# Thread: Direct Acting Spring Damper

1. ## Direct Acting Spring Damper

Hello everyone,
I am Ritwik Das from BITS Pilani,India.Our team ran a Pushrod and Rocker system for their last car. However this year we are considering to use a direct acting spring damper system. The reasons to consider so are as follows:
1.Low Weight
2.Less Complexity
3.More Testing Time
4.Bellcrank geometries are not usually changed very often to give different wheel rates.
5.Multiple shock pick up points may be used to give adjustability instead.
6.No significant aero drag on an FSAE car.

We have spring dampers that we have used for the last car. I need to determine whether these will be suitable for the direct acting setup.

What I know & what I have
The force and velocity curves specified by the manufacturer ,so my guess is that the damper shock forces seen in the curve should be less than we would face when the spring is directly mounted.I am also attaching the calculations used for determining spring rates and force velocity curves which were used last year.
We currently have the Quarter Midget Series Damper from KAZ Technologies. This is the link
http://www.kaztechnologies.com/quarter-midget/

2. It's unlikely that you will be able to achieve that MR with direct acting. Work out how the shock will package first and see what range you have available. What are the shocks? Some teams run an extended spring hat to allow them to have more options for motion ratio.

3. We are not trying to achieve a MR of 1 but less than 1(MR = spring-damper-change-of-length/wheelprint-vertical-displacement), the reason being to lower the effective unsprung mass.
We currently have the Quarter Midget Series Damper from KAZ Technologies. This is the link
http://www.kaztechnologies.com/quarter-midget/

4. For some context, for a ride frequency of approximately 2-2.2 Hz at both ends of the car (for a 288kg car with driver included) 45/55 Weight distribution and motion ratios of 0.5 at both ends of the car i was going to purchase the damping code (compression 3, rebound 2) for both front and rear.

Maybe you could compre with that and see how your shocks suit the situation. Personally i would have liked ratios higher than 0.5 but my situation prevented me from doing so. 0.7 would be fairly good if you could reach that i think.

5. Originally Posted by Mitchell
It's unlikely that you will be able to achieve that MR with direct acting. Work out how the shock will package first and see what range you have available. What are the shocks? Some teams run an extended spring hat to allow them to have more options for motion ratio.
For a example, here is what Delft used to do when they where using direct acting.

6. Is that(the above picture) a custom made spring damper with the rod extension for the team ? The valve code currently obtained using new calculation doesn't match the old ones. So can the dampers be revalved to change the force we want OR do we have to buy completely new dampers because the latter is too costly. Also I came across this note by a member of Monash FSAE

"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."

7. Is that(the above picture) a custom made spring damper with the rod extension for the team ? The valve code currently obtained using new calculation doesn't match the old ones. So can the dampers be revalved to change the force we want OR do we have to buy completely new dampers because the latter is too costly. Also I came across this note by a member of Monash FSAE

"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."

8. If you will take a look at the photos from 2015 FSAE Michigan, you'll find the car that was 1st in both autocross and endurance used direct acting dampers (Rejoice Z!! We've been listening to you! Brown gokart!). GFR used rod extensions to get the motion ratios we wanted. The use of multiple shock pickups on the chassis is a great idea, and could be coupled with exchangeable extensions of different lengths to maintain ride height.

Ritwik, GFR uses non-adjustable dampers. Our vehicle dynamics folks have learned to re-valve them, it's not hard.

9. bob.paasch
When I saw your car on facebook page with direct acting, the first person came into my mind was Erik .

10. This is really a silly question so please bear with me ,
Ours is a steel spaceframe chassis , so to have multiple shock pickups won't we need multiple tabs on the chassis at different heights which have to be welded. Also how can rod extension alone create a variety of motion ratio without coupling it with multiple pick-up points since Motion Ratio is defined as "sin(Spring Angle)*B/A where B= Distance of CA mounting point on the chassis to the shocker mounting point and A=distance of CA pivot point to the chassis. So how does using rod extensions of different length without changing the spring angle affect the motion ratio. What am I missing?
Sorry again for the silly question but I can't just figure it out.