Shocker Inclination Angle

[chinchin]

I was wondering how to decide the Shocks Inclination Angle for my Formula Car. I searched a lot but didn't find any basis of deciding the angle at which shocks are to be mounted.

I have seen a lot of universities mounting the shocks horizontally. Does this provide some additional advantage or is this completely governed by the space available?

[Mehul Botadra]

A lot of force goes into the pushrod/pullrod and then most of it is transmitted to your damper spring assembly. Do a force analysis and decide your angle. Usually a horizontal damper is preferred. But it totally depends on your calculations. You might wanna add another member at the damper node to dissipate the force, only thing you're adding weight! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[The_Man]

The right question to ask here is why the damper and spring is mounted in a particular way?

I will assume you have a push/pull rod system here.

Firstly you need to understand the effects of the angle of the push/pull rod on your suspension geometry. Remember this affects your motion ratio so it is more important than you think.

Next you need to place your damper in such a way that the plane of the damper axis the bell-crank(rocker)and you pull/push rod pick up point is in one single plane, else you will have all kinds of bending and compliance that you do not want.

Keeping it very simple. Now you have to find a solution where you can package the entire suspension somewhere on the chassis. The components must be in plain and as you are changing the plain and the angles/size of the rocker your motion ratio is affected. You need to select a position there the whole system is optimized in terms of packing, weight and CG height without compromise on your suspension geometry requirements. The dampers can be mounted vertical, horizontal, below the chassis, anywhere as long as you can justify the selection.


Originally posted by chinchin:
I have seen a lot of universities mounting the shocks horizontally. Does this provide some additional advantage or is this completely governed by the space available?

It is mostly governed by space available but you can provide some additional advantage with it if you work smartly.

[exFSAE]

Or you could just ditch bellcranks entirely and have direct-acting shocks... removing some weight and complexity.

[Mehul Botadra]

That would mean too much force going into your dampers! If you're using cycle shocks, bell cranks are advised considering a 70kg driver! Then you'd have stiffer springs and suspension settings seem like Rocket Science!

[exFSAE]

Force primarily goes into springs. You still have total control how much goes into the damper (velocity dependent) portion.

At least one team has done it for a couple years. After I took a second to think about it.. on these cars, it's really not a bad idea.

[Mehul Botadra]

So your damper force does depend on the Spring Stiffness and the force applied via the bellcrank that is coming as a component of the bump on the wheel! Though I do like the idea of having a direct damper mounting just that I'm apprehensive of burning my shock fluid!

[The_Man]

Originally posted by exFSAE:
Force primarily goes into springs. You still have total control how much goes into the damper (velocity dependent) portion.

At least one team has done it for a couple years. After I took a second to think about it.. on these cars, it's really not a bad idea.

I have seen quite a few teams do it too and they do it in Baja all the time. Makes me wonder why we do not see direct actuation of the shock absorbers that often. Time for some thought-designing (like thought-experiment). I am going to try list down a few reasons why I will and will not have it. See if this discussion takes us somewhere.

I will not have it because:
<UL TYPE=SQUARE> <LI>I will need a damper that is long and heavy. The Baja cars I have seen it often on have much smaller wishbones(in front view) and also the roll cage is wide giving ample of opportunity for a nice mount for the chassis pick up of the damper.
<LI>With suspension travel, the angles the shock will make will vary dramatically. This will cause the motion ratio to change significantly. This not necessarily a bad thing but I will tend to (as far as I am imagine) soften with travel which is exactly what I do not want. Changing that will mean awkward mounting of the shock.
<LI>Shock and Drive shaft will most definitely intersect unless you mount the shock really high up and on the upper wish bone. I do not like that too.
<LI>I will loose so many adjustments in the suspension. Changing the ride hight will change my effective stiffness. There is no control over derivative of stiffness with travel. Basically, Too few variables to play with in the suspension.
<LI>I will not be using my damper travel well.
<LI>Lastly for the sake of argument aero(Flow into radiators).
[/list]

Why I will have them:
<UL TYPE=SQUARE> <LI>Super simple design. No complications, a pick up point here, one there and you are done.
<LI>May be I save weight, if my larger dampers are not too heavy. Also, I might need bigger springs.
<LI>Manufacturing easier no bell cranks, no slots in body works for spring travel, no push rods.
<LI>Safe, No moving suspension parts near drivers legs. Also means more leg room for driver.
<LI>Saves me from looking like a zombie after hours of intense concentration in front of the CAD trying to figure out the right plane for the inboard suspension. When I do think I have found the perfect plane, the Template won't pass.
[/list]

In the end for me I think for me still I will not have direct actuated suspension.

[The_Man]

One more argument in favor of the inboard suspension.

The ARBs how do I mount those with direct actuation of dampers? Only thing I can think of is to connect them with pick up points on the wish bones since there is no bell-crank.

[exFSAE]

Personally why I think it isn't very prevalent.. is that students in this series have a tendency to think they need to be building mini F1 cars and are often leery of even trying alternative solutions.

Mount the ARBs like you would on any road car. This of course assumes the need for ARBs. Seen many a FSAE car with so much spring thrown at them that the bars don't contribute much if anything. Often wind up being tuned just fine purely with spring, and the bars removed for weight. Also.. the dampers do not need to be long nor heavy.

It's just a nice simple solution, and it works.

[GianVioli]

I did some simulation at ADAM's and the optimal Shock angle is 400 degreess.

[Mehul Botadra]

Originally posted by exFSAE:
Personally why I think it isn't very prevalent.. is that students in this series have a tendency to think they need to be building mini F1 cars and are often leery of even trying alternative solutions.

Mount the ARBs like you would on any road car. This of course assumes the need for ARBs. Seen many a FSAE car with so much spring thrown at them that the bars don't contribute much if anything. Often wind up being tuned just fine purely with spring, and the bars removed for weight. Also.. the dampers do not need to be long nor heavy.

It's just a nice simple solution, and it works.

Works at the cost of what? I dont have Michael Schumacher on my team to push the car to the limit!

[Mehul Botadra]

Originally posted by GianVioli:
I did some simulation at ADAM's and the optimal Shock angle is 400 degreess.

400? http://fsae.com/groupee_common/emoticons/icon_confused.gif

[The_Man]

Originally posted by Mehul 'Chelsea' Botadra:

Works at the cost of what? I dont have Michael Schumacher on my team to push the car to the limit!

On the contrary the fact I do not have Nico Rosberg (:P) on my team, I would like to have a simple system rather than a million adjustments. Because my driver doesn't know what to do with them.

So "Nico Rosberg not on my team" goes to my 'Why I will have direct suspension' list.

[Mehul Botadra]

Originally posted by The_Man:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mehul 'Chelsea' Botadra:

Works at the cost of what? I dont have Michael Schumacher on my team to push the car to the limit!

On the contrary the fact I do not have Nico Rosberg (:P) on my team, I would like to have a simple system rather than a million adjustments. Because my driver doesn't know what to do with them.

So "Nico Rosberg not on my team" goes to my 'Why I will have direct suspension' list. </div></BLOCKQUOTE>

Its like having 50% knowledge and still going with the simpler system even though you know your 50% knowledge system is much more efficient. The amount of weight you lose using smaller dampers is good enough and using light bell cranks can also mean the mean weight is lesser than one outboard shock. I'd prefer giving the driver a good car to drive rather than making it simple just because he cant give efficient feedback.

Using outboard shocks is as good as using struts! Its Formula SAE, not Formula Tank!

[exFSAE]

SMH...

You can use the SAME shocks on a direct-acting setup as you would with them inboard with a bellcrank. You don't have to use ones that are any larger.

[Fil]

Direct acting shocks can work without being digressive and too heavy. With the track widths getting narrower and frames getting wider it does make sense to go direct acting.

as for formula tank, stop looking at f1 or any open wheel category for inspiration, you are an engineer, figure out what works best for your rules and competition, don’t look at what others are doing and compare it, do a detailed study into the benefits of each system and then see what works best for the requirements you have devised in your design strategy/methodology. See the alumni thread from a few months ago about this...

also we managed to win the aus comp last year including design with direct acting suspension... there are so many threads on this forum about this, search inboard vs outboard. I did a study into direct acting in 2005 after Z (where is he hiding) talked about it and guess what it actually made sense.

[oz_olly]

Z doesn't post around here anymore, he got sick of people not wanting to think outside the box. Now days he must be too busy writing articles for Racecar Engineering, they just published part 2 of a multi-part series on 'anti' forces. He finally gave up the goss on his idea's about kinematics and motion screws in the article. He's actually a really friendly guy, I spent hours on the phone to him a couple of years ago talking about vehicle concepts. He said he would be more than happy to have a chat about design over a cheese burger or two at his local McDonalds if I ever find myself in Sydney with a few hours to spare.

As for direct actuating dampers, I say WINNER. It really is nice and simple and reduces the number of nodes and load paths required for your chassis design. Just don't tilt your dampers too far from vertical, you end up with a rather variable motion ratio.

Cheers

Olly

[Richard Pare]

Shock inclination angle all on its own doesn't mean anything. What IS important is the motion ratio - you want it as high as practical.

With a pushrod/bellcrank setup, it will be fairly easy to get the MR right around 1:1, and possibly a bit higher (shock travels more than the wheel).

Direct acting MRs can be pretty low, depending on the inclination.

For the shock to work well, and also to allow lighter springs (both in rate as well as in mass), the MR needs to be somewhere near 1:1. Modern formula cars strive for 1:1 for the front (if not higher), and even higher for the rear. This allows the springs to be of lighter rate and mass, and also pumps more fluid for every increment of wheel movement.

The more fluid that the shock can pump - especially with the high wheel rates we have on modern cars - the easier it is for the shock to be fine tuned for good spring and tire contact patch control. The difference in capability of the same shock moved at a 1:0 MR compared to a .5:1 MR is dramatic.

[Big Bird]

I'll add my support for the direct acting shocks route, especially given current cockpit template rules. Most teams would benefit more from the simplification of design / manufacturing / testing, than from any perceived "perfectness" that they think rocker suspension offers.

There is a psychology thesis on loss aversion that could be written about some FSAE'ers and their obsession with F1 solutions, ("If it ain't like what they do in F1, then we've got no chance..."). So the usual over-commitment to fancy design, and the subsequent /inevitable underdelivery of a fully functioning vehicle.

I've just taken a look at the Michigan results, seems again that only around 33% of teams finished all events, and only 700 points would bury you well inside the top 10. It doesn't take F1 design features to get you a good finish, just a complete vehicle. And if a team is failing to score 700-800 points regularly, then I'd argue the payback of simple design / shorter delivery times would more than offset any percieved tuning advantages of a mini-F1 car. (Apologies to those old-timers around here who have read this argument of mine too many times now!)

Chelsea, the wording of your argument seems to imply direct acting shocks are crap because they are somehow unfashionable. I'd sincerely hope you don't think such an argument is going to get you far in a design event. They'll crucify you. A good design argument will recognize a number of competing criteria (e.g. motion ratios / shock travel etc for contact patch control, packaging, cost, time for design and manufacture, available resources, etc), and will rank the importance of these based on your own team's particular position and design problem. If you are a well funded and resourced, well organized top team who is delivering a race-ready vehicle very year, and need an extra 1/10th of a second to beat Stuttgart et al, then motion ratios may be your critical factor. But if you are failing to deliver a complete car each year (which most teams are) then better payoff comes from simpler and more robust design. With due respect, Orion Racing isn't challenging for overall wins yet - so you'd be better off lowering ambitions and delivering, rather than aiming high and failing.

If I had an opportunity to do all this again, one of the design features would be direct acting shocks - along with a lot of other "unfashionable" features that the tech-heads would cringe at. But that is another thread....

Cheers all,

[Charlie]

If you can get the motion ratio you want with a direct acting shock, go for it. It is the simplest solution. However, I think it would be difficult. Every car is different though.

[ben]

Originally posted by Mehul 'Chelsea' Botadra:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by The_Man:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mehul 'Chelsea' Botadra:

Works at the cost of what? I dont have Michael Schumacher on my team to push the car to the limit!

On the contrary the fact I do not have Nico Rosberg (:P) on my team, I would like to have a simple system rather than a million adjustments. Because my driver doesn't know what to do with them.

So "Nico Rosberg not on my team" goes to my 'Why I will have direct suspension' list. </div></BLOCKQUOTE>

Its like having 50% knowledge and still going with the simpler system even though you know your 50% knowledge system is much more efficient. The amount of weight you lose using smaller dampers is good enough and using light bell cranks can also mean the mean weight is lesser than one outboard shock. I'd prefer giving the driver a good car to drive rather than making it simple just because he cant give efficient feedback.

Using outboard shocks is as good as using struts! Its Formula SAE, not Formula Tank! </div></BLOCKQUOTE>

How did Monash win design at FSAE-A and the comp overall then? Bribe the judges...

Isn't it more likely that the design choices made were well justified and the system gave good dynamic performance on the track?

Inboard suspension with bell-cranks and push/pull rods are a no-brainer where aero drag reduction is vital and/or wide track widths are used.

FSAE has no need for drag reduction, the track widths are narrow and the chassis' are proportionally wide relative to those track widths. The chassis templates make outboard shocks even more appealing.

As others have already pointed out this isn't a mini-F1 design comp, it's about finding the best solution to a set of rules.

Monash think outboard shocks have merit and proved it in competition - have you proved your ideas anything like that well?

Ben

[The Stigg]

Hi Guys,
interesting post as it leads to a design approach that faces a design problem with time, money and complexity restrictions. And this approach fits way better to real world conditions - where most of you will work for the next 30 years.

Regarding the outboard shock discussion I would focus on the crutial parts (shocks - that in most cases need to be bought and don't fit your requirements out of the box) rather than the complete system.

If someone builds amazing shocks with a affordable price that fit to your design requirements than I would buy them and concentrate on the right implementation and in most cases the shocks will give you the answer which way you should go.

[Demon Of Speed]

Now that we are completely off the subject of the original post I would like the add that the higher the MR the less force that is put into the frame where the Coil-over dampener attaches.

[The_Man]

Originally posted by Demon Of Speed:
Now that we are completely off the subject of the original post I would like the add that the higher the MR the less force that is put into the frame where the Coil-over dampener attaches.

You are just redistributing the load. With different MR the load from the suspension transmitted to the chassis through the 2 points (the bell crank pick up and the shock mount) just redistribute. It might be a good idea if your shock mount is less stiff compared to the bell crank mount. Total force as logic(Newton's Laws) dictates should not change.

If you are talking about the direct acting shocks the change in force is probably because of your suspension geometry rather than the motion ratio. If you can imagine this that if your motion ratio is low then your spring is almost parallel to the wishbones so at lot of spring force is actually stretching out the wishbones which causes the higher loads at the pick up point. However the total load that goes through the suspension subsystem on that corner to the chassis will be the same irrespective of the motion ratio.

[flavorPacket]

Hold on, the_Man. check this out:

1st scenario: 0.5 MR (spring to wheel), wheel rate 100 lbf/in

you have 100 lbf on tire, tire moves 1 in, spring moves 0.5 in. Spring has load of 200 lbf assuming no preload (spring rate is 400 lbf/in). Thus the loads on the bellcrank are from the pushrod (a component of the 100lbf wheel load) and 200 lbf from the spring.


2nd scenario: 1:1 MR, 100 lbf/in wheel rate

now you have 100 lbf on the tire, tire moves 1 in, spring moves 1 in. Spring has a load of 100 lbf (spring rate is 100 lbf/in). The loads on the bellcrank are the pushrod load and the spring load, now 100 lbf.

So the bellcrank loads are indeed reduced by using higher motion ratios, assuming all else is equal in the geometry. The spring is simply being leveraged more effectively for the same wheel loads.

[dazz]

flavorPacket is right on the money there, though neglecting speed dependant loads induced by the damper.

Which brings us to the damper unit (in the shock absorber), no good designing for a really high MR if you can't tune your compression and rebound adjustments of your shock to suit. And obviously the total required wheel travel X the MR can not exceed the available stroke of your damper unit.

[jpusb]

It's really a nonsense - endless discussion about if direct shocks or bellcranks and which one is best. IMO, it ALL depends on YOUR GOALS. That is something everybody needs to understand. Someone said it already but not this way. Someone in the thread said something like, if you're racing Stuttgart, then you should really go for adjustments and complex stuff. If you are just aiming for the top 15 or top 20 you just need a car in 4 wheels that holds its wheelbase/track and is drivable.

At the end, I think NO ONE has problems with either the bellcranks or the direct mounts, so it's not a matter of reliability. It can be about manufacturing time, but maybe it is easier for a team to design/build a bellcrank configuration than a direct mount one, nobody knows the situation or methods of a particular team... whatever. For me it all depends on your needs and your reality. If you are running a $600 shock, and you went through hell to get that money for your shocks 'cause you are a limited budget team that is fast, you don't want it damaged by rocks and stuff hitting it on the track (many of us practice at karting tracks, not huge parking lots where nothing happens if you go off track or worse, have an accident). If you crash with an inboard shock, it's probable you just have to make the A arms (mild steel?), if you crash with an outboard $600 shock, that's not cool for many, many teams. If you practice with 39 C ambient temperature, and 53 C track temperature (like we do here at our country), radiator flow is really critical, and a shock in the way is a big problem. So, as you can see, FOR US, it is better the bellcrank option, and through the years it has become really easy to re-design, adapt, manufacture, and fine tune. However, that is NOT necessarily the situation for every team on the planet.