Rising Rate Suspension...Help!!!

[Cricketboy]

MSC,

Ive been set an assignment from uni to design a suspension system that has adjustable rising spring rates. Basicly we are not allowed to use a progressive spring, and the system must be cheap/light for use in the Fromula Student events.

So far research found has been minimal. We know what rising rate is but are struggling to work out how it is achieved on race cars. Any knowlage of any systems be it road/race, car/bike please direct me to where you found it so i can read up on the subject.

If anyone knows exsisting solutions to rising rate systems can you help us out?

Thanks
Ash

 

[frank]

you could angle the gometry so that the arm acting on the shock/spring is throwing an arc eh ash (like a conrod moves the piston very little @tdc, and moves the upstroke far more if the crank is offset)

 

[reflexuk]

A pushrod from the wheel end actuates a rocker which actuates one end of the spring/damper. Due to the radius(s) of the rocker the spring sees an action that progressively increases. Like my model car.
http://lmgtfy.com/?q=ttech+predator+front+suspension
switch the results to images to see how it works.

 

[Bombardier]

Dirt bike rear suspensions have a linkage system that creates different rates.

 

[Cricketboy]

So its mostly just suspension geometary? What about dual spring shocks and progressive rate bump stops. Also can a system be rising rate with one spring and damper for both front wheels? Ie a monoshock system.

So far we have identified a few concepts of suspension geometary as you guys mentioned. However we are struggling to find any information on these alternate solutions

 

[mckenziecz]

what about horizontal suspension setup with one shock on the middle. would that not be rising rate? just an idea

 

[reflexuk]

A monoshock can only be a rising rate when both wheels on an axle experience the same compression bump. In body roll one end is being compressed while the other is released ie no compression of the spring at all! This design tends to control roll with anti roll bars.

Variable rate bumpstops are a crutch to help a inadequate spring design.

look up the engtips forum for specialised advice

 

[mckenziecz]

A monoshock can only be a rising rate when both wheels on an axle experience the same compression bump. In body roll one end is being compressed while the other is released ie no compression of the spring at all! This design tends to control roll with anti roll bars.

I thought of that afterwards. Silly me.
I got another idea in head. I'll draw it and post it later. Working atm

 

[Cricketboy]

cheers fellas

 

[mckenziecz]

hope that make sense. just another horizontal setup with another arm that would have two rods from each side of the shock pulling a collar at the bottom of the spring as the suspension goes depresses.

 

[Cricketboy]

is this rising rate though. if the spring is linear wouldnt this set up maintain that linear spring rate.

all the set ups ive seen have a bell crank that translates the vertical motion of the wheel into rotational movement of a link between bell crank and spring/damper. the rotation means for small travel the spring is compressed a little. as the crank pivots the link moves in an arc and causes larger deflection at larger wheel travel, hence causing rising rate.

other than this bell crank set up are there alternatives like i said orginonally, dual spring or provessive bump stops.

Any ideas?

 

[reflexuk]

Small correction - ALL motions in wishbone bases suspension designs are rotational.
All geometries cause some amount of rising or falling of the rate of compression of the spring - sometimes even one then the other.
You have to watch the angles between the links to establish whether the rate is rising or falling. Most of the angles in the line drawing are closing as the wheel bumps therefore causing a falling rate. But, because both ends of the spring are being actuated the net result might be a rising rate. It would take some maths to work it out.

Ive been set an assignment from uni to design a suspension system that has adjustable rising spring rates. Basicly we are not allowed to use a progressive spring, and the system must be cheap/light for use in the Fromula Student events.
If this is the brief then the cheap simple solution is the single rocker, with slots on the arms to move the pickup points. I would say it needs a ballrace for precision and smooth operation, perhaps a single race would be cheaper than two. Rose joints to pushrods and damper shaft. You need a sturdy pivot pin on the chassis too. Perhaps the rocker is constructed from two Carbon Fibre plates,(fulfilling the light part in the brief), held apart by the rose joints with a tubular insert to serve as a bearing carrier. It's already been designed of course but your brief doesn't ask you to come up with something new or revolutionary, and because it uses the key word cheap you can carry it off. You therefore appear to be overthinking it and strictly, your brief is easy to fulfil. It may be even cheaper with Ally plates and a greased bronze bush, the ally being the light part.

IIRC most of the formula student cars are this way already, no.

Now if you really want a challenge, one with a potential positive financial outcome, you should have taken on the LSD for formula student brief. That's the most expensive part of the car isn't it? Many teams would wholeheartedly embrace ie purchase a cheap purpose built one wouldn't they? Don't take me up about that, I think I've mentioned all I know.

Remember, design, like art, when done well, is almost effortless. Advances in basic sciences is what usually permits advances in design. The mobile phone had been conceived, designed and was utilised in the original Star Trek series in the 1960's. It took advances in basic science to make it possible.

 

[Cricketboy]

I understand what your saying, as a group we have knowlage on this type of set up. However there are other ways to achieve rising rate as mentioned above. These alternative set ups we have less or no knowlage on, hence why i am asking for some help if anyone has experience of these types of suspension systems.

And yes most Formula student teams buy the diff, and yes it would be more effective to research into LSD's. However we only bulid the formula student next year. This assignment is a 'warm up' excersize to get us thinking about our design methodology.

As I first said. does anyone have any knowlage of the following rising rate systems

Monoshock
Dual spring
Progressive bump stops

Thanks guys