Shocks from scratch

[Nbruno]

Hey guys. My school is restarting our FSAE program after a ~10 year hiatus. I've been put in charge of shocks, springs and swaybars on our new car. This is my first time doing this kind of design work, so I'm trying to gather as much info as possible. I've been looking into different manufactures, and as far as I can tell, only Ohlins and Penske make specific FSAE shocks, but the standard model for those are quarter midget shocks. Is that accurate? I've also heard that mountainbike shocks are also used, but I've been advised against using those because of the differing requirements for the two dampers.

The shocks on the most recent car are Carrera (I think). It would be great if I could re-use these, since we are a new team and on a limited budget...but I also don't want the car to be a dog. Is there any sort of middle of the road setup out there that works well? Also, is it a good idea to re-use the 10 year old bellcranks and swaybars? Thanks guys, sorry if I sound like a noob.

[BMH]

Check out shocks made by Tanner Racing. They work pretty well for our application and are lightweight. They run about $180 or so a pop, with the spring being about $25 or so from the same supplier. You can probably find springs cheaper somewhere else if you looked hard enough.

[Demon Of Speed]

The cost report index has a list of most all the shocks people use. You names the ones most people use. It is true that the mountain bike shocks are not the best, but they do work and are a lot cheaper then the specific FSAE shocks. If you have time to test you can re-shim/re-valve the mountain bike shocks (if they are made to be taken apart).

I would look into getting your old (or any new) shocks dynoed so you know where they are at, and can tune from there. I would also get your springs rated so you know where you are with them. I wouldn't recommend using old bellcranks and antirollbars, but that is just me.

[exFSAE]

Who advised against using mountain bike shocks because of "differing requirements" ? Did they have any or legit reasoning to back it up? Or was it just BS handwaving? Handwaving is unbelievably rampant in this series, btw. I don't care who says something.. if it's Joe Schmo from a team, or Claude or Doug or whoever. With pretty much anything, prove it to yourself first.

Few things here, from the 'what its worth' department.

You're in charge of some pretty critical items in terms of getting the car to drive well. Ultimately though you need to prioritize and have a design cut off date.

In terms of priority.. in my experience shock tuning is low-ish on the list in this series. Many teams can't even get their alignment and basic steady state balance set well. I think you'd be more hosed if you miss the ball on that, than on crappy dampers. MTB dampers might not be "perfect" but they've worked just fine for many teams for many years. I've seen top 20 cars (and have been on one of the teams) running absolute garbage shocks which for all intents and purposes could probably have been removed from the car all together!

Some other thoughts-

You can have a pretty well balanced car without swaybars. Get a range of springs from soft to stiff. One way or another you're trying to tune the load transfer split between the axles... springs and bars both do this. IMO the only reason bars are nice on these cars is because when designed PROPERLY, they give you nice quick and meaningful "trim" adjustments instead of big changes by going up in springrate.

You don't need to have bellcranks either. Direct-acting shocks help reduce complexity, and gets rid of a REAL pain in the ass of the design process (bell crank motion). Can be done with reasonable weight as well. Pitt has done it recently.

At a minimum, start getting a feel for who does make shocks that will fit at all on these cars. Get some feedback from teams as to which to stay away from in terms of availability and quality. Then, see if you can figure out a rough idea of what kind of damping you'll need the shocks to produce. Then.. see how well the available shocks meet those requirements.

[rkraft]

Hi Nbruno,

The Ohlins shock is an adapted twin-tube mountain bike damper. The Kaz/Penske damper is an adapted monotube Formula 1 damper. Penske does make a monotube 1/4-midget damper that some teams use, but that is a different model from the FSAE shock.

Based on the goals of your team, I think exFSAE has laid out a great place for you to start. You need to 1) build your car and 2) learn some stuff about the car you're building. And pretty much any damper will allow you to do those things.

Bad shocks are not going to make a first-year car a 'dog'. In fact, they probably won't make any difference at all in your laptimes unless they're really bad (read: empty). I'd wager that aside from not being able to pass tech, you could build a car with no dampers capable of winning this competition. But that's hardly the point of FSAE. By designing around dampers, you will begin to learn how they influence the dynamic response of a vehicle. If I were you, I'd look into using the Penske 1/4-midget sold by Kaz. It's cheap, valved in the ballpark for almost any FSAE car, easy to maintain and modify, and non-adjustable, so you won't waste precious testing time turning a bunch of knobs. Check it out at http://www.kaztechnologies.com...-Adjustable-165.html (http://www.kaztechnologies.com/Penske-Adjustable-165.html)

FYI exFSAE, there are many reasons why mountain bike dampers are not suitable for FSAE use. It's certainly not handwaving. Aside from the massive difference in force-velocity characteristics (most MTB shocks run a TON of rebound force), the basic construction of a mountain bike damper does not lend itself to responding well to lower-amplitude, higher-frequency inputs. Furthermore, the design of a mountain bike damper does not prioritize minimizing friction in the same way that a proper racing damper does. This is a discussion for another thread, PM me if you want to keep going.

Ryan Kraft
ex-Kaz Technologies

[RobbyObby]

Lots of teams (including us) run on Fox DHX-RC4s. Albeit, our team manager works for Fox so we get the "demo" models donated. We personally have had no problems with them in the past. However, I believe the lowest spring rate available for them is 250 lb/in if im not mistaken. So if you need to go any lower for any reason, that may be a problem.

[blister]

Also Bilstein produces now a FSAE specific damper. I think only one team ran them in Germany. If the price is not too bad than it could be a good alternative.

http://www.motorsport.bilstein...a-Student.481.0.html (http://www.motorsport.bilstein.de/MDS-Formula-Student.481.0.html)

Concerning "Mountain Bike Shocks s.ck for FSAE":

1. The movement for a MTB shock is mainly low speed/low amplitude (pedals) or high speed/high amplitude (jumping from a rock). For FSAE it is more high speed/low amplitude or low speed/mid-high amplitude

2. On a bike there is only one shock (for the rear wheel). So manufactures only need to be in the ballpark of adjustment accuracy. For FSAE you need 4 dampers with at least the same slopes

Both things can be sorted out and while sorting out you learn more about dampers than with looking for the perfect solution

An important thing is also to define what is really low and high speed for FSAE, what for MTB and what for F3 etc.

All in all I also think dampers are not as important as in other series but understanding dampers helps you a lot understanding your suspension. But seeing TU Munich drive on the track in Austria with its phenomenal traction made me wonder if its damper related (they ran professional F3 dampers, like Stuttgart and Karlsruhe) or just very good setup work

[Crispy]

The quarter midget Penske 7800s (not the Kaz ones) are about ~$180 each and you can get a huge range of Eibach springs at ~$25 a pop. We get them through Chris Billings (FSAE design judge and very helpful resource) at the "shock-shop". I think you can get them from Penske in the ballpark of what you need, but they can also be rebuilt. They are not externally adjustable, which is good or bad depending on who you talk to. The damper is definitely good enough to make a competitive car.

The Kaz Penske 7800s are cool too, but a bit spendy.

[Demon Of Speed]

Read:
"The Shock Absorber Handbook" John C. Dixon
The best/only book I have found on automotive dampers.

[exFSAE]

FYI exFSAE, there are many reasons why mountain bike dampers are not suitable for FSAE use. It's certainly not handwaving. Aside from the massive difference in force-velocity characteristics (most MTB shocks run a TON of rebound force), the basic construction of a mountain bike damper does not lend itself to responding well to lower-amplitude, higher-frequency inputs. Furthermore, the design of a mountain bike damper does not prioritize minimizing friction in the same way that a proper racing damper does. This is a discussion for another thread, PM me if you want to keep going.

Oh I agree that from a "legit" vehicle dynamics perspective.. MTB dampers are not the proper choice for this platform.

But "suitable" for FSAE as a series is a little different, IMO. I've always felt that being a strong performer isn't so much about being perfect as it is about just being less f'd up and broken than everyone else.

So from that perspective, I call MTB shocks "suitable" for these cars given that you can put together a pretty decent car with them and have a strong dynamic performance relative to everyone else.

[Zac]

For a first year team I think the two goals should be to build a car and to acquire the resources and knowledge to build a better car next year.

In terms of shock selection, as others have noted there are some options that work better than others, and some options that cost significantly more than others. Doing a cost benefit analysis is pretty straight-forward, but don't forget that you can reuse shocks from year to year. Throwing down $2000 on the ideal set of dampers is a big investment, but is throwing $500 at a set of Risse's that will leak as soon as you take them out of the box a good use of your limited resources? Obviously you need to focus on just getting a car done, but don't be afraid to splurge on a couple items that you can reuse next year.

[js10coastr]

Originally posted by exFSAE:

In terms of priority.. in my experience shock tuning is low-ish on the list in this series. Many teams can't even get their alignment and basic steady state balance set well. I think you'd be more hosed if you miss the ball on that, than on crappy dampers. MTB dampers might not be "perfect" but they've worked just fine for many teams for many years. I've seen top 20 cars (and have been on one of the teams) running absolute garbage shocks which for all intents and purposes could probably have been removed from the car all together!

Some other thoughts-

You can have a pretty well balanced car without swaybars. Get a range of springs from soft to stiff. One way or another you're trying to tune the load transfer split between the axles... springs and bars both do this. IMO the only reason bars are nice on these cars is because when designed PROPERLY, they give you nice quick and meaningful "trim" adjustments instead of big changes by going up in springrate.

You don't need to have bellcranks either. Direct-acting shocks help reduce complexity, and gets rid of a REAL pain in the ass of the design process (bell crank motion). Can be done with reasonable weight as well. Pitt has done it recently.



X2 This was a main reason for me going with the Penske 7800 dampers. They are cheap compared to the Ohlins, of comparable weight, with a range of spring rates readily available. I also ended up with the direct actuation of the dampers after figuring out the history and reasons behind a pushrod/bellcrank system.

[Kirk Feldkamp]

I just picked up a set of used Penske 7800's in fine condition from a FSAE team for $100 a pop. I've had other friends get them for around $100 from online quarter midget classifieds/forums too. They're out there, you just have to look for them.

-Kirk

[sbrenaman]

Originally posted by Crispy:
The quarter midget Penske 7800s (not the Kaz ones) are about ~$180 each and you can get a huge range of Eibach springs at ~$25 a pop. We get them through Chris Billings (FSAE design judge and very helpful resource) at the "shock-shop". I think you can get them from Penske in the ballpark of what you need, but they can also be rebuilt. They are not externally adjustable, which is good or bad depending on who you talk to. The damper is definitely good enough to make a competitive car.

The Kaz Penske 7800s are cool too, but a bit spendy.

Happy Shock Shop customers here as well.



Also, regarding the type of input our cars see...

What do you guys mean by low/high frequency inputs? What does that mean in terms of velocities?

I'm a little bit suspect of our cars seeing mostly high-velocity inputs. We run on pretty smooth tracks, and for every high speed shock movement, there's a corresponding low speed movement at both the top and bottom of the movement. Of course, I haven't actually figured out yet what exactly "high velocity" means in terms of track surface irregularities.

[RollingCamel]

Originally posted by sbrenaman:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Crispy:
The quarter midget Penske 7800s (not the Kaz ones) are about ~$180 each and you can get a huge range of Eibach springs at ~$25 a pop. We get them through Chris Billings (FSAE design judge and very helpful resource) at the "shock-shop". I think you can get them from Penske in the ballpark of what you need, but they can also be rebuilt. They are not externally adjustable, which is good or bad depending on who you talk to. The damper is definitely good enough to make a competitive car.

The Kaz Penske 7800s are cool too, but a bit spendy.

Happy Shock Shop customers here as well.



Also, regarding the type of input our cars see...

What do you guys mean by low/high frequency inputs? What does that mean in terms of velocities?

I'm a little bit suspect of our cars seeing mostly high-velocity inputs. We run on pretty smooth tracks, and for every high speed shock movement, there's a corresponding low speed movement at both the top and bottom of the movement. Of course, I haven't actually figured out yet what exactly "high velocity" means in terms of track surface irregularities. </div></BLOCKQUOTE>

In optimumG seminar Claude asked this question. Where do you think FSAE cars at high speed or low speed? We talked about track surface type and etc but it turned out that you are much more in low speed than in high speed even if in a bouncing Mini-Baja. To get into high speed shock movement you have to pass through the low speeds. It showed clearly on the data histogram.

[Nbruno]

Thanks guys! I definitely did not expect this many replies so quickly

I'll check out the Shock Shop. <$200 Penskes sound much better then $700 Penskes. I'll also check out the other brands. I come from the F2000 world so I am partial to Penske though http://fsae.com/groupee_common/emoticons/icon_smile.gif

As far as the mountain bike shock thing goes, I was advised against them by a former FSAE team leader who now runs his own racing suspension company (and is conveniently a Penske dealer). According to him, the bike shocks are designed for much higher shaft velocities they one would ever see in a FSAE car. Probably won't make or break the handling of the car, but if there are better, reasonably priced options out there, why not just go with that?

Thanks again!

[rkraft]

Originally posted by exFSAE:
So from that perspective, I call MTB shocks "suitable" for these cars given that you can put together a pretty decent car with them and have a strong dynamic performance relative to everyone else.

I'm sure that it is possible to build a car with hockey pucks for dampers and win every dynamic event.

And Scott, don't get velocity and frequency confused. They are separate but related properties of a waveform.

Typically, high damper velocity comes from a high-amplitude input, so you can think of a high-velocity input as a big bump. However a high-frequency input does not necessarily cause high velocity: think about running on a line of small coins spaced every 3 feet at 30 mph: high frequency, very low amplitude, low velocity.

To understand high-frequency vs. high-velocity as it applies to your vehicle, I recommend examining the inputs you see on track in both the time and frequency domains. From there you can decide what sort of damper is best-suited for your car.

[Zac]

I'm sure that it is possible to build a car with hockey pucks for dampers and win every dynamic event.

Akron's 99 car actually isn't that far off from this.

[sbrenaman]

Originally posted by rkraft:
And Scott, don't get velocity and frequency confused. They are separate but related properties of a waveform.

Typically, high damper velocity comes from a high-amplitude input, so you can think of a high-velocity input as a big bump. However a high-frequency input does not necessarily cause high velocity: think about running on a line of small coins spaced every 3 feet at 30 mph: high frequency, very low amplitude, low velocity.

To understand high-frequency vs. high-velocity as it applies to your vehicle, I recommend examining the inputs you see on track in both the time and frequency domains. From there you can decide what sort of damper is best-suited for your car.

'twould be nice to have some pot's on the dampers. I'll design the provisions in this year's car but I doubt we'll be able to utelize them. I'll see if I can get my hands on some Motec damper pot data for an FSAE car. I've always wondered what the majority of inputs the shock sees.

[Demon Of Speed]

Originally posted by sbrenaman:

'twould be nice to have some pot's on the dampers. I'll design the provisions in this year's car but I doubt we'll be able to utelize them. I'll see if I can get my hands on some Motec damper pot data for an FSAE car. I've always wondered what the majority of inputs the shock sees.

We run pot's on our dampers. One problem we have is when we attached them we made an extension on the bell crank which changed the motion ratio and motion gain on the pot compared to the dampers. So the read our data they needed adjusted, which might not be as accurate as we would like. Long story short, think about how you are mounting them, and mount them so they are directly measuring what you want.

[sbrenaman]

Good point Demon - The idea would be to mount them using the same mounting locations as the damper. There's some pretty slick long bolts you can buy/have made that will do the job. I've got a pic in my OptimumG seminar notes, can't find a pic online in about 1 minute.