Rising rate ..wheel or rocker??

[Eklavya Singh]

I have a slight doubt regarding rising rate. Does it refer to only the wheel rate or also to the Installation ratio of the rocker.

Rising rate - Increase of wheel rate with increase in ride (bump).
Installation ratio = Spring travel/Wheel travel
Installation ratio falls with ride
Travel at spring end falls with ride.
Much more travel required at wheel end to provide same travel at spring.
Thus it seems that a falling Installation ratio would be ideal.

But ,
Wheel rate = spring rate * (Installation ratio)^2

I'm confused as to whether the installation ratio should be falling or rising?
With my current geometry I'm getting a falling installation ratio, which I feel is making the wheel go up more for a definite amount of spring travel. This seems correct but then I'm getting a falling wheel rate. Please help. Correct me wherever I'm wrong.

Thanks

Eklavya Singh
Technical Head
Formula Manipal

[Eklavya Singh]

I have a slight doubt regarding rising rate. Does it refer to only the wheel rate or also to the Installation ratio of the rocker.

Rising rate - Increase of wheel rate with increase in ride (bump).
Installation ratio = Spring travel/Wheel travel
Installation ratio falls with ride
Travel at spring end falls with ride.
Much more travel required at wheel end to provide same travel at spring.
Thus it seems that a falling Installation ratio would be ideal.

But ,
Wheel rate = spring rate * (Installation ratio)^2

I'm confused as to whether the installation ratio should be falling or rising?
With my current geometry I'm getting a falling installation ratio, which I feel is making the wheel go up more for a definite amount of spring travel. This seems correct but then I'm getting a falling wheel rate. Please help. Correct me wherever I'm wrong.

Thanks

Eklavya Singh
Technical Head
Formula Manipal

[Silente]

Hi,

i would not think to the problem in term of wheel or spring movement.

You just know that :

Motion Ratio = Wheel movement/Spring movement = MR

It's just the opposite value than your installation ratio:

Installation Ratio = Spring mov/Wheel mov =IR

Wheel Rate cab be said to be( we are taking as negligible a derivative term of second order):

Wheel Rate = Spring Rate/(MR^2) = Spring Rate*(IR^2)

So to have a rising rate at ground (which is what you actually are looking for) you just have to look for an "MR" which is falling down with wheel movement or a "IR" which is rising with wheel movement.

Normally you can obtain this changing a little Rocker design, in order to keep the same suspension geometry.

[Eklavya Singh]

Thanks

One final thing - (Spring displacement / wheel travel) i.e. Installation ratio should rise with bump or droop?

With my rocker geometry its rising with ride = 50mm and falling with ride = -50mm.

P.S. - I'm using WinGeo 3

[Silente]

Bump.

Normally with this kynd of design you want to make smaller body movement in braking. something like a bump stop.

WinGeo has also a lot of good analysis features which can be used to check suspension behaviour.

Good Luck!

[Eklavya Singh]

Are you sure?

I'm getting a falling IR with bump. I tried various iterations with bellcrank geometries. Doesn't seem to increase in any case. And it's a very simple geometry. Right angled bellcrank with pushrod, bellcrank ad spring in the same plane and a .935 static IR. The chassis goes down 50 mm and it falls to .545

[SNasello]

You can make a simple calculation to prove to yourself whether a rising/falling IR leads to a rising falling wheel rate.

Wheel Rate = Spring Rate/MR^2 or Wheel Rate = Spring Rate * IR^2 where IR = 1/MR. Motion ration is defined as wheel displacement/spring displacement and the opposite for installation ratio. So, if you have a 200 lb/in spring, with a static IR of 0.935 that falls to 0.545 at -50mm travel ( i would assume this is jounce, or spring compression, or bump, whatever you want to call it) you would have:

Static WR = 200 lb/in * 0.935^2 = 175 lb/in
WR @ full jounce = 200 lb/in * 0.545 = 59.405 lb/in

This would appear to me to be a (severely) falling rate suspension (wheel rate changes by more than 60%) . For a rising rate, your wheel rate would increase with more jounce. So what you are looking for is either a decrease of motion ratio with jounce or an increase of installation ratio with jounce in order to have a rising rate suspension. Make sure you are using proper sign convention. Chassis moving down or wheel moving up puts the spring into compression so this would be the jounce or bump mode.

[Eklavya Singh]

Ya I just realised. Thanks for the major design error correction.

P.S. to Silente - I went to Modena this time when we came to FSAE Italy. Really awesome place.

[Silente]

yeah, good place to study and to work.

Ferrari has a great appeal here around.

But there other more beautiful places in italy, trust me!

Best wishes!

[Eklavya Singh]

Yeah I know. We went to Rome, Vatican, Pisa and Maranello obviously for the competition. Very beautiful. Will see you guys at da competition next year. Ciao

[-francisco-]

Hi Eklavya, to determine the type of rocker motion you have (falling rate or rising rate), you can easily visualize it like this:

1. The rocker starts (full droop) at a certain angle to the shock, for instance 70º and as it compresses the angle increases towards 90º:

-FALLING RATE (the wheel becomes harder to compress)

2. The rocker starts in relation to the shock at 90º or more and as it compresses this angle increases:

-RISING RATE (the wheel becomes easier to compress)

[-francisco-]

One question:

What percentage of bump/droop do you guys think it's the more appropriated for FSAE cars and general circuit racing single seaters?

I come from a mountainbike background where we use up to 8" of rear travel with 30% of SAG (suspension compression with rider on the bike). But at formula student i couldn't find that info anywhere so by the rules i gave it 50% bump, by the rule of 1" travel each direction, but know i know it's not the ideal.

Any tips?

Thanks

[Eklavya Singh]

I think 2 inches either side is good enough. We designed for that.

[Drew Price]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by -francisco-:

-FALLING RATE (the wheel becomes harder to compress)


-RISING RATE (the wheel becomes easier to compress) </div></BLOCKQUOTE>



This is backwards.



Best,
Drew

[Eklavya Singh]

OK now I'm thoroughly confused. Let me write what I've inferred and tell me if it's right.

I'll talk about IR = spring travel/wheel travel

Angle between rocker and damper increases from say 70 to 90 deg
IR increases
Wheel rate increases
Good
Rising rate

Angle between rocker and damper increases beyond 90 deg
IR decreases
Wheel rate decreases
Bad
Falling rate

I think that francisco is talking about Motion ratio and Drew is talking about Installation ratio.

Conclusion - We want the rocker and shock to move towards a 90 deg angle during bump. This increases the wheel rate and everybody's happy.

Please correct me if I'm wrong anywhere.

Eklavya Singh
Technical Head
Formula Manipal

[Pete Marsh]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by -francisco-:
One question:

What percentage of bump/droop do you guys think it's the more appropriated for FSAE cars and general circuit racing single seaters?

I come from a mountainbike background where we use up to 8" of rear travel with 30% of SAG (suspension compression with rider on the bike). But at formula student i couldn't find that info anywhere so by the rules i gave it 50% bump, by the rule of 1" travel each direction, but know i know it's not the ideal.

Any tips?

Thanks </div></BLOCKQUOTE>

What your asking about is called ride, or heave, frequency in many common VD books. Try RCVD by Miliken and Miliken.

Pete

[-francisco-]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Pete Marsh:
What your asking about is called ride, or heave, frequency in many common VD books. Try RCVD by Miliken and Miliken.

Pete </div></BLOCKQUOTE>

Ok, thanks a lot. I've researched a bit more and found people is aiming at 1/3 droop, 2/3 of bump. I'm only designing the pushrod system of our car because we're running on 1-200 class. So i have to work with previous year's A-arms.

The thing is, with my previous 50/50 travel expectations i was running 60mm of total wheel travel, 30mm each direction. But if i go by the rules and give it 1" of droop, i get 3" of total WT with a 2" shock. Do you think the judges we'll bitch if i run only 20mm of droop?

[-francisco-]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Eklavya Singh:
I think that francisco is talking about Motion ratio and Drew is talking about Installation ratio.

Conclusion - We want the rocker and shock to move towards a 90 deg angle during bump. This increases the wheel rate and everybody's happy.

Please correct me if I'm wrong anywhere.

</div></BLOCKQUOTE>

Yeah i was talking about MR. People talk about a rising IR, but most aim at a linear rate, so the wheel movement becomes more predictable for the driver.

A linear rate is virtually impossible so people go for a slightly progressive one less than 10% of progressiveness they say. On our car we have the front suspension working on the same plane and i designed 120mm rockers to our 200mm shocks, that bottom it out at 90º. That makes it relatively linear.

By by what i studied from pirate4x4.com and The Shock Absorber Handbook (rapidshare), the bigger the rocker the more linear it is. (the varion of the angle between rocker and shock during the motion is smaller)