm/30mm made for me by Steerforth Ltd.

Knowing the force that will be delivered through the TRE's also makes it straightforward to design steering arms which will not twist under full load.

Si

[BIG DAN T]

The 6 bolt is not the Adwest heavy duty box as it is not made by Adwest but by Gemmer. The 3 bolt is described as heavy duty but to my knowlede was never fitted to Defenders. It has all imperial pipe fittings and so would need custom hoses. I am not sure about its strenghth at the most extreme end of things but can say that i have never seen a broken shaft in one but have seen them fail due to the box caseing itself splitting when used with uprated PAS pumps and fiddled with pressure valves.

[mmgemini]

Aquote from a very famous car designer.

"The Akerman principle {SP} of steering is the greatest fraud ever perpetuated in the history of vehicle design"

Colin Chapman

[simonr]

There are lots of different opinions about Akerman setups which have different merits depending on the application and suspension geometry.

IIRC 6 Bolt boxes are no "stronger" than 3 or 4 bolt, just that they use a ball screw design which exhibits less wear and tends to be longer lasting which people interpret as "stronger"..

Si

[B reg 90]

Variuos issues involved in this.

Steering boxes break mainy (in my view) because the center of the tyre contact patch is usually way beyond the point at which the king pin line strikes the ground, i.e. you have a huge scrub radius. On 4x4's we need big tyres, so we space out the tyre with a bigger offset on the rim or spacers so that we still get a decent steering lock. In actual fact we need to use wider axles coupled with wheel rims with very little offset. Next time you are in a car park look at all the cars - you will find that ALL vehicles have the tyre centered on the king pin line, i.e. minimal tyre scrub. Look at the wheels - the mounting flanges will all be close to the outside edge of the wheel.

Now have a look at the front end of your modified 4x4. I'll bet that the swivel housing is easily seen (should be hidden in the wheel rim) and that the king pin line (imaginary line through top and bottom swivel bearing) doesn't even hit the tyre, never mind the center of the tyre contact patch. Now look at your wheel rims and see the monting flange way away from the outside of the wheel rim....

When you go for a fast drive and hit a bump/rock with a front tyre with:

The car from Tescos - The tyre contact with the rock is in line with the steering swivel line, no kick back created in steering

Your 4x4 - the tyre contact is offset from the steering swivel point and hence wants to sweep the tyre backwards - steering kick. You have to resist that at the steering wheel. This shock loads the steering box and the rest of the system and the driver. I belive this is why the majority of steering box failures occure on fast events like Croatia (3 new boxes in a week!!!).

Solution - Get your tyre contact patch on the steering swivel line. Regain your steering lock by widening your axles. You might find that the steering boxes then last longer.

However that does not cover being stuck in a bog and simply not having enough steering force to turn the wheels. The solution to this is to fit a disco 2 steering box- They give 40% higher tourque output. However this needs a 40% higher pressure from your pump - you need to fit a disco 2 power steering pump to go with it.

The above would in my view give the best survivabilty of a steering box system.I'm fitting full hydro....... However all the above still applies.

Adrian

[landroversforever]

So something like the ashcroft Force9s would help the situation as they are wider?

[Daan]

So something like the ashcroft Force9s would help the situation as they are wider?

The force 9 would probably makes things worse because the kingpin axis stays in the same position, or at least near. The Disco 2 axle, however has the kingpin axis set in further inside the wheel to reduce the above effect.

Although all of the above is correct and the 3 boxes broken in 1 week is not good, that shows that you are driving like a gorilla and therefore dont last 8 days no matter what car you turn up with. all you need to do is take it a notch back and you will finish. Alex started at pos 24 in Croatia, is now leading.

Hate to play the miserable old b@st@rd bit, but thats how this works.

Daan

[B reg 90]

The force 9 would probably makes things worse because the kingpin axis stays in the same position, or at least near. The Disco 2 axle, however has the kingpin axis set in further inside the wheel to reduce the above effect.

Although all of the above is correct and the 3 boxes broken in 1 week is not good, that shows that you are driving like a gorilla and therefore dont last 8 days no matter what car you turn up with. all you need to do is take it a notch back and you will finish. Alex started at pos 24 in Croatia, is now leading.

Hate to play the miserable old b@st@rd bit, but thats how this works.

Daan

Daan,

Agree to a point. To win at that level you need to push on where you can and know when not to push. However as moroe and more people get competitive the normal level of driving needed gets higher stressing the car more. Deliberately having a design defect and not driving as hard is a strategy, but not the best one in my view.

Check out Motorsport.com, go to offroad and select pictures. You will see a picture of Alex's multi million £ unlimited budget car. The alloy wheels have the mounting flange at the outside edge of the car, no scrub radius. Find a side view of Jim's car - Force 9 axles, so wider, but he still has wide offset wheel rims, i.e. loads of scrub radius - FAIL imho. I'm not saying that Jim would have not bust his steering with a lower scrub radius, that depends on how he drove. HOWEVER, he would have had to hit something harder than he did to cause the same breakages. That makes the chance of component failure lower and the chances of finishing the event higher.

Adrian

[fozsug]

.

Check out Motorsport.com, go to offroad and select pictures. You will see a picture of Alex's multi million £ unlimited budget car. The alloy wheels have the mounting flange at the outside edge of the car, no scrub radius. Find a side view of Jim's car - Force 9 axles, so wider, but he still has wide offset wheel rims, i.e. loads of scrub radius - FAIL imho. I'm not saying that Jim would have not bust his steering with a lower scrub radius, that depends on how he drove. HOWEVER, he would have had to hit something harder than he did to cause the same breakages. That makes the chance of component failure lower and the chances of finishing the event higher.

Adrian

Some great pics over there!

[fozsug]

http://www.motorsport.com/#!/offroad/photo/main-gallery/not-the-nest-place-to-get-the-winch-cable-jammed/?i=64&id=1317772&s=2

[Daan]

Agreed with your points regarding scrubradius, no doubt. Jim ran disco 2 axles last year I believe, and had no problems with this, obviously this year, he has. I am just saying leading on day 1 (like steve 90 last year did for example), means squat when you brakedown soon after. almost all the big budget guys are now out of the race, Alex is probably big budget, but drives very conservative, and has very little horsepower. People are turning this into a power race, and I dont believe it is about that at all.

Daan

[Steve 90]

So Daan, Your saying that Ackerman is not that important. Quite interested in that and If its correct will make the fitting process/my life easier.

My main concern was that when turning at tighter angles (When the ackerman is at its greatest) on a low grip surface the last thing I wanted was the tyres fighting against each other and destroying the small amount of traction we do have, causing the front end to push on where it may have turned.

The fact that the outer wheel can turn through a greater angle with parralel steering before the inner wheel hits the radius arm is something I hadn't thought of. Would the wheel fighting against each other cancel out any advantage gained or has it proved to not be an issue. Any experience anyone?

Very interested to hear more about the possibility of parralel steering and if you think it will cause issues when turning tight angles on poor surfaces.

Scrub steer, You are correct. The scrub steer on my truck is terrible. More so running Boggers, less so on the Simex (which we ran in Croatia) but still quite bad. I completely agree that the increase in load through the steering box due to this is massive and steering box failure would probably be almost a thing of the past if we ran center point steering. But, I dont know of a strong axle set up that will correct this without going down the line of custom Spider trak or similar.

So, a steering solution is probably the most cost effective/straight forward fix at the moment.

As far as driving style is concerned. These events are getting faster and tougher (Which is exactly what I like, which is why I want to do them). We have broken boxs with very hard hits, sometimes driver error, Sometimes bad luck. We have also had broken box's that have never had any shock loading that I wouldn't expect it to have to take through normal driving on this style of event. Probably all comes back to the scrub steer issue, but it is what it is so we'll deal with it.

If everyone ran a defender box then driving to the strength of a defender box and staying on pace would be an option. But, we are traveling at speed on tracks we have never seen before. Even the most careful driver is going to take a hit if they want to run at the front. As soon as people build trucks with stronger steering systems they are going to get away with pushing a little harder. You either have to let them go, Risk a DNF and keep up or build the truck to survive.

A few years ago people were having the same discussion about standard half shafts and driving in a style that will not risk breakage, Things move on, so do the trucks. If someones truck is stronger you cant sit back at a safer pace waiting for them to break something..............and most of all, whay would you want too whan you could have strong steering and drive fast :D

So, the threads not about driving within a defender steering box's capability, Its about making the truck stand up to the inevitable hits it will take on these events and mainly the geometry when fitting Hydraulic steering.

I think the question is now, Will Parallel steering be ok???? I hope so!

Steve.

[steve200TDi]

Daan,

Agree to a point. To win at that level you need to push on where you can and know when not to push. However as moroe and more people get competitive the normal level of driving needed gets higher stressing the car more. Deliberately having a design defect and not driving as hard is a strategy, but not the best one in my view.

Check out Motorsport.com, go to offroad and select pictures. You will see a picture of Alex's multi million £ unlimited budget car. The alloy wheels have the mounting flange at the outside edge of the car, no scrub radius. Find a side view of Jim's car - Force 9 axles, so wider, but he still has wide offset wheel rims, i.e. loads of scrub radius - FAIL imho. I'm not saying that Jim would have not bust his steering with a lower scrub radius, that depends on how he drove. HOWEVER, he would have had to hit something harder than he did to cause the same breakages. That makes the chance of component failure lower and the chances of finishing the event higher.

Adrian

Which one is Alex's?

Steve

[B reg 90]

Which one is Alex's?

Steve

I'm reffering to this one:

http://www.motorsport.com/#!/offroad/photo/main-gallery/alexey-goubev-russia-22-minutes-to-hunt-down/?s=2&id=1320929&i=8

Adrian

[B reg 90]

My main concern was that when turning at tighter angles (When the ackerman is at its greatest) on a low grip surface the last thing I wanted was the tyres fighting against each other and destroying the small amount of traction we do have, causing the front end to push on where it may have turned.

Steve.

In my view you want akerman steering. Colin Chapman was circuit racing where the steering angles are only a couple of degrees. When your driving on the limit (tarmac) the rear is slipping any way, the difference between akermann and non akerman steering angles is fractions of degrees etc, then I can see the logic in the arguement.

However, we are looking to operate:

1/. Larger steering angles

2/. get traction at larger steering angles

3/. not tear up a boggy surface (i.e. float across the top on a peaty bog as in Ladoga) when steering

Having non akerman steering will result in one tyre scrubbing when steering, i.e. usinging up some of your precious traction, tearing the ground surface, etc. Therefore if you can manage it you want 100% akermann.

That said I am in the process of fitting hydrosteer to a volve C303 axle. I can't get 100% akermann with out compromising other issues/within the constraints of the design. I am therefore going to get as much as I can.

Adrian