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Might be more for one of the scaler forums but it's been a long time since I posted on those and I'm not even sure I still have my login deets.

Anyhoo, I know there's a few good custom rig builders on here so maybe some good advice for me :)

I'm building a Class 1 scaler using an old Bruiser body, Maverick Scout axles and transmission and TCS X-trail chassis rails.  It has 85mm shocks and is just on the tall side of "about right" for the type of look I'm going for.  I've mostly replaced the Maverick links with custom (or modified the existing) to get the right wheelbase.

At rest the truck looks perfect.  The wheelbase is spot on and there's plenty of ground clearance under the middle of the chassis, even with fairly small 90mm KRT tyres.

40685277975_bd43d6e1fd_k.jpgIMG_20180420_114847 by Mad Ax, on Flickr

Ignore the kinky link in this photo - that's stock, I'll be replacing it with a straight link when I adjust the wheelbase

The problem is, at full compression, the axles move laterally and push the wheelbase out.

27708391938_c8708dd6a0_k.jpgIMG_20180420_114907 by Mad Ax, on Flickr

I know this is generally a problem with 4-links due to the arc described by the lower link, it's always going to shift the axle a little bit under articulation.  Now I've been trying to engage the 3D modelling aspect of my brain, but it currently seems to be out of action (it's probably suffering an incompatibility issue with the nighttime baby support bioware I installed a year ago) and I can't bend my brain around how to minimise the issue.

So, do my bottom links look right?  Should I relocate my top links to reduce the problem, raise the mounting point on the chassis or lower it, or will I just have to adjust the wheelbase to get a good compromise?

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2 hours ago, Mad Ax said:

Now I've been trying to engage the 3D modelling aspect of my brain, but it currently seems to be out of action (it's probably suffering an incompatibility issue with the nighttime baby support bioware I installed a year ago) and I can't bend my brain around how to minimise the issue.

Priceless!:lol:  Anywho, while I'm no expert on 4-link geometry, a question occurred to me that may have bearing (or not) on the situation. Are all your links the same length? Thinking back to all my 4-link monster trucks, all the links are the same length and I don't recall any radical wheelbase changes upon suspension compression with those. I'm totally shooting from the hip here so I'm not sure the difference it makes. Hopefully some more knowledgeable folks will chime in. 

PS, I agree that the arc swung by the lower link dictates that there will be some wheelbase extension regardless however. Nature of the beast, I suppose. 

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The top links are a fair bit shorter than the bottom links.  This is partly due to where the upper links mount on the axle and mostly due to where they mount on the chassis - the TCS X-Trail chassis is a kinda scale chassis rail setup and you wouldn't normally have rear links disappearing up into the passenger area.

Part of the issue is possibly the amount of suspension travel - a 1:1 truck probably wouldn't sit as high as mine does without some serious suspension mods, and really mine is only that high because that's the size of shocks I had (I fitted those cheap alu 85mm shocks today, up to now it's had a really old set of 95mm inner-spring shocks from TCS or RC4WD or something like that, but the springs were way too stiff for this rig and I think they're out of manufacture so I can't get any softer springs).

It's probably mostly compromise - either have it look a bit short on full extension or fit shorter shocks to make it a bit more scale.

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Yeah, I think the different link lengths may result more in axle angle change, i.e. rotational (caster and/or kingpin inclination type of thing) more so than wheelbase change.

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It comes down to simple geometry, and how you have mounted the links... See, with one end of the link being fixed to the chassis, it creates a pivot point, so the other end does not travel straight up and down, it travels in an arc or circular motion from that fixed point.. If you have a spare link, place it on a piece of paper, hold one end of the link on one place, and then put a pencil in the other end of the link, and move the link up and down, and you will see that it creates an arc (it will work like a compass)..

Basically, your truck changes wheelbase as the suspension moves, and its an inherent issue with a 4 link suspension..... Shorter links are worse, longer links are better..

To lessen the problem, what you need to do is lower the pivot point on the chassis so that the links are level or parallel to the ground when your suspension is at half travel..

If you then want to take it one step further.. Setup your shocks to have some droop (down travel), and set your ride height so that the links are level .. Then when your suspension compresses or goes to full travel, the wheelbase will shorten slightly..



 

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Thanks @Backlash - that's awesome.  I had my head around the 'describing an arc' or 'working like a compass' bit - it was the relationship of top and bottom link that I was struggling to work out.  The bottom links are quite long but the top ones are shorter, it might be that the top links are pushing the top of the axle out under compression.  I'll give it a closer look next time I get some workshop time (maybe tomorrow evening).

Unfortunately the TCS X-Trail chassis doesn't allow me to mount the lower links horizontally unless I want next to no travel - it seems a bit limited like that.  I only used it because I had one over from a previous project.  Annoyingly it's also a little too long - the front shock are going to be angled forwards slightly to reach the shock hoops.  The only solution I can see is to cut off the stock hoops and mount some homebrew ones further back.

I'm trying to go for a little static sag but it's hard to tune the suspension when I only have random collections of springs lying around.  I was originally using long internal-spring shocks from another project, I got them about 10 years ago and never used them, but the springs were way too stiff - even fully loaded the truck wouldn't articulate at all.  Not only are the springs too stiff but the shocks themselves bind - they are air shocks, there's just a film of lube inside.  I used some Associated Green Slime on the shafts which helped (that stuff is magic) and even drilled the piston to reduce damping.  The internal spring is an odd size barrel spring and I couldn't find anything else that would fit.

The shocks I've used are cheap alloy units that came on another project.  One has never had oil in it, was had residue but was empty, the others had a very stiff oil (not sure why, they were on a CC01).  They'd been installed with (I think) standard CC01 springs that were too short and not giving full travel but I found some very soft longer springs, not sure what they're from.  With all shocks running empty they now give just a little bit of sag.  I'll probably add some weight to the chassis, especially up under the bonnet where there's a lot of space.  I've left some big bits of delrin there so I have something to screw my mass to.

:)

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The other option you have is to go full droop on the suspension... That would mean that the truck was effectively sitting on its bump stops (no up travel) at normal ride height, which "should" allow you to set the wheelbase exactly to suit the body, and also lower the whole truck (thus lowering its overall COG) which will make it sit like a real late 80's early 90's Hilux still sitting on its original saggy springs.... Any down travel in the suspension will shorten the wheelbase slightly, but it will also increase the gap between the top of the tyre and the wheel arch of the body, so it should not be as noticeable...

The one negative thing that running full droop will do, is make it ride rough (bouncy) when driving on the trails, as there will be no up travel in the suspension to absorbs the bumps (a bit like the original Hilux and Bruiser 3 speeds.. LOL.).. Some softer tyres may help it ride better though..

Its probably not going to matter much on a crawler, but if your top links are a lot shorter than the bottom links, just check to see that the diff is not rotating (excessively) as it moves up and down.. Rotating the diff can change the pinion angle and also alter the caster of the steering...

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