New for Suspensions!

[melkohil]

Hey guys,

I am new for suspensions design and I have two questions: How to adjust the ride height? what are the consequences if I raise it beyond the normal?

Thanks!

[Zac C]

How to adjust the ride height?

You probably need to make something either shorter or longer.


what are the consequences if I raise it beyond the normal?

The car will flip and you'll (probably) kill your driver.

[Pat Clarke]

Melkohil,

It is good manners to firstly introduce yourself and your school.
Then it is considered good manners to do a modicum of research before posting questions.
Failure to do so usually gets dismissive answers as above.

Pat Clarke

[JT A.]

Give this thread a read

http://www.fsae.com/forums/showthread.php?8638-How-many-eggs-do-I-need-Context-and-etiquette-on-these-forum-boards&highlight=eggs

[Claude Rouelle]

Melkohil,

What about introducing yourself? Just a question of being.....not even courteous....simply polite.

So unimpressive...

As far as your question, you wrote "...if I raise it (the ride height) beyond normal" How do you define and quantify "normal"?

[P^squared]

Raising the ride height beyond normal will give you better acceleration times from a standstill.
Because higher CG --> greater load transfer --> more grip at the rear --> less wheelspin
( just in case no one's following the FSG thread :D )

[Claude Rouelle]

P^squared,

1. Just a note: if you have a FWD or a 4WD (which is the case of more and more Electric FS cars) your consideration could be slightly different.

2. All other things being equals you are right in principle on a RWD for acceleration consideration: but...... then why not having 500 mm of ride height for acceleration? What is the limit? Is the rear tire longitudinal grip linear with the vertical load? What would be most efficient: vertical load or slip ratio control or tire pressure or tire temperature or toe or initial weight distribution, launch control loop, or aerodynamics parameters or simply mass, power, traction control, damping. ....? Or simply practical consideration such as ride height adjustability limit? If you do not have at least a theoretical answer for the influence of these other parameters stay away from intuitive or emotional (kind of the-winner-do-this-so-it-must-be-right" considerations): this is an ENGINEERING Design competition Also stay away from any multi input parameters simulation that is not backed up by comparative test data validations.

3. Do have the same thinking process for braking efficiency?

[P^squared]

Claude,
I suggested ride height adjustment to improve performance only after all the design and manufacturing has already been done. That is the reason to have any kind of adjustment built into a system.

I wouldn't follow the same thinking process as long as we have All Wheel Braking instead of just one axle.

I don't think FSAE suspensions are designed to allow such amounts of travel : 500 mm , let alone keeping the tyres properly oriented at such extremes, I.e. if you want a lowish CG for other events ;)

[Claude Rouelle]

P^squared,

Ride height and ride height variations are 2 different things, except that for a given suspension stiffness the higher the CG the more suspension movement you will need. You kind of elided my questions or observations. 500 mm was used a provocative number; I could have said why not 50, 100, 150, 200 or 250 mm or 1 meter! How do you make that choice?

Claude

[P^squared]

What I meant was that while having a low ride height for Tilt and other dynamic events, it won't be possible to increase the ride height for acceleration above a limit , as the wishbone pivots will only allow a limited change in wishbone inclination.( unless you've made provisions to take care of that )

The theoretical upper limit to this kind of crude adjustment would be till the car " pops a wheelie " ( all weight transferred to the rear axle )

The lowest ride height would be that where you're confident in the sprung mass not touching the track under any dynamic maneuver. This, when you don't have an undertray/ wing in ground effect.

For most of the dynamics I'll aim for the lower ride heights

[Claude Rouelle]

P^squared,,

It seems we are on the same wave length

Just a simple question (answer of which can be useful to many readers here): How will you change the ride height?

Claude

[P^squared]

For suspension systems employing a pullrod / pushrod actuation mechanism, ride height adjustment is simply a matter of varying the length of the pull/push rod ( vary the distance between the bracket on the wishbone, and the bellcrank mounting point ).

This task becomes easier if you have opposite handed rodends at the two ends of the tube. Then it simply becomes a matter of turning this tube to vary its length.

Similar adjustment can also be achieved with direct acting dampers as well.

Using spring preload to get the same effect would limit your droop travel, while ( in extreme cases ) also altering suspension characteristics in bump.

[Claude Rouelle]

P^squared,

I assume you are thinking abut a car with pull or push rod, no direct activation

In that case

1. IF you use the spring perch position to change the ride height you will effectively change the amount of your damper possible droop and bump travel but you will also change the position of your rocker and in some case that will tremendously change the motion ratio and therefore your wheel rate (remember that the wheel rate (or if you want suspension stiffness) is related to the SQUARE of the motion ratio) If I was you I would not adjust your ride height with the spring platform (some call it perch) unless you really know that you are doing and have beforehand tested all different adjustments on a setup pad with scales

2 . I have seen teams in FS competitions changing the front and rear ride heights for the acceleration, apparently putting the ride heights back where they wanted them for the skid pad event and not finding back the car handling they were used to. Static corner weight and crossweight can be VERY sensitive to pull or push rod length change. on a FS car 0.5 kg (1 lb) between the LF and the RF static corner weight can be a huge difference) I have seen students and professionals completely screwing up their car handling and making it very asymmetrical (with for example understeer in left end corners and oversteer in right end corners and systematically looking, again for example, the LF wheel in left hand corner entry....) by changing the pull or push rod length with a difference of only a 1/2 turn, if not less. That is why a) unless I am working with a very experienced team I would not change any push or pull rod unless the car is on setup pad with 4 scales) b) I think there is an advantage of using shims (instead of rod end length or turnbuckle) to adjust the ride height with push or pull rod: usually less mistake are made as you can always double check the length of the installed shims stack, while many students do not meticulously measure the eye-to-eye (rod end center to rod end center) of their push or pull rod length (PS: I assume that your chassis and the rest of your suspension is symmetrical in both geometry and stiffness..... with no asymmetry in rod ends and damper residual friction) but that is another - big - chapter!!!)....

Here is a proposed solution to avoid any of the mentioned pit falls; when you design your rocker make sure you have 2 holes for your push or pull rod: one for low (or "normal") ride height and one for high ride height as for acceleration. When you change for one to the other ride height you will have less risk of creating an asymmetrical car. Be aware though that you motion ratio will be different.

If you have direct actuation you can imagine having 2 pick up points on your chassis for your damper/ spring units.

Hope this helps

Claude

[P^squared]

Claude,

There are potentially better ways of doing it with a direct actuating damper.

http://www.fsae.com/forums/showthread.php?12087-Direct-Acting-Spring-Damper/page7

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