best setup for hitting sides

thanks for all the help and suggestions

they should rename this the engineering forum, this seems to be where they all hang out......


im building a sider as we speak, its gonna stomp the competition all year..... hopping competitions are dead, its the side hitting comps that are the shit these days.....

Originally posted by bdroppeddak



im building a sider as we speak...

mmmm... I love sider. My favorite is Dickens brand.

I hit pretty good sides and i have a wishbone with super pivot's.

Edited: 12/11/2006 5:54:40 PM by grim

Originally posted by BioMax



Originally posted by grim



Edited: 12/2/2006 9:33:10 PM by grim

By design a triangulated 4 link is supposed to stress under side flexes to hold a vehicle stable. In other words, by design they are not for hitting sides.

I'm not sure what you are tring to say here, but parallel or triangulated will allow the same amount or articulation. The "twist" that the bars see is the same. The only difference is that with a watts-link the roll center is at the center of the link it self and with triagulated bars the roll center is at the instant center of the triangulated bars (simply speaking.)

If triangulated 4-links didn't work well for articulation then the off-road guys would be in a heap of trouble, because a giant watts-link isn't the most efficient means of limiting a long travel suspensions' lateral movement.

Hmm..

Your response brings an interesting argument to the table for me, one part of your view goes against design theory I've studied - let me know what you think of this?

I understood your stance as this - "the only difference in the 2 designs is that a watts linkage limits travel and changes roll center to a different location" (def. correct me if I'm wrong in my summery). The part that catches my eye is not the correct roll center fact and lift limitations under extremes (like your offroad vehicle) but the - "no other difference" stance. Let me paint the picture from my side of the world, and compare theory so we might not confuse others with different conclusions.

With the upper links the same distance pivot point to pivot point (front to rear) as the lower links (to avoid pinion angle change throughout articulation) the upper bars on a triangulated design are longer by design due to the angle of the bars. From my research I have been led to believe when a side articulation is stressed on the design the longer upper bars have a stress reaction on geometry that is different than the stress reaction of an all equal length design (parallel). This leaves me with a question for you.

From your research why do the longer angled links not effect the rear end or stress differently when a side stress is pulling the rearend rear to front on one side from the non angled shorter bottom link?

I think I am following what you are asking.

The effective length of a bar is only that which is measured from a purely side view. From that view you can not tell if it is a 4-link, 3-link or triangulated. Reguardless what the final bar length is it is only as efective as measured front to back.

You are correct in assuming that the longer bar will effect things while "hitting sides." It is called roll-steer. I have already talked about the Satchell link and it's roll understeer advantages, but not really why. This ^^^ is why. On a properly designed Satchell link the upper and lower bars are angled in opposite directions, done properly, creating a stable roll understeer condition.

All designs have roll steer. Period! Some have more, some have less, but all have it, even every independent design. All 4 bars being parallel will mostly be effected by the travel of the suspension and only slightly by lateral bar movement. An all parallel 3-link has even less roll steer.

Roll steer on a foward 4-link (or 3-link) of just about any design, where the bars are parallel to the ground at half travel won't really have enough roll steer issues to concern yourselves with. But taking it into consideration can make for a very stable/confident feeling vehicle.

I hope this answers your questions and/or makes sence.

I'm sorry I should have also clarified my statement about the only difference between the watts-link and a triangulated link being the roll center. There are many other differences, but none having to do with bars or joints in a torsion load.

Watts-link or triangulated, both systems will "see" the same torsion loads on the bars and joints.

Okay one more...

A foward facing link system can be designed to excell at whatever you may desire and after chewing on what grim has said about triangulated 4-links for a bit, I am going to agree that the parallel 4-link with a watts-link (properly set up that is) would be the best for hitting sides (as long as it uses heims or super pivot joints.) ONLY because the roll center can be placed exactly between the upper and lower bars to eliminate as much roll steer as possible.

But realize that the vehicle would be a "sider" first. It would lack in handling (comparitavely) and any benefits that having a foward facing instant center would offer. I feel that any well thought out system can hit sides and offer other performance advantages without such compromises.


But ultimately grim is right about the best design.

Edited: 12/12/2006 8:41:24 AM by grim

[ 12-12-06 - I've added in Parenthesis with clarifications to keep semantic confusions to a minimum - I wrote this quickly but now that we are comparing ideas & clarifying in a scientific analysis I'll clean it up with added notes to help everyone follow the debate - the best of luck to everyone who has been looking for this kind of info on your builds]

By design a triangulated 4 link is supposed to stress (geometry) under side flexes to hold a vehicle stable. In other words, by design they are not for hitting sides (geometry wise, as much as a parallel system on a watts linkage). In order for a side to side motion to take place one of the triangulated bars must stress (geometry) at some point between its articulation pivot points. In my experience a properly setup fwd facing parallel 4-link and a correctly setup watts linkage is the best for about anything (modestly adjustable that's) rwd. The bars may rotate a bit upon hitting sides but by design it wont try and bend the link bars (I'll address that right now below) or stress the mounts (on the rearend) in a pushing motion (geometry). And the watts linkage will keep your rear end centered like a triangulated setup to avoid side shift issues or dog tracking. But that's just my experience, you'll find many build opinions; take your time, and good luck on your search.

Also I roll a slip drive shaft on anything adjustable with a carrier barring, just to keep any tension out of em'. -----------------------------------------------------------------------------------------------------------------------------[Hey thanks for your time and knowledge Max, this is a great learning debate for minitruck design. This fundamental topic over the years I've seen (as I'm sure everyone else has) turn to heated debate more times then I can even count. I'm sure this topic will put a lot of heat to bed and leave the better risk to benifit analysis as the deciding factor in many design discussions]

If you wouldn't mind I'd like to address your point on "the effective length of a bar is only that which is measured from a purely side view". I totally agree with that in terms of what engineering takes place in a simple side view (a straight up and down un-tilted motion wouldn't be effected by bar length because pivot distance remains the same front to rear). But I've read conflicting opinions back and forth on this -one view has seemed plausible to me-> that the 2 longer angled links effect each other in a strange way on side motions, what do you think of this next paragraph? For example when roll-steer starts taking place the longer bar on the side of the forward roll steer seems to me to cause a pushing motion on pinion angle from the top angled link. When this pushing motion starts to occur the opposite angled link seems to be under tension in addition to normal rotation stress. This theory is how in the past I have explained to myself the habit of triangular designs breaking off link tabs of the opposite link during a side-to-side motion. What have you discovered about this?

I see what you are trying to say now.

What you are thinking happens, doesn't. The triangulated bars act more like a wishbone does. The wishbone can not do what you are thinking because it is a triangle and shouldn't change shape to create the situation that you are concerned about. The triangulated bars act the same way. They can not change the shape of it's set triangle (unless the tabs break) to cause the binding you feel happens. The rear end swings around the point set by the trianulated bars the same way a wishbone set-up does. The lower parallel bars are just along for the ride. By changing the angles on the lower bars you can change the roll steer effects. The pinion angle is only slighly effected by all of this.

As for the tabs breaking, I think it is probably a case of an under engineered part. The builder not appreciating the loads placed onto the tab to keep the vehicle from moving front to back and side to side.

Does this all make sence?