Developing a Proper Suspension Model

[Shankenstein]

Originally Posted by Calum

I downloaded that to my phone the other day, expecting to read it at work. I tried to open it today but it's blank. I've tried to open it on my work computer aswell, nothing. Is anyone able to read what Andy linked to?

Yep! Roll Center Myths by Mitchell. Good read, but I can't say I fully understand it yet.

http://www.scribd.com/doc/55617287/F...Centers#scribd

[Calum]

Originally Posted by Shankenstein

Yep! Roll Center Myths by Mitchell. Good read, but I can't say I fully understand it yet.

http://www.scribd.com/doc/55617287/F...Centers#scribd

I just realized it's behind a pay wall. I'd love to read it, but I'm not paying $9 for 6 pages.

If anyone's willing to help me out a little, send me a pm, please.

[Amaya]

Originally Posted by Calum

I just realized it's behind a pay wall. I'd love to read it, but I'm not paying $9 for 6 pages.

If anyone's willing to help me out a little, send me a pm, please.

This is a free download of what I believe is the same thing. Also I remember reading something similar from the SAE website a couple years ago (may have been the same thing). ptmaynard.com/file_download/19

[Calum]

Originally Posted by Amaya

This is a free download of what I believe is the same thing. Also I remember reading something similar from the SAE website a couple years ago (may have been the same thing). ptmaynard.com/file_download/19

[Racecomp Engineering]

Weird, I had it open to share on my google drive. Did it work for anyone else? I can email it if needed.

- Andy

[fika84]

I had it open on my computer and it worked just fine.

[Shankenstein]

For science!

I ran across the manual for the Ohlins TTX. Of course, there's some marketing... but they go into alot of detail about the history and hardware in their high-end gear.

Long story short. Adjustable poppet valves with shims, low gas pressure, and high flow pistons. This allows for short stroke, but high force without generating much hysteresis. It's not rocket science, but it's elegant engineering.

http://www.ohlinsusa.com/files/files...A4-Europe1.pdf

[Captain Snooze]

Originally Posted by Shankenstein

For science!

I ran across the manual for the Ohlins TTX. Of course, there's some marketing... but they go into alot of detail about the history and hardware in their high-end gear.

Long story short. Adjustable poppet valves with shims, low gas pressure, and high flow pistons. This allows for short stroke, but high force without generating much hysteresis. It's not rocket science, but it's elegant engineering.

http://www.ohlinsusa.com/files/files...A4-Europe1.pdf

I've not read it all but I'm thinking if I won lotto a (relatively) small amount of money would be spent on buying some TTX dampers then a truckload would be spent on paying the engineer to make them work.

I was surprised by the 40/50 click low/high speed adjustment range.

[Shankenstein]

Originally Posted by Captain Snooze

I've not read it all but I'm thinking if I won lotto a (relatively) small amount of money would be spent on buying some TTX dampers then a truckload would be spent on paying the engineer to make them work.

I was surprised by the 40/50 click low/high speed adjustment range.

From what I can tell, the low speed adjustment is a needle valve that bleeds pressure.


The high speed adjustment seems to be a simple poppet-style check valve that you can vary the preload on its spring.


When I was in R&D at an oilfield services company, we used a ton of these check valves. The stankiness and corrosive nature of an H2S well were pretty impressive.

[Shankenstein]

Pistons!

Everybody's got em, some people know what they do, and a select few people in the world are cool enough to design them.

Shock absorber pistons exist to limit and direct fluid flow, in a way that dissipates energy. In most modern shocks, the piston has shims on either side to control fluid flow. Since shims are flexible, they will open when enough bending moment is applied.


If you look at the full piston assembly, you will see that the center is fixed, and the outer edges experience bending




There are different types of pistons, which play with the shape and location of the compression and rebound ports. First a summary, then details.


1) Linear Pistons:

A well-designed linear piston will expose the face shim to relatively uniform loading. It will also have plenty of space for shock oil to enter. Target market is usually racing, since there isn't any high-speed blow-off.

2) High Flow Linear Pistons:

As you'd expect from the name, if you're moving alot of fluid and want linear response... this is your piston. High force and high stroke applications benefit from this type of design.

3) High-Frequency Pistons:

Note the tiny holes around the perimeter. This was Ohlins' compromise that provided similar performance to a linear piston, but with a slight deadzone that moved high-frequency, low-amplitude inputs to the tires using bleed (un-shimmed pressure relief). Link to smart people discussing it

4) Linear/Digressive Pistons:

For street applications, a linear compression setup can be a bit "crashy" if you choose an aggressive initial damping rate. This allows suspension engineers to blow-off excess force that would normally build up on a linear piston. All of the compression force is concentrated on the outside of the shims, which causes more bending moment. Also, many manufacturers include a degree or two of pre-load on the compression shim stack.


5) Digressive/Digressive Pistons:

If you want softer performance at high speed rebound, you can do the same thing as above. Blow off excess force, once sufficient pressure has built-up. Typically the rebound stack is allowed to build more force before blow-off, since this is less noticeable to the driver.

6) Velocity-Dependent Pistons

These pistons typically have a progressive-linear-digressive shape... which allows you to keep the nose of the dyno graph narrow while still providing for the high speed rolloff.

Other notes:
- Many pistons have a spot for inserting a variable bleed screw. Similar to the high frequency design, some bleed helps to minimize hysteresis by equalizing pressure during low-speed damping ... but it sacrifices some high speed responsiveness.

- You can build 2 or 3 layer shim stacks. As higher piston velocities are reached, the deeper layers will reinforce the stack. This allows for fine-tuning of high speed behavior.


For those interested in shims, I'm still learning about the different interactions. As the guys on ReStackor say, it's both simple and complex. Check out their site for some amazing explanations: LINK

[RBbugBITme]

Thanks for posting mostly Penske pistons. Its much of the reason Penskes are better than the rest. They are important.

[Shankenstein]

Originally Posted by RBbugBITme

Thanks for posting mostly Penske pistons. Its much of the reason Penskes are better than the rest. They are important.

Werd. Ohlins has some interesting designs, and I applaud their effort with the R&T series for mainstream sportscars. Bilstein is an incredible value propostion. Penske is the bleeding edge of the R&D and track support, and obviously racing teams find alot of value with that.

It's a quite similar to my previous profession (oil & gas services). It only takes a few brilliant engineers, quality technicians, and a machine shop with top-notch capabilities... and the incremental performance becomes big $$'s.

Do you have any recommendations on choosing shim stacks for a VDP?

VDP feels like the "way of the future" since it allows for low-friction and street-friendly materials... but tuning seems like a nightmare. My best guess is that it requires a soft low-speed stack and a moderately stiff high-speed stack. I can't imagine a 1-stage solution working for all applications, since the piston face geometry is fixed.

[RBbugBITme]

Originally Posted by Shankenstein

It's a quite similar to my previous profession (oil & gas services). It only takes a few brilliant engineers, quality technicians, and a machine shop with top-notch capabilities... and the incremental performance becomes big $$'s.

Do you have any recommendations on choosing shim stacks for a VDP?

VDP feels like the "way of the future" since it allows for low-friction and street-friendly materials... but tuning seems like a nightmare. My best guess is that it requires a soft low-speed stack and a moderately stiff high-speed stack. I can't imagine a 1-stage solution working for all applications, since the piston face geometry is fixed.

You hit the nail on the head. Penske customer support is unmatched and everyone copies the piston designs.

I'll get back to you with an educated answer on the VDPs. I've yet to develop a shock for a sports car that used a VDP but I know it's done. I will say the main piston design is very dependent on the track and situation. Usually we're looking to absorb bumps so you can attack corners so a progressive high speed curve is a bad idea. Our digressive/regressive performance is why we took the top 7 spots at the Tudor race at Sebring and top 5 in Continental Tire. Of course Sebring is uniquely bumpy.

[RBbugBITme]

So the low down on VDPs. Their use can be very driver dependent, it is mostly a driver feel decision between the VDP and the digressive piston. When you're fully loaded in a corner and you hit a bump, you'll blow off a lot of force with a digressive piston and it can feel like you've lost your platform. If a driver doesn't like that he can use a VDP piston to build some more high speed force into the shock after it blows off.