Let's talk about up to date coil and air shock options

[jpusb]

Hi to all,

Sincerely, I am a bit out of date on road racing shocks utilized in FSAE (for example, know very little about Penske options, I can see what they offer at the webpage of course, but I don't know how they perform and the tricks). But, on the other hand, I am currently very deep into bike shocks (downhill racing shocks, most of the time, but enduro also), and I often ask myself the question, why aren't people considering these, way more cheap, shocks for FSAE?

Most of them are 4-way adjustable, but still many of them are 3 or 2 way (which is enough for A LOT of teams), they are light as they can get, easy to service and shim, reliably AS HELL, the range of tuneability is amazing, clicks and knobs are tool free (and look cool http://fsae.com/groupee_common/emoticons/icon_wink.gif) , and few of them are even now offered with an airspring (air sprung suspensions have improved SIGNIFICANTLY over the last 3 years I would say).

I am yet to see (in pictures now, since sadly I don't get to go to FSAE competitions), a good nice car with Rockshox Vivids (for example). Even Cane Creek unveiled their air sprung double barrel in 2011 (I have no data on this shock for now). Air forks (which is not the case here, but anyway) have been winning races since 2006 (then they were ok, today they are increibly good), and even now in Supercross (the competition level top-teams in Supercross is insane) top teams were running air forks this year (2 pound weight saving for them).

And after working with air suspension for a while now I can say the weight savings and adjustability (zero $ and time to change spring rate to exactly what you need) they offer are simply too good to pass, at least IMO. But even ruling out air options, you still have a lot of way cheaper, very good options out there.

What do current people designing suspension in FSAE teams think?


Cheers to all and good luck at MIS
JP

[Markus]

Not a suspension guy so I won't dive that deep into this...

But Cane Creek (aka Öhlins) double barrel downhill bike shocks are a quite common choice for FSAE teams around the world. You can even buy them with valving suitable for FSAE.

Air shocks on the other hand sound like an idea to consider and it's nice you're bringing it up. I'd think it's possible to run these shocks with nitrogen? As the shock temperature might change considerably during endurance - arguably depending most on shock positioning - but it's hard to quantify the effect without testing.

Also the progressive rate of air suspension, an advantage on downhill biking and motocross, has its pros and cons on a "racecar" solution.

[jpusb]

I know CCDB are used a lot in FSAE, that's why I mentioned the air sprung option (in fact, our team has been on the coil double barrel for the last 3 years). But they are not cheap IMO.

I agree with you about air heating up, and this is the main reason why they are not the standard in DH racing (although they are being used more and more). But I don't think shock air temperature would be a problem in FSAE, these shocks are designed for a lot more punishment than road racing. In 4 minutes of downhill trail riding the shock can see shaft speeds a lot higher than FSAE, and also gets cycled a lot more, so heat dissipation is an important factor in bike shock design. In FSAE you run at average higher road speeds (read, cooling air speed) so.
Progressive rate is true but this has been attacked in many ways by shock manufacturers and it is now virtually gone (unless you want there to be, usually you can tune this, sometimes by opening the shock, but sometimes through knobs). Anyhow, now its linkages that are giving the progression part while shocks are getting more linear. Also, what is known as "negative air spring" has a great effect on this (and many more things) and works great.

I would love to see a top team on cheapbutstillequallygood shocks.

JP

[Luniz]

Hi JP,

the two main challenges you will run into when using mountainbike shocks are:

1.) They are overdamped for the application. Usually you would take a 200/57mm shock and have a MR close to 1:1. In the Bike application, those shocks are installed with a ratio of a lot more than that, 2-3:1, and much stiffer springs. So even if they are super adjustable, the base valving might be too much.

2.) As you normally only have one shock on your bike, the accuracy of damping rates according to clicks on the adjusters is not of a very high importance. But you will have trouble setting up your car with four shocks that all behave a bit different

Air shocks look like a good possibility to save some weight, but additionally to all the above, you have one more setting to control and to set up, which makes it more complicated. Also, you can not change spring preload on an air shock without also changing the "spring rate" so setting up corner weights might be a bit tricky...

[Francis Gagné]

Luniz, I do agree of most of what you said. And JP, the price tag for a CCDB is not much lower than a Ohlins TTX25. Probably not worth the risk and uncertainty in damping ratio. (Maybe 200$ to 300$ less for a set of 4) and that is with a 10% off from MSRP, which is the same for both damper at 650$. (In fact the TTX25 is a revalved CCDB, but is available coiled only through Ohlins, http://www.ohlinsusa.com/files/files/Features.pdf)

The weight saving of an air shock is around 2000g per set of 4. Though, it would need calculations to see if there is a motion ratio that can use the stock spring rate and damping adjustment range of these while using its travel. And you still have the risk of overtemp, damping and/or spring rate change with temperature, etc.

If a team really would want the mass saving of air shock they might be able to revalve the CCDB AIR with a TTX25 revalve kit, but it is not on the "cheap" side anymore.


Originally posted by Luniz:
Also, you can not change spring preload on an air shock without also changing the "spring rate" so setting up corner weights might be a bit tricky...

If you use a push/pullrod setup this is not an issue is you have a length adjustment on it.

[jpusb]

I was not suggesting to buy CCDB instead of Ohlins because as you say the price difference is really low.

I was suggesting something more like the Vivid. The normal price of the Vivid is around $343 per shock. They can offer this price because they sell hundreds and hundreds I guess, you can even find them in discounts for $230 (new). The vivid comes in three different tunes, and the low damping tune is for motion ratios that you would see in a FSAE. Anyhow maybe you need to re-valve rebound, but as I said working on these things is piece of cake.

Have you dyno tested a set of these bike shocks lately? I haven't, so I won't say that they are all equal in terms of tuning, but I wouldn't say the opposite just because they are cheaper. I don't think manufacturing qualities are any different than those from Ohlin or Penske, regarding tuning knobs. The build quality of these things is amazing now, because the market is growing so fast and the competition is really tight. I would say it is one of the sports with more tech improvement from year to year, it is crazy (and a bit nonsense really).

The preload issue, as Francis said there are many solutions for this both to correct ride height and corner weight, very simple.

My guess on this is that teams that would really benefit from these shocks (mid field, mid budget teams) don't want to jump out of what is known and people use in FSAE/Fstudent. Happens in every single race series and even in towns (with food and what people do/sell), every body firsts looks at what people are doing.

I would love to be a design judge just to have real insight on how many people on expensive shocks are really using what the shock offers.

Let's see where this thread goes.

[Luniz]

Back in 2008 we had a few shocks on the dyno, and they were all different, up to 20% if I recall correctly. They were Fox DHX 4.0 so also not really the cheapo stuff...

Another thing that crossed my mind: The Öhlins/CCDB is the only shock with eyelets big enough to fit balljoints, all the others run on DU bushes. The balljoints give you a lot more otions in terms of installation...

But I do agree with you that most people just run the Öhlins because everyone does and they aren't really using them. When we were doing setup for our 2011 car, we managed to reduce laptimes over 15% just with different shock settings, pretty impressive. But that shows that you can have an awesome shock and completely ruin your car's performance with it if you don't have a proper setup ;-)

[jpusb]

That is really cool insight you shared about the DHX 4.0. However, as I said, things have changed A LOT since then. Still, I wouldn't use a DHX for a FSAE because settings are far from independent from each other, you change one setting and alter a bunch of other stuff. I don't know how is the RC4 in this regard, but what I do know is that you can ask Fox for custom tunes (when buying the shock new) and that could be really useful for FSAE.

I'm with you in that you can certainly ruin the setup with bad shocks (or gain a lot, specially in consistency and long sting runs I think).

I prefer ball joints myself, but all these shocks can be run with needle bearings without issues.

[Markus]

The problem I see in needle bearings is non-forgiveness to angular errors.
It's doable but requires really tight chassis and suspension build tolerances, which might be a big trouble especially with a steel space frame.

[Luniz]

That's exactly what I meant with "more freedom/options in terms of installation" ;-)

[Thrainer]

In 2012, autocross at FSG and FS Austria was won by a car with air shocks. So it seems they can work in FSAE. AMZ is using DT Swiss air shocks and I've just seen very similar shocks on the Zwickau car today.

[jpusb]

AMZ is running air shocks for FSUK 2013.

[jpusb]

FS UK overall winner and 2nd place were both on air bike shocks. Would be cool to have input from someone of these teams on the topic.

Awesome cars.

[mmcdermott]

The spring rate of a gas spring varies nonlinearly with deflection. Therefore, the roll stiffness at each axle, the LLTD, and the understeer/oversteer also vary nonlinearly with deflection.

With enough analysis and testing you may be able to use this "feature" of gas springs to your advantage. Or you may decide that you don't need the added complexity of tuning handling with variable rate springs.

Bicycles and motorcycles do not have to deal with roll stiffness and LLTD. (I like gas springs for off road motorcycles and have run gas over soft coil springs for over 30 years.)