Air suspension for adjustable ride height

[JulianEdgar]

I don't know if this has come up often, but of all the aftermarket suspension systems, using air suspension most easily allows you to adjust ride height.

The whole topic of air suspension (different types of springs, spring rates, compressors, etc) is a bit beyond a simple discussion group post, so I thought I'd do it in some pics.

The relevance to aero is that changing ride height influences both drag and lift/downforce, and air suspension allows easy adjustment, both 'on the fly' and statically. It also means you can maintain a constant ride height front and rear, irrespective of load.

So, on my Gen 1 Honda Insight...

Front struts (note rolling lobe air springs - not donuts):



Rear air springs (again rolling lobe):



Compressor and tank (normally hidden behind bumper):



Valves:



Front height sensor:



Rear height sensor:



Controller (Adaptronic e1280s engine management ECU with custom program):



Gauge screen on controller software (used only during set-up):



The controller automatically maintains correct ride height, irrespective of load. It also progressively lowers the car at speeds over 80 km/h (50 mph) and then when I slow down to 80 km/h, it pumps up very fast (because I don't want to hit a local floodway crossing at ultra low ride height).

I also have a manual driver over-ride for height control, and display the front-left, front-right and rear ride heights on my dash.

It was a lot of work to do all the modifications but I literally haven't touched it now in years.

[JulianEdgar]

Having full control over the ride height is interesting.

First, I found through measurement that my preferred 'over 80 km/h' ride height was already best for both drag and downforce. (That height is just a little lower than the standard Honda ride height.) That's interesting, because even the standard Insight ride height is terrible in terms of ride and handling on my bumpy roads! (With steel springs I improved both by going to a higher ride height.) But with adjustable damping and rising rate springs (as rolling lobe air springs are), I can get away with it.

Second, lowering the car clearly improves stability in gusting crosswinds. That was more the case before I added the rear fins - before those, the better stability with the ride height drop was very noticeable.

Third, one of my favorite drives is on Highway 1, the main highway that joins Australia's capital cities on the east coast. (It's very much like the US interstates that I loved so much.) Highway 1 is the quickest route to my nearest major town, and so I jump on it, manually dial in lowest ride height, turn on the CB and listen to the trucks. Very funny.

There are also opportunities to explore things I haven't yet done. For example, at speed, I could program the system to lower the front more than the rear, ie change rake depending on speed. However, I have set up the auto changes in ride height to give me best ride and handling - and that in turn means maintaining enough bump clearance at all speeds (so no rake change).

[JulianEdgar]

But it was almost aerodynamically serendipitous that fitting air suspension also meant that I had installed front and rear height sensors.

Why? Because when I fitted the front and rear aerodynamic undertrays, I started getting dash warnings for low ride height!

Huh? Let's take a few steps backwards.

When first setting up the air suspension, I'd been worried that something would go wrong without my noticing, so I had configured warnings like:

• low tank pressure
• high tank pressure
• low ride height
• high ride height

The Motec dash allows these warnings to be configured in plain English, and in addition I can flash red lights when they occur. I set all this up, and it all worked fine - no warnings.

Then, when I fitted the undertrays and went for a drive, I immediately got the 'low ride height' warnings! Repeatedly, and at any speed over 100 km/h (~60 mph). That's when I realised the undertrays were providing real, measurable downforce.

After that, I realised I could disable the 'automatic' aspects of the air suspension (auto lowering, etc) and directly measure lift and downforce - just by looking at the dash display.

(Dr Wolf of Porsche, when giving feedback on my book, also commented that Porsche had used the same approach - measuring ride height on the road to determine lift and downforce.)

[JulianEdgar]

I did start off using a relatively simple PIC-based controllers for the air suspension, done by my friends at eLabtronics.

This sort of programming:



...and this sort of hardware:



But I found I really struggled with it. One issue was that the re-programming wasn't 'live' (you had to upload the complete program to the controller before you saw the new settings in action) and another was that it didn't have enough software 'building blocks' for my purposes (eg no PID block). A final issue was that there were no gauges in the software - so you couldn't see what was actually happening.

I got more and more frustrated, and then called on someone I know who has developed full programmable engine management ECUs.

He said: "Sure!" and then helped me massively in churning out custom stuff for a brilliant ECU* - but programs written in flow-chart software that I can understand (I cannot write code, so I need flow charts).

So I tried his approach, and very quickly got better results than previously. When I got stuck, my buddy would help me out with some new programming - eg averaging a road speed input. (My buddy's name is Andy Wyatt, and he has now moved to Haltech.)

Within an unbelievably short time (weeks, not months) the system was up and running and working well.

*the ECU is designed from the ground up as a completely customizable controller. It doesn't seem to have been a commercial success, but it is absolutely awesome. If I could afford to, I'd buy five - control my (yet to be built) solar house, any industrial process you name, etc.

As I said, I haven't had to now touch the programming in literally years - something that I was starting to think was a bloody impossible dream!

(I think the air suspension system (engineering, hardware, electronics, software) is the single hardest car project I have ever done - but air suspension per se doesn't have to be this hard!)

[JulianEdgar]

And while I like electronics, I also like pneumatic / hydraulic controls systems, that over the last ~100 years have reached a really high level of sophistication.

So height control could be maintained by a valve like this:



..and I imagine, though I haven't done it, that an rpm-based pressure (ie a pump) could operate a speed-controlled lowering (a bit like the speed input in an old auto trans).

My old W123 Mercedes has a hydraulic/nitrogen self-levelling system at the back that works exceptionally well.

And 1950s / 1960s Citroen are still state of the art for oil/nitrogen springing and self-levelling - although I don't think any lowered themselves at speed.

[skyking]

Brilliant!
That's the thing about the automatic ride height valving, it doesn't allow for adjustable heights. The electric potentiometer is better suited for that.
I want to do air suspension on my truck but it's going to be quite complicated and more along the lines of what you did.
I'm looking for a total of 8" (200CM) of suspension travel, with about 4" of practical ride height change. That presents challenges with steering geometry.
The 1500 Ram trucks have OEM air suspension options on higher trim levels.

[JulianEdgar]

Took a pic of the dash on the highway today:



Ride heights on bottom line - front left, front right, rear. Measurements are in centimetres and should all be zero at this speed after 15-20 minutes. So rear ride height is 1mm high in this pic.

FYI:

21.8 = AFR
5 = gear
103 = km/h
73 = kPA manifold pressure (must have been going slightly downhill)
Bar graph = tacho

Bottom line can show lots of different data, switched through via a button.

Blue light (just visible) shows I am in lean cruise mode.

[Taylor95]

Very cool. What sort of fuel economy do you get with highway driving?

This seems like a very complex system to set up, especially with the programming. Is this feasible for someone without connections to anyone in the auto industry?

Can you provide links or more details on the struts that you used?

[JulianEdgar]

Originally Posted by Taylor95

Very cool. What sort of fuel economy do you get with highway driving?

High Threes in litres/100km (about 60 US mpg). Drive it really, really gently and 78 US mpg is possible; drive it hard and 36 US mpg is possible.

That's all with about double normal power and no IMA.

I seldom drive it gently.

This seems like a very complex system to set up, especially with the programming. Is this feasible for someone without connections to anyone in the auto industry?

Yes it is a complex system. I'd say that at minimum you'd want to have mapped and wired programmable engine management or similar. Note that the air suspension installation also required quite a lot of fabrication, welding, etc.

Someone in the UK is fitting air suspension to the rear of his Insight. He has my book on air suspension but as it's only a little book, he has been asking me for more details. He and his son are developing an electronic control system based on the PIC micro (similar to the first system I used).

[JulianEdgar]

Originally Posted by Taylor95

Can you provide links or more details on the struts that you used?

They are Airlift struts - something like these.

Jig with original strut on it:



New strut on jig (I didn't weld with the air spring in place!):



Old and new, prior to welding:



Finished lower half before damper installation. Note extra hole for 'fast rack' steering arm position. Tapered holes meant I had a machine shop make the steering arms.



Finished struts before paint: