Hot Rods Wishbone 3-link information please

Reaching out to @Kerrynzl, @ELpolacko, @exwestracer, @Ned Ludd and other gurus who will be able to shed light on the following subject. Please don't feel rejected if I have not named you above. All hambers with accurate and relevant information are invited to please join in. I hope this thread will be illuminating and educational not only to me, but many others as well.

I am chasing info and guidance on a wishbone 3-link rear suspension setup I would like to try (if it is possible and will even work).
Bottom wishbone link (1 of) attached inside frame on each side and under the middle of diff centre. This would effectively be a triagulated link and negate the need for a panhard bar (???).
Top links (2 of) attached outside frame running back to mount above axle tubes at outer ends.
The following images are the general idea, but I want to switch the top and bottom arm setups. Is this able to be done? Application is for low to mid power street use. Must handle well and be a reasonably comfortable ride. Info on arm geometry and lengths would be appreciated, especially with regard to recommended anti-squat and roll characteristics.

So, what is the concensus. Really hope we can keep this on track and informative. There are lots of us out here that are all ears and ready to learn.

grumpy65

 

Anti squat would be determined by the angle front to rear of the links as viewed from the side, lateral roll center should be determined by the height of the lateral locating link, if you hook a wishbone to the bottom of the axle housing you will have a very low rear roll center and I suspect the car would tend to understeer or " push" as we call it in dirt modified racing.this should be able to be overcome with a rear antiroll bar .

 

If you type an @ before the member name, it creates a "mention" of the user, so that they are alerted to the post.

Like this: @Kerrynzl, @ELpolacko, @exwestracer, @Ned Ludd

 

Definitely not a guru in any sense, but without going into roll centres, centres of gravity etc., simple geometry would suggest that you are going to get a major pinion angle change through suspension travel, which can't be a good thing.

Bear

 

Thanks for the input @King ford. Will look into rear (and front) roll centre and antiroll bar as you suggested. If front roll centre was also low would it help? More reading to do.

Thanks also to @gimpyshotrods for the posting tip. Most helpfull. See, I am learning already.

@Surf City, thanks Bear. If the setup was done so the upper and lower links were the same length (when viewed from the side), and running close to parallel with the ground and each other, would it not perform sort of like a parallel 4bar and keep pinion angle constant? Of course anti-squat would also be incorporated with arm angles. Is this reasonable or am I missing something?

 

Hey gimpyshotrods, should I redo with the @ or will what you have done do it for me.
Thanks again.

 

King ford, is the low roll centre caused by having the centre arm mounted under the diff centre? I will be able to set this up the other way and have the single link as the upper if needed. Will that help?

 

He's already done it [I got the alert, and learned something here also]

Having the triangulated link pointing rearwards [1st photo] will keep the roll centre height consistent regardless of ride height [ the RC is attached to the rear end ]
Having the triangulated link underneath lowers the roll centre which will soften the rear and induces some understeer but stiffening the rear roll stiffness would remedy this [or softening the front].
Having the roll centre low increases traction on the outside tyre during cornering [overturning moment] so this is generally more desirable.

 

Thanks Kerrynzl. Glad I got you. I have been reading a lot of your posts on other threads and think you will be a big help to me. You have talked about this setup as being the same as lotus cortina. Will there be any issues I should consider using this in a street use application?
Steve

 

@Surf City, thanks Bear. If the setup was done so the upper and lower links were the same length (when viewed from the side), and running close to parallel with the ground and each other, would it not perform sort of like a parallel 4bar and keep pinion angle constant? Of course anti-squat would also be incorporated with arm angles. Is this reasonable or am I missing something?[/QUOTE]


Sorry, misinterpreted your original post. Thought you were considering running the top links rearward (running back) from the rear end to the frame.

Bear

 

Sorry, misinterpreted your original post. Thought you were considering running the top links rearward (running back) from the rear end to the frame.

Bear[/QUOTE]
No worries. Thanks Bear.

 

<<<< My modified runs a 3 bar + panhard bar rear suspension. Two bottom bars go forward to under the frame rails mounts. Upper runs down the driveshaft tunnel, next to and parallel with driveshaft and goes to trans mount crossmember. The panhard controls sideways movement. Works great.....my seats are sunk way down in the area where most split wishbones/triangulated links are taking up space.

 

Have seen pics of yours on another thread and the seating position you have acheived is one of the things I am after. I think this can be done with a rearward pointing wishbone at the bottom and straight top links on each side. With the low rear roll centre this will create, would I need to keep the front roll centre very low as well (lower than rear)???

 

The only issues the Lotus Cortina had was the body "coke canned" because it wasn't designed to take loads from coil-overs [mounted in original shock mounts]
Lotus was obsessed with weight savings so everything was engineered to a minimum.
On the Cortina the outer links pivoted at the original front spring eyes, and at the other end they were mounted directly in front of the axle [not below as in conventional 4 link style]
By mounting the link in front, the outer links transfer ALL the thrust loads to the frame during acceleration
The lower triangulated "A" frame only needed to handle compression loads from pinion torque reaction [minor] and much greater tension loads from brake torque.

The Lotus Cortina had a very low roll-centre which made it a very successful racer due to the fact it kept both rear wheels planted [without a slipper diff]

 

Sounds like a system with benefits. Knowing that the rear roll centre will be low, what should I be aiming for at the front. Assuming I should try to keep the front roll centre low as well to maintain overall balance, but would then need higher spring rates all round to compensate? Will this make it handle well but let go unexpectedly at some point towards the limits?

I have the option to have the wishbone arm at the top pointing backwards if needed. Would this be a more user friendly way to go for general driving? This is being designed to go in a T-bucket which will already be light and have a relatively low centre of gravity. I don't want lateral grip to let go like a light switch.

 

Firstly, GRIP and HANDLING are 2 different things. Many bad handling cars can be simply improved with better tyres. But a high performance tyre with low slip angles will let go "like a light switch" once it's limit has been exceeded.[old X-ply's "squeal like a pig" way before letting go]

A slightly understeering car is more desirable than an oversteering car. you will have a tendency to apply the brakes in panic situations. And if you have the skills , apply throttle oversteer on corner exit [this always makes me grin]

Now a common myth is bodyroll is bad and novice designers will use a high roll centre to prevent bodyroll. This applies lateral forces to the tyre footprint without adding vertical forces [so the tyre will slide more easily]
The laws of physic are always there. You will always get lateral acceleration, so try and harness these forces as increased grip.[hence the low roll centre]
The front and rear RC's actually make up a roll axis which the CG [centre of gravity] rotates around.
Roll resistance should be via the springs and bars [anti roll bars ,or "ARB's"]

Having a wishbone at the top creates a high roll centre, you will need a very low roll centre in the front AND soft roll resistance in the rear [the ratio of front to rear roll resistance is normally called "roll couple"] This high rear RC low front RC is common on most domestic cars which is why they normally have only a front ARB [and the myth that you should have an ARB on the front only]

On the front, I have always considered using a P & J type 4 link BUT triangulating the lower 2 links in the centre of the I beam [a Lotus Cortina type suspension in the front] You can use longer shackles on a crossleaf spring to prevent binding and "shackle wobble" would almost be eliminated by design

 

Thanks Kerry. Starting to get my head around it, I think??? Doing a lot of reference reading in between posts, and your patience and expertise is greatly appreciated.

The front triagulated 4 link you mention would put the roll centre at the centre of the axle, correct? If a parallel 4 bar system was used, could the front roll centre be adjustable somewhat through placement height of the panhard?
Would either of these systems get the front roll centre close to where it should be to match the low rear for balance (and where do you recommend the front roll centre should be)? Thinking that if the centre of gravity axis was level, roll axis should be level or slightly down at the front. So should roll axis be parallel to CG axis ( or slightly down at front), or am I on the wrong track?

 

Doesn't sound like any traditional build going on here. The closest would be a arm running from a banjo center section forward to strengthen rear end when removing torq tube and going with a open drive shaft. This will get you in the ballpark.
http://www.how-to-build-hotrods.com/3-link.html

 

The roll axis and the CGH [ or CG axis ] do not need to be parallel.
Weight transfer is : Wt x CGH x G's ÷ Tw
Body roll is: Wt x [CGH- RCH] x G's ÷ Tw
[ G's is lateral acceleration or G forces. Tw is Track width measured at the outside edge of the tyres]

Now here is [hopefully] where the penny drops.
If there is a greater distance between CGH and RCH there will be more "overturning moment" [bodyroll],
And the lesser distance between CGH and RCH there will be less "overturning moment"

If the front has a lower RCH it will try to bodyroll more degrees than the rear, But it cannot because the front and rear CGH are tied together in the same chassis.
So the way to balance this is to have more roll stiffness in the front [either stiffer springs or stiffer ARB]
OR............. Get the rear roll stiffness to support the front [through the frame]

Now if we go back and look at the tyres [in particular , the slip angle] The more weight you load onto the tyre "AT THE LIMIT" the greater the slip angle.
So a front heavy car with a high FRONT roll stiffness will try to go straight ahead on a corner [Understeer].
If that same car had a lower RCH or softer roll stiffness it would bodyroll more degrees at BOTH the front and rear. This would transfer more weight on the outside rear tyre which increases the rear slip angle [Oversteer]

The roll stiffness of the front and rear [Roll couple] must be matched to the appropriate CGH's and RCH's. [which is why racers carry a s***load of springs for testing, and have adjustable ARB's ]
-----------------------------------------------------------------------------------------------------------------
A triangulated link isn't favourable with racers, because it is either above or below the rear-end.
A popular method is the Watts-link [with a 4 link] but they cannot be mounted too low because of the Bell-crank.
There is a modified "Watts-link" called a "Wob-link" that can get very low RCH's [designed by one of the greatest suspension guru's..........Arthur Mallock]
Attached is a drawing of the "Wob-link" [the short arm has a greater Arc than the long arm, which is cancelled out by the bell-crank causing a vertical path]

 

Thanks 1946caddy. Pictures are food for thought and the link is definately useful. No, I guess it is not very "traditional", but the info Kerry and others are supplying is relevant to all builds and if we all learn and benefit from it, it will make everyones rides more pleasurable and maybe safer. A good thing surely, traditional or not.

 

Have been thinking about this all day at work and picturing how it all relates and works together. What you have explained above is where I got to (finally). Kerry, thanks for going through all this. It is no doubt basic and low level stuff to you, but we all have to start somewhere. I am sure that anyone following this thread is appreciating the knowledge you are sharing.

I was talking to a guy today that does some circuit racing and ran it past him. He asked what I wanted to acheive with the T-bucket. I said that if I could get it to ride and handle at least as good as my XR6 Falcon, I would be happy (for all in the US, a large 4 door sedan with a mild sports suspension package fitted from factory). He laughed and stated that I was setting the bar very low. He said it should be very easy to acheive this in a T-bucket without worrying about race car technologies. He then politely called me "obsessive". This is a word I hear my wife use regularly as well. I think it means "easy going", or maybe "good looking". I might look it up when I am not so busy making sure everything is in order and done right. Anyway, long story short, am I possibly over-thinking this? It won't be a race car, so will the basics do?

 

Kerry, should the suspension be set up to load the outer front tyre more than the rear in cornering, or does this happen naturally due to CG momentum (inertia) and CG being towards the front of the vehicle anyway?
Or should we aim for equal load to both outer tyres?

Also, is it an advantage to have fixed roll centres (as opposed to floating such as in a wishbone independant suspension)?

 

Pay no mind.

Poor handling cars are not only very traditional, they are dangerous, and miserable to ride in on today's roads, with today's drivers.

Some think that this should be an enforced condition for owing a rod or custom.

It should not, ever.

 

Was not meaning to tear a strip off 1946caddy. If it appeared that way, I apologise to him and also to all who are along for the ride on this thread. Each to their own. We should all appreciate the whole spectrum of rods, from traditional to (how shall I put it) "outside the box". There is surely room in our sport/culture for all, some as "custodians of tradition", and some as "seekers of advancement". One is no more important than the other. The primary concern should surely be to build what YOU want to drive and enjoy. Selfish attitude? Maybe. But if potential newcomers are coralled into one train of thought, they may never build anything. That would be a shame.

 

That is what "roll-couple" is all about. If you load the outer front slightly more, it will understeer. If you load the outer rear more it will oversteer.
For slalom type racing or twisty hillclimbs it is preferable to have the car more "pointy" [oversteer].
In just about all other situations a slight understeer is preferable. This allows the use of the throttle on corner exit etc.

Getting a T-Bucket to "behave" [not just handle] as good as an XR6 is a reasonably high goal. You are referring to a modern road car that is quicker around Mt Panorama than Alan Moffat's race winning GTHO.
Setting up a bucket would be similar to setting up a Lotus Super 7 [weight distribution]

The biggest mistake in building a car, or setting up a car is the "Wrong Choice of Springs" [but is also the easiest to tune, if you don't mind swapping parts]

 

Just been thinking of a great man with great drive and vision. He was obstinate and stubborn in his ideas, and forged ahead on the path of his choosing despite what others thought about the way he did things. He was definately a "ground breaker" and marched to his own drum. We should all be very thankful that this man went against the grain and did things his way. His name was Henry Ford. (July 30, 1863 - April 7, 1947) Where would we be without him???

One of his quotes really sums it up.
"If I'd asked my customers what they wanted, they'd have said a faster horse".

 

Ok, Thanks again Kerry. Will keep the thinking cap on. I thought my goal was high but, hey, shoot for the moon!

Test time.
Centre of gravity will be where it ends up. Try to keep it as low(ish) and as centred in the wheel base as possible. Roll centres kept low. This gives a greater roll moment (think "lever") and will induce more roll due to the lateral force rotating the CG around the RC. This is good as it places a downwards load on the outer tyres in the turn. The actual shift of the mass of the body does not contribute much at all to the equation, it is more about the lateral force acting on the "lever". Tune the application of the downward force front and rear with spring rate and damping. Tune the body roll to taste with spring rate and antiroll bars. This will be acheivable with my build as I intend on using coilovers in the rear and torsion bars up front.
How did I do???

When I get all this drawn up, do you mind if I pm you so I can send it to you to have a look over?
Regards,
Steve

 

I love this thread. I have no idea how to do the math. But it's starting to make more sense. Gary

 

Build it as low, wide and lightweight as possible and the G's will increase [Weight transfer is : Wt x CGH x G's ÷ Tw]
Now I am going to give you something to cause insomnia
Roll stiffness should be treated separately from suspension stiffness [frequency]

If your car has a limit of 1G cornering, it is the same weight transfer at 30mph or 100mph [at 30mph would be around a tight hairpin, 100 mph around a sweeping curve]

Now the faster you go. the stiffer the suspension is required [the car will be hitting bumps at higher speed so the springs need to absorb more energy]

Roll stiffness is usually a combination of suspension stiffness and an ARB
At lower speeds with a softer suspension, the car needs a stiffer ARB to control the weight transfer.
At higher speeds with a stiffer suspension, the car needs a softer ARB to control the same weight transfer.
A common mistake is too add a stiff ARB to stiff suspension.

By all means PM me

 

I guess I may have passed the beginner's test as now you seem to be stepping it up a level. Fortunately, the more we get our head around a subject, the more we are capable of advancing up the ladder. Still, give me a bit of time to research and understand this new piece of witchcraft you have put forward. I have no doubt it makes sense. I just need to have it make sense to me, and I will. Bring on the insomnia. It will give me more time to read and think!!!