Jump to content
landroversforever

3 Link Questions, discussion and a brain dump

Recommended Posts

Excellent, keep us posted with pictures!!

Will do. I'll probably keep the 3 link discussion here rather than in my build thread :)

Share this post


Link to post
Share on other sites

Currently playing with the 3 link calculator with some of the fixed numbers to see where the others would like end up.

Not got this far yet, but do I need to reverse the +/- of the roll axis angle the calculator spits out? As it's designed for a rear suspension setup and I'm using it for a front design. 

Mike (DiscoMikey) has said it's a good idea to aim for the roll axis angle to slope to the front for helping turn in. 

So, I need to get some more numbers nailed down and see what it looks like! 

Share this post


Link to post
Share on other sites

My limited understanding is that with a high CoG and a low roll centre height, you get more body roll, less weight on inner wheels in a corner, and more chance of toppling over.

This is why when you lift a truck, it wants to fall over, you have moved the CoG up, but the roll centre has likely not moved at all.

Having roll centre and CoG the same gives you best solution, but whether that is possible on your truck is another thing... so getting closer together is likely all you can really do, maybe?

In theory with your roll centre lower than your CoG you could almost cancel out lean, but like I said, my understanding is limited, but may put you towards the right path.

:)

 

Share this post


Link to post
Share on other sites

Close... the reason a lifted truck tips is mainly just because of the raise in CoG, imagine a triangle the base is the wheels where they touch the ground the top point is the CoG. To make the triangle tip you have to tip it up enough for the CoG to wheel contact line to go over vertical now make that triangle taller (lift, bigger wheels, rollcage etc) notice how it has a steeper angle to the side of the triangle... doesn't take as much to tip 

Now because real world isn't as simple as that add in bigger wheels have more sidewall deflection so will drop that down hill corner of the triangle a bit more, lifted springs have a softer spring rate (more travel) etc, your suspension pivots.... this is where rollcenter comes in.... your suspension creates a point that the body rotates around in relation to the axle (each axle has a different height based on its suspension geometry) to calculate this you must look side on to the pivot points of the suspension so for us with beam axles that means side on to the truck (with independent suspension you are front on, this changes how rollcenter reacts against a tipping load.... and why a higher rollcenter can work for us) 

Lets go back to our triangle, the bottom is the wheels on the ground (lets assume they don't move) then we have the body ontop with the CoG as the top point of our triangle.... somewhere between is the rollcentre pivot point, now lets start with a low rollcenter as you tip the triangle the body is going to tip on the springs rotating around the rollcenter pivot moving the CoG out getting closer to tipping the triangle.... now if the closer the rollcenter is closer to the CoG you get less body roll swinging the CoG out to the side 

Now there is something that Bill put me onto about this relationship but I've got to get to work and I'm late now lol will add it tonight 

 

Share this post


Link to post
Share on other sites

That makes sense :), Thanks!

Now as I'm not relocating the Panhard in any way, my roll centre will end up being (for the moment I should add) about 1" above the axle centre line, and ~18" below the CoG. The only change here would be any suspension lift, which I'm going to try my utmost to avoid.

Share this post


Link to post
Share on other sites

Right just finished playing lol what a cracker night nothing like being an idiot... 

Anyway back to the thing Bill noted, as you make your rollcenter higher he found he lost feel for the tipping point and when it went it went faster with a higher rollcenter..... apply the model I noted above low rollcenter the body tips on the out till it gets to the piont the CoG is outside the triangle then it lifts the uphill wheel and falls, so your able to feel the body moving before the wheel lifts, on the flip side the body moving is what has moved the CoG out far enough to finish tipping it 

Now with a high rollcenter the body doesn't move as much untill the wheel is lifting, but you will gain some extra angle because the body isn't tipping 

 

Bill also noted that a lower rollcenter gave better climbing traction particularly in ruts, this is a bit of puzzle to me (not that he noted this, but as to why).... best guess is a form of suspension lock out due to the fact as you move the rollcenter up you increase the amount the axle swings sideways as it articulates, I'm guessing this sideways motion in articulation is being resisted by the side of the rut stopping the axle articulating making the opposite wheel light on the ground allowing it to break traction but this is a guess lol 

Share this post


Link to post
Share on other sites

Thanks again DeRanged.... all useful information to help me (and others) understand :)

Share this post


Link to post
Share on other sites

Another couple of questions:

Antidive: What sort of number would you guys be shooting for? I've got a load of link position measurements I'll be looking at this afternoon after work and with three adjustment holes on the Chassis end of the upper I have a choice of: 79.9, 69.9, 59.5. I've got a 49 as well, but I don't think that link position is possible from memory.

Materials and sizes: I'm looking at 2" OD 7075 aluminium for the links as that's being used in the Ultra4 builds. (On a theoretical 1000mm link it saves about 1.5Kg). Now for the top it would be useful if I can get down to a 1.75" Dia.... is this going to be too weak around the thread? (makes it roughly 1/4" wall).

Share this post


Link to post
Share on other sites

Lol your little two paragraph post is so loaded 

AD is something that really isn't that discussed online and books on the subject when it applies to a solid beam axle.... to be honest I've yet to find any that offer up any usable advice for long travel so the following is my personal observations and some comments I've found from respectable forum members 

I really only consider this something worthwhile for road going or off road racing vehicles for the simple fact of traction.... to gain the bonus of the virtual weight transfer to the front axle when braking you need a good hard surface.... this is what AD does is create a jacking motion that forces the front up as the torque loading from the brakes is applied to the axle and for every force there is an equal and opposite force so if the body is forced up there is a similar force on the tyres.... more weight on the tyres means potentially more traction which you can use to stop better 

It all boils down to traction now if your on mud, wet conditions or your tyres are full of mud whats the advantage.... there isn't one 

Now there is another bonus to AD again more for road going trucks this is the jacking effect can be used to counter some of the front end dive that comes from the inertial weight transfer that comes from deceleration 

I have come across some posts on pirate about AD and usefulness and there are a few that believe lower numbers are better for traction in 4wd better at climbing the axle over stuff (rock climbing).... in my yrs of  watching and tuning 4wd's offroad I haven't seen an advantage or disadvantage to do with different number and I concern myself with fitment more than a set of numbers and try where I can to put in adjust ability and in about the same range as you have selected for no other reason than that is the done thing lol interestingly most don't adjust them and the few that do I have some that liked it higher and others that prefered it lower and both for the same reasons "brakes better when ....." lol 

As for links and size lol if they are running that for Ultra 4 then they will be sweet, I can't comment on alloy links I haven't used them before alloy is very expensive and hard to source here and not as user friendly as steel     

Share this post


Link to post
Share on other sites

Not sure how its loaded? :unsure: 

I'll stick with trying to keep a reasonable amount of adjustability in the AD. Just thinking out loud here as I type... If the links are setup for AD, when under power climbing over something is the power going to 'lift' the axle?

My thoughts with the links are that if the 2" Ali can cope with being hammered in a racing situation it will get a nice easy life under my 90.

 

A development after last nights workshop session... 4" above the axle centreline is my absolute max for the upper link. I had been playing with 6" in the calculator, but unless the link was about 10" long and at 45o I would be going straight through the centre of the engine mount. 

Share this post


Link to post
Share on other sites

Its loaded because it is personal opinion lol 

In theory I agree it should lift, this is the odd thing the pirate guys who I refered to found the opposite they were running scores like 15-20% and found it was able to climb the axle up steps better, they didn't care about dynamics just results and they built alot of rigs and mod'ed them all the time and just changing AD on a truck was the difference between climbing certain tracks or not .....  I wouldn't setup a road truck like this as it would be a bit scary lol 

Offroad I have never been able to see a worthwhile gain.... going downhill you have gravity on your side and stopping on the way down is more about the level of traction the ground will give so having the ability to jack the front end to me is mout, to climb things you are either going real slow using engine torque or you are accelerating in which case the front is unloaded due to inertial weight transfer 

Onroad or racing where your on gravel roads or hard dirt traction is better and to me the AD is worthwhile to make the truck more stable by stopping the nose diving under brakes and I normally setup or recommend a median of 80% but it depends on the weight that transfer a rear engine truck for example I'd want less,  now these figures are what I'd shoot for on the front I'm more concerned that the links are as long as possible (reduces changes in handling due to large changes in angles as the suspension extends or bumps) is the most important and fitment around motor etc next then the score 

The other thing is don't forget link geometry is only a part of whole spring rate, shocks, swaybars etc all play a part  

Share this post


Link to post
Share on other sites

I've got the numbers I want to post, but photobucket is down for maintenance at the moment. 

Another thought I had had was regarding descents and a low AD. I'm thinking a higher AD would be of benefit in the case of a steep descent? As a low AD would exacerbate any issues of braking on a steep descent.

Share this post


Link to post
Share on other sites

Yes you are right but the bit so many people forget is when your decending your going slow, the steeper it is the slower your going.... next as you tilt the truck on its nose the links carry more and more of the weight instead of the springs which means your front end lifts, and so does the CoG (as the suspended weight goes from the springs to the links the springs will push the body out, both front and rear will do this so it is the same effect as discussed about rollcenter triangle and all except the base is wheel base instead of wheel track) 

So to me having more jacking effect from AD doesn't gain anything, that said the lowest setup I've been involved with was still around %50 (CoG was an estimate so not a reliable figure) and that was acceptable, driver didn't notice any change in feel 

I still don't know about the low score crowd on Pirate 10-20% I would need to play with that before I'd be willing to comment on it as anything other than extreme  

Share this post


Link to post
Share on other sites

So these are my current numbers I'm playing with. For those that are used to the 3 Link calc, I've added a few cells to give me some easy 'read outs' such as the ground clearance etc. The lift cell gives me an easy way of changing the height of all the chassis boxes in one go. 

In the changable boxs, the green text are items that are fixed, and the orange are ones that are pretty much fixed. 

56BABA6D-56AF-424D-8CBA-FA15C511656A_zps9omq6sgz.PNG.2ea7d727a6466e0005bda82916a7ce87.PNG

2688EC08-8310-4397-8B1A-0656CC7D0333_zpsuw2ysiui.PNG.855e52b16c82c5dfef796bea85b22e4f.PNG

Share this post


Link to post
Share on other sites

Sorry about taking my time to answer this.... been pulling some long hrs with work and its hard to find the motivation to think at night, and a bit of an issue with back pain and sleep lol, but I've had a day off to sleep so 

I'd be happy with the AD%, the short instant centre is a bit of a concern... give me your shock travel measurement and I'll work out you pinion angle change as it cycles 

Share this post


Link to post
Share on other sites

No worries, we've all got lives outside of the forum! 

What effect does the instant centre have on the handling? They're 11" if I remember correctly... OME +5" travel.

Share this post


Link to post
Share on other sites

Sorry long hrs and limited sleep due to ongoing pain that has been bad enough to sit me in A&E till 3:30am one morning (still went to work the next day lol) but anyway my head hasn't really been there... why I haven't replied till now one bad statement is enough lol  

A short IC on short travel isn't an issue, on long travel it means as the suspension cycles the axle needs to go forward/back this can cause some lock out of the suspension on high speed impacts.... don't stress your IC is low enough this wont happen.... I didn't pay enough attention when I made the comment 

There is another side effect of a short IC and that is diff roll is increased, I have run your links in CAD assuming you run 5" up and down travel and 1" of bump stop or jounce.... at drop you'll rotate your pinion down (clockwise sideways as per calculator) 3 degrees  on the end of bump stop 6" up travel you will rotate pinion up by about 8 degree's  

Estimating placement of transfer case drive flange near to the chassis mount for your lower link that gives you a Drive shaft plunge of slightly more than 3/4" and a total drive shaft angle change of 28 degree's, not certain on the rover UJ's but thats workable numbers for hilux stuff on a standard slip joint 

Oh and just on a side note the roll axis angle is nice angling up at the rear this means the front is slightly more responsive than the rear giving you a little bit of under steer slowing the steering a bit (a good thing on a lifted truck)

Share this post


Link to post
Share on other sites

Right, time for some discussion around the chassis ends of the lower links. I want to make the brackets bolt on as you all know so they can't be perfect. The following are my ideas for the moment.

First things first, please excuse the awful sketches... sausage fingers + ipad = messy!!

15226429_10154548096345837_1435478176_n_zpse8q6vnkv.JPG.063a03df4335631558203f851f8d9c38.JPG

The three bolt heads you can see are the bolts for the gearbox mounts. As standard these are M8, I'm going to open these up to M12. Does anyone know (to save me looking) if these have any kind of anti-crush setup? they're not tubed as far as I know but I would assume Richards put in some kind of folded structure.

 

15182495_10154548098135837_1806174466_o_zpsbndpde0d.thumb.JPG.7e1dc8a052efb9c1b4a1a7038378c45a.JPG

 

15233639_10154548098220837_924343118_o_zps9oti4yan.thumb.JPG.eb8582efe9fc50947a8d9c1ccbf6b308.JPG

 

 

15182441_10154548103790837_1130185515_o_zpsdg5zeqkk.thumb.JPG.55d374e1d37df891cdcab7aaa00236f6.JPG

 

 

15225406_10154548098310837_949306404_o_zpshac8evek.thumb.JPG.31b044881971e59df87c1babcc4b18f4.JPG

So the above picture will be an extra crossmember to tie in the two inner plates, its not a replacement for the normal gearbox crossmember.

Share this post


Link to post
Share on other sites

I'd welcome any comments on the design. Materials would also be useful, do those in the know think I'd be better off with something like a grade of Domex to let me thin the material?

Holes such as the ones for the suspension joints will be done with a large washer to increase the material thickness around the hole or some kind of webbed affair on the two chassis plates each side, to tie the 3 bolt holes and the joint hole together. Without adding a the weight of a complete second or thicker plate.

Share this post


Link to post
Share on other sites

Material wise, you won't gain much by going too exotic. BS EN 10025 S355 will be more than enough providing you design it correctly. - remember, standard chassis is only basic grade mild. You can make your structure as strong and stiff as possible but it's a lost caus if the structure it is attached to is made of cheese/a weak design

Would be worth ringing up Richards and asking them if there are crush plates in these holes. It is a must for anything as critical as this! 

Remember, (granny sucking eggs I know but..) thick flat plate is not stiff, and will fatigue much worse than a thinner plate which is shaped to increase stiffness. Use soft edges wherever possible on the weld detail. Start simple when you actually get round to designing it, get the structure right and then lightweight it, rather than trying to get fancy straight away. 

I wouldnt necisarrily say calculate the forces involved because they are a complete unknown, but take the basic concept and draw arrows along where you think the forces will travel then you will know what isn't stiff enough/is too stiff/where cutouts can be made.

will be happy to offer my opinion and advice wherever required, as you already know.

Share this post


Link to post
Share on other sites

As mentioned, those holes need tubing to take big loads. The main rails are only 2 mm thick.

I would completely support the front axle using the removable crossmember, like the qt 3 link system. Have you thought of a 1 link? That would solve a lot of problems.

 

Daan

Share this post


Link to post
Share on other sites

The construction you have drawn wants to turn around de bolts, so is completely dependent from the clamping force and not by a desired closed shape construction.. would not be my choice.

 

I understand you want to mount the link further backwards.

I would use in a way the original link mount hole to mount a sort of little subframe to and use the three M8 bolts for the gearbox mount to keep it in the right orientation.  So the horizontal strain will be taken by the original link mount.

So far I know the M8 holes are backed up inside by an anti-crush setup

Edited by Carloz

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy