Inboard v/s Outboard

[Shashi]

Hey guys.I would like to know why nearly all teams I checked out have an inboard suspension design.Any specific advantages?I would also like to know whether there is a decrease in the unsprung mass by the implementation of an inboard suspension.

[PatClarke]

Shashi,
You already asked this question over a week ago on the FSG forum (not that you cannot post here too) My response there was....

Shashi,

Before deciding whether you want 'inboard' or 'outboard' suspension, you need to determine exactly what your suspension aims are and how you intend to achieve those aims.

Firstly, mounting suspension units 'inboard' does not reduce the unsprung weight, and part of the spring and damper as well as the operating rod and bellcrank will still move with the wheel.

Professional open wheel race cars usually mount the suspension out of the airstream (Not necessarily 'inboard') for aero drag reasons, and to clean up the airflow to other aerodynamic devices.

Other advantaces of using a rod and bellcrank mechanism to operate the suspension units include ease of setting up the chassis weights or ride height without interfering with spring preloads etc. Changes in the bellcrang geometry can give varying rates of suspension. It is usual on successful FSG cars for the front WHEEL RATE is linear or rises slightly (progressive) in bump, and the rear WHEEL RATE is linear or falls slightly (regressive) in bump. This can be adjusted much more easily and cheaply by changing the bellcrank geometries than by trying to get variable rate springs that suit your requirements.

Note above I referred to WHEEL RATE, NOT to SPRING RATE!
Wheel rates can be altered with the same spring by varying the motion ratio of the bellcrank.

It is possible to use bellcrank operated suspension that is still 'outboard' and many FSG teams do this to avoid having to accommodate the spring/dampers and bellcranks in the same part of the chassis where their knees live.

Direct acting suspension, as used on historic Formula Junior cars etc, is simple to do, but does not permit much in the way of adjustment. Usually, these systems have a regressive wheel rate, something that is not desireable on the front of your car.

Whatever decision you make regarding your suspension design, be sure to understand WHY you made those decisions. Document your calculations. Excell spreadsheets etc will be useful when the Design Judges question you about your design decisions.

Regards
Pat Clarke

[Shashi]

Thank you Pat.Just getting some more opinions before I begin.Can you please take a look at another post of mine and advice me on the same.

[PatClarke]

Shashi,
If you mean the one about the Ohlins ST44 shocks, why not get the handbook from this site?

http://www.ohlins.com/Portals/0/documents/manuals/07425-01.pdf

It took me all of 10 seconds to Google it.
Neither this site, nor the one at FSG is a shortcut where you can get easy answers. FSAE is an engineering competition where you are supposed to design and build a car and demonstrate it at the competition. People on these forums dont mind helping newcomers get started, but they do get pi**ed off when you can't help yourself with the simplest of matters.
Pat

[Big Bird]

Pat,

I'm not sure if you are still reading this thread, but a quick query regarding your above comments, about rising / linear / falling rates.

I understand the philosophy behind falling rate at the rear (under acceleration and rearward longitudinal weight transfer coming out of a corner, the roll stiffness of the rear drops relative to the front - and therefore we have overall better grip at the rear end to transmit engine power. At least that is my interpretation of the theory). My question is - why does the front necessarily have to be rising rate? Or even linear? If you had a falling rate at the front, but a rate that falls at an even greater rate at the rear, you would have the same relative effect in regard to your roll stiffness distribution.

I understand that the suspension system also deals with pitch, and that a rising rate front would assist a formula track car in keeping the pitch attitude level (obviously important on an aero car in particular). But personally I think pitch performance isn't as great an issue in this comp as it may be in others - we enter most of our corners with a reasonably level pitch attitude. My understanding of the relative rate theory revolved around the roll stiffness and cornering performance.

Only asking because I reckon you could design a direct acting system that gives the desired relative effects - rear rate decreasing relative to front rate - with less mounting points on the chassis (less complexity, less points for flex).

Any input most appreciated!

Cheers,

[PatClarke]

Hi Geoff,
I'll be in Melbourne next week. Maybe we can discuss it over a beer =]
I have developed some new (for me)theories about suspension geometry =]
Cheers
Pat

[NetKev92]

I think the aero concerns are the main point. Caterham noted a drag and nose lift reduction when they re-located their front coil-overs inboard on the Caterham CSR. The question now is whether the re-vamp of their 40 year old classic will be enough to save the company since they let Westfield take the lead in independent rears and other refinements for so many years of gradual development.

The spring stuff mystifies me a bit. Going tail low and nose high seems like bad news for a car with an aero package. Without, it still seems like it would be a secondary effect. Reducing rate springing though does seem easier with a rocker than trying to make a spring do the trick directly. Seems like the soft part of the spring always compresses first, leaving the stiffer part for the second half of travel.

Besides aero and spring trickery, easy position for adjustment and the cool factor play a part because all the big boys in formula cars have them. Most of the serious guys though run on tracks a lot faster than a typical autocross course, and we get back to the aero...

[Brian Evans]

Here is my take on rising rate for the front suspension. The wheel rate is there to fundamentally keep the car off the ground. If the suspension acts in bump, it's because the dynamic load is higher than the static load. If you go to an inch of bump and your wheel rate is 100 lbs/inch, you've added 100lbs of dynamic load to the corner. Since the balance of the car is closely related to the ratio of wheel rate to corner weight, I like the idea that the wheel rate of the corner is rising as the corner weight rises. Plus if the dynamic load is about to crash the chassis into the ground, I like the thought that the rate is getting stiffer, help to keep the chassis off the ground...http://fsae.com/groupee_common/emoticons/icon_smile.gif

I don't like rising rate for the rear suspension because it rarely goes into bump when an increase in rear stiffness would be a good thing...how about a really loose car on acceleration, or a car that loves to oversteer when it hits a big bump...

I have never figured out how springs with dual rates in series or with graduated coil diameter or with graduated coil spacing or with tapered wire work. I always get back to the spring's rate is the sum of the various rates in series, and that's just one single rate at the end of the day. The only way I can figure out to make a spring act in rising rate is to progressively coil bind the spring, making it shorter (and stiffer) as the load increases. Maybe that's what they are doing...

On outboard suspended cars, the spring will either be rising rate or falling rate depending on if it's installed leverage increase or decreases as the suspension goes into bump. the simple way I think of it is if the angle of the installed spring to the suspension arm is greater than 45 degrees, the rate rises, if smaller than 45 degrees it falls, but that is a pretty simplistic generalization. It's easy to plot, though. On rocker arm cars, if the installation angle of the pushrods to the lever is approaching 90 degrees, the rate is rising, if it is moving away from 90 degrees it's falling.

Brian

[jdstuff]

Originally posted by Brian Evans:

I have never figured out how springs with dual rates in series or with graduated coil diameter or with graduated coil spacing or with tapered wire work. I always get back to the spring's rate is the sum of the various rates in series, and that's just one single rate at the end of the day. The only way I can figure out to make a spring act in rising rate is to progressively coil bind the spring, making it shorter (and stiffer) as the load increases. Maybe that's what they are doing...

Brian

...and they do just that! Often automotive progressive rate springs are flat-wire (versus the traditional round) that progressively coil bind as they are compressed.

[Fil]

Hello Gents,

Regarding the direct acting suspension, we ran with this system last year and are sticking with it this year. I'm very happy with it and have had none of the problems you have mentioned.

In terms of regressive wheel rates for direct acting suspension, the problems that are brought up are complaints teams (F1 etc.) had back in the 50's and 60's. They were originally running fairly small track width cars with large damper angles, however as the pace increased they began to increase the track width of the cars and as a consequence they ran the shocks at very shallow angles due to narrow chassis, large tracks and similar sized dampers to the narrow track width cars, then the digressive rates became an issue.

As for FSAE cars where the track widths are generally small there shouldn't be an issue with digressive wheel rates. I am able to comfortably get linear rates within 1% in 2 inches of travel and motion ratios of 0.7.

Also regarding adjustability on our car we have multiple shock pickups on the chassis, each point has a 10lb/in wheel rate change from one another. This means I have 4 sets of spring and the range is from 100lb/in to 220lb/in in 10lb/in increments. It also has coupled damper adjustments so if I need less damping but the same wheel rate I drop the shock to a lower position and put a different spring that makes it equivalent, this is done with a pin that is retained by an R-clip and this change can be done in less than 2 mins. When I did the design for the 06 car I found it was easier get the different motion ratios I wanted from the direct system than from bell cranks. Multiple bell cranks would need to be made or long bell cranks with multiple hole without the sort of adjustment I wanted, this system worked out far better.

Pat what do you mean by "does not permit much in the way of adjustment"?

The corner weights aren't a major issue and the amount of preload that is dialed in is not that significant. One issue is there isn't a neat way to package an anti roll bar like on a bell crank.

One of our major points for going to direct acting was the shock load were all in plane rather than off plane loadings seen in most pull/push rod designs and it meant our chassis was far lighter and more efficient because we used our roll hoop and engine plate to react the shock loads. The system requires no complex jigging and much less manufacturing.

Aero is the main reason why you would go to a push/pull rod design, but we have done tests in the wind tunnel with the 06 car and have not found a significant increase in drag. We get no flow down the side of the car due to the large camber on our front wing which kicks flow up to driver's shoulders height. The amount of time saved in making this system is well worth it.

[Brian Evans]

Fil. did you make your multiple pickup points such that you could change the wheel rate and retain the same ride height without further adjusting the spring? That would have been cool!

A friend of mine has a Lola T460 Formula Atlantic car (1976 vintage), it has slotted front upper A-arm pickup points so that the front roll center can be changed from a "dry' to a "wet" setting in about 30 seconds, without changing toe, camber, ride height or anything else.. I thought that was kind of cool too.

Brian

[Fil]

gday brian,

unfortunately I was unable to do that. The change in ride height is about 20mm so we just adjust the spring platform accordingly. Its not too big a deal, the real benefit is in testing when we want to make quick changes, which we did last year in the limited ammount of testing that we had.

Fil