Jump to content

The Suspension Thread


Recommended Posts

THIS THREAD APPLIES MAINLY TO SETUP OF A GRIP CAR BUT SOME THINGS DISCUSSED CAN APPLY TO DRIFT ALSO

 

Also, this thread is mostly aimed at AE71/86's and KE70's, but most of this info can be used on other cars.

 

I've spent a bit of time looking through the different suspension related topics, mainly on ae86dc, and Ive noticed that nobody has accually posted much good info up on why things do what they do. Every body seems to put up posts along the lines of "use cut down falcon springs and suzuki seirra front shock and you'll be sweet" nothing about what real effect this has on the vehicle just that thats what you need to do. As such i thought it would be a good idea to start a thread to educate people on how and why suspension systems do what they do and hopefully save people some time and money in the process.

 

so...

 

1) a really common one i've noticed on here is that nobody seems to leave room for droop, a car need droop (the amount that the suspension hangs from static height) in the suspension system, whatever that system is, having zero droop will cause the car to be excessively bumpy and destroy shocks and other components. The wheels can only rise towards the vehicle but not fall away at all, so when the u go thru a dip of any sort, the whole car drops into it and gets hammered on the way out of said dip. In a racing/drifting/fast street circumstance, when you go through a corner, say a left, the right side of the the car should compress and the left should extend. With no droop, the right will still compress but the left will not and the car will hang wheels in the air. Whilst this may look cool, if the wheels are in the air the tyres cannot provide grip. A good amount of droop, no matter what the car or what the hieght is 2-3.5inches

 

2) Lower is not always better, all cars, the aim should be to get the suspension arms/links/whatever parrallel with the ground to get the most balanced roll centers, or as close to it as possible. This is what RCA's correct on the front of ae's and ke's are for. If the arms are at to great an angle from parrallel (this goes for raising or lowering) the roll centers get out of whack and the car then suffers front bumpsteer and general poor handling characteristics.

 

3)bumpsteer, This i caused when the LCA and the tie rod end run at different arcs in there travel and cause toe change, wheather by a + or - no. this causes stability issues under heavy braking mainly but all through the suspension range in some cases (see datsun 1600). With standard hieght cars, if any bumpsteer occurs it is usually out of the normal range of suspension travel and can't be felt. On a lowered car it is genrally fixed by raising or lowering the tie rod end mounting position or movung the rack mounts, theoreticly, it could be fixed by modifying the steering links so that they all run on the same radius. KE70's and AE86's VERY RARELY will get bump steer. What is normally felt on a ae or ke that is confused with bumpsteer is...

 

4)Incorrect scrub radius, to picture this, run an imaginary line from the top of your front shock through your ball joint and onto the ground with the car at static height, front the front. Now on a KE/AE, most of the front tyre will be outside this line, what this causes is whenever u hit a bump, instead of just going over it, your wheel will be pushed back around the axis of your strut, which u feel as steering through your hand. Newer vehicles (with newer strut designs) do not experience this as much as AE/KE's due to a better scrub radius. I personally think that this may have something to do with the front end vibration several KE/AE's get at around 80kp/h. There are ways to fix this but they generally cause several other issues that are much harder to deal with, You now know what the feeling is, deal with it :rolleyes:

 

5) cars can be to stiff, jap drifters/circuit cars are way to stiff, they have access to circuits and even streets that are pool table smooth, we don't, run softer settings and go faster. Have a look at the gent on this site that has the valvoline sponsored ae86, his car has plenty of body roll, why? because it is needed to make the tyres work. If you don't run slicks, you don't need stupid spring rates (drifters may not apply here).

 

6)negative camber, massive neg. will not neccessarally make your car handle better, if you don't have enough grip in your tyres to cause the tyre to roll onto its face in a corner then you have to much neg, simple as that (drifters may be different here) a road tyre cannot deal with massive neg and it will slide/understeer. Circuit style camber designed fro slick tyres will not work with a road tyre. It causes poor stability and poor braking due to a reduced contact patch (running 185's stretched over a 8 inch rim whilst looking cool, does not help this eather)

 

7)caster, in laymans terms, this is camber that happens when you turn, both front wheels lean over during a turn due to the strut being pulled forward at the bottom or rearward at the top in a static position. On a street car this is more effective than negative camber in creating grip in a corner without regularly destroying tyres due to neg. However exccessive caster can cause high speed instability.

 

8)track, a wider track does not neccesseraly mean a better handleing car, AE/KE's are blessed with a near on perfect wheelbase from factory (again drift may be different here) of length = 1.7xwidth. Massive track can make the car overly square and when done by using massive offset wheels, causes more problems with scrub radius as discussed before.

 

Understeer/oversteer, in laymans terms, oversteer is when the tail kicks out, understeer is when the front slides and tends to go straight ahead instead of turning. generally, understeer scares the driver and oversteer scares the shit out of the passengers. In alot of cases over/understeer is caused by factory wheel alignment settings or incorrect spring and swaybar settings, if the rear is to stiff, or the front to soft then the car will oversteer and vice versa.

 

Rollcenters, as a car goes through a corner, it will lean, the inside of the car will extend and the outside will compress its supension. If you could picture an imaginary point about which the car rotates, this is the roll center. Rollcenters can be changed depending on suspension types but the best way to measure change is on a track. Making a sprinter have adjustable rollcenters is not overly difficult, we all know about front roll center adjusters and having several different thicknesses of rca will alow you to try different settings. The pic below shows how to work out front roll centers, this must be done on both sides to find the actual rollcenter where the lower lines intersect. As pictured, changing the angle of the strut or the track will effect the roll center but changing the angle at which the LCA sits will cause the most change. Now generally speaking, raising the roll center will cause more roll, lowering it will cause less. However, having no roll means that the cannot roll onto its tyres and will understeer, having it to high will cause the car to roll over to much and also cause understeer. A sweet spot must be found through testing of the car. There is no right or wrong here as different ride hieghts and driving styles will mean different sized RCA's.

post-1650-1214327077.jpg

For the rear it gets more complicated as the angle of the 4 link bars from front to rear has a bearing on the roll center as shown in the 1st pic below and much like the front, rear roll centers are dependant on driving style, horsepower levels, and ride height. the easiest way to make the rear system work properly is to make the 4 link bars equal length, parrallel with each other and the ground. This dials out all roll steer and as shown in the 2nd pic below, makes them no longer a factor in roll centers. Once this is done then a panard bar arrangment that can be moved up and down at both ends, or an adjustable watts linkage setup, can be used to adjust rear roll centers. The only other thing to remember when running a panard bar is that if the bar is not parallel with the ground (as in both end mounts are an even distance from the ground) the roll center will move left to right with suspension movement, it will also change to a different location between left and right corners, and cause left/right diff movement through suspension travel. Again there is no right or wrong here, a high hp car may run a higher roll center in the rear to force the cars weight onto the drive wheel in a corner and provide better drive where as a drifter might run a really low rear roll center so that the rears don't load up and cause eccesive grip or tyre wear.

post-1650-1214327098_thumb.jpg

post-1650-1214327107_thumb.jpg

 

Roll steer, i know there are other terms for this but its the term in using for this topic, roll steer is steering of the car through the rear wheels when the car leans through a turn, this is built into factory cars to induce understeer so grandma can get to the shops safely, not so good for us revheads. It is usually caused by having a setup that causes the diff to turn in relation to the car, in the case of sprinters, it is caused by having shorter upper arms in the rear 4link arrangement. Because the upper and lower arms have a different radius of movement when the suspension moves up and down, it causes the top mounts of the diff to be pulled foward on compression and pushed rearward (to a point) on extention, when the car is driving straight, this causes pinion angle change caused by the diff twisting, in a corner, 1 side is compressing and 1 side is extending so, its causes 1 side to be pulled foward and the other side to be pushed rearward. This is how the rear steering effect is caused. While most of this effect can be dialled out using swaybar/spring and shock settings, the only way to properly correct this is by running equal length 4 link bars, in doing this you effectivly lose your rear seats. It really depends on what you want more, rear passengers or proper handling.

 

Pinion angle, this is the angle that the diff sits at front to rear. there is alot of argument over what the right pinion angle should be, for a traction setup, it should be a 0 degrees when the car is under heavy acceleration, that means that at rest, the diff should be angled slightly downward so that when the nose of the diff tries to rise under acceleration, the diff twists up to straight. The amount that the diff needs to be angled downward depends mainly on what bushes you are running, if they are spherical bearings (rose joints, pillow balls, rod end, heim joints etc) there will be littl or no movement so a lesser angle will be required as opposed to rubber or poly bushes.

 

Tyres, tyres need movement, fullstop, if you don't have any sidewall flex or tread deflection the tyres can't work properly, whilst a 185 stretched over an 8-9 inch rim looks cool, its wont provide proper grip. A near solid tyre with no sidewall flex will slide and is great for drift but on the curcuit it will not provide proper grip and on the street it is dangerous. Tyres are not just cheap, black round things for burnouts, they can provide a massive amount of information about what the car is doing and what may be wrong with a given setup. Next time your at a circuit meeting, talk to a race tyre technician, he will be able to explain what can be done with a depth indicator, temp gauge your eyes and hands. One of the few things of the top of my head that can be measured easily is camber setting effectivness. If you have access to 1 of those infared termometer guns, borrow it and go for a hard hills run or track day. Use the heat gun to measure the inside and outside temps of a given tyre, say right front. If the outside of the tyre is very hot and the inside is cool then more negative camber must be added, if the outside is cool and the inside hot then you have to much neg. simple as that. I will be adding more here soon.

 

Aero aids, aerodynamic aids at road speed really are mostly a waste of time, however, having a front spoiler that is as low to the ground as is practically/legally possible will stop air getting under the car and aid high speed stability. Most rear wings are a waste of time, they do not genrally provide downforce unless they are higher than the roofline as if the are within the frontal area of the car the wing will pass over them (there are exceptions to the rule). However, if a rear wing is designed more like the TRD ducktail style rear wing, the flow of air off the rear of the car is interupted and the turbulence that normally happens at the edge of the bootlid, happens further back and causes the vehicle to be more stable.

 

ill edit post when i think of more and have more time, thats it for now though.

feel free to voice your opinions.

 

constructive critisism and debate will be noted and or discussed, flaming will be ignored.

 

Some of the pics and information in this thread come from "how to make your car handle by fred puhn" its a good book, i suggest that everybody get a copy. Other info is from experiences ive had, people ive worked with and places ive worked.

 

The pics used below are from AE86DC, i can't remember who i got 1 of them off but the other is an spja for sale item, if anybody has any issues with pics i use, let me know and i'll take them down.

Edited by Fat Bastard Customs
Link to comment
Share on other sites

Members dont see this ad

There you go jake, you can read it now ;)

 

Another thing i'd like to throw out there.

 

I'm currently trying to figure out a way to at least reduce the scrub radius problems that a KE70 has, i have my own ideas but if anybody has any thoughts i'd really like to know. Basically the way I'm doing mine at the moment, the ball joint will be replaced with a spherical bearing as will the inner LCA bush. The castor rod will be rosejointed and there will be a pivot point where the castor rod currently bolts to the LCA. Similar to the below pics.

post-1650-1214328164_thumb.jpg

post-1650-1214329009_thumb.jpg

The only way i can see to improve the scrub radius problem is to move the balljoint closer to the wheel.

 

Please, discuss.

 

And yes, this thread is from ae86dc, but i figured that there would be more grip orientated drivers on rollaclub.com, took me a while though :(

Edited by Fat Bastard Customs
Link to comment
Share on other sites

yeah, the images are from fred phun's how to make your car handle.

 

1st time ive ever done a big write up like this, i'll put a credit thingo in with the sources ive used.

 

my bad, i couldnt be bothered drawing the pics up myself :P

 

And i find that that rule you mentioned works, but not in all cases. I had 50mm of droop in the rear of the KE70 and found that if it was being used for circuit only i wouldve been fine, but when being driven on some of NSW's awesome roads (yeah right) it would hit full droop. So i gave it more, not much, but it doesnt hit full droop as easily now.

 

Horses for courses where thats concerned i think.

Edited by Fat Bastard Customs
Link to comment
Share on other sites

  • 2 months later...
  • 3 years later...

Such a great thread. Great information. Wondering if anyone has more to add? Information on Watt's linkages etc. There's a little write up on Wikipedia, can anyone confirm the info there as being correct?

Sorry for the thread dig...

Edited by willis
Link to comment
Share on other sites

×
×
  • Create New...