Air suspension on Lotus 7

[hssss]

I am building a Lotus 7 type car with a 351 which will be used primarily for street use and some solo 2. The suspension will be primarily MII front with longer CAs and 06 Mustang with longer UCA. has anyone used or nkow someone who has used air suspension in a handling setting. My donor car is a MKVII that has the entire system intact. It would seem to allow a low ride height and still be able to allow crossing those pesky speed bumps, train crossings, and driveways without a CLUNK.

[bret]

I don't have any personal experience with the Lotus suspension, but I do have considerable experience with air suspension in general and have spent some time addressing the handling aspects of this type of suspension. I've done 2 autocrosses this spring...the Year One event and a local SCCA event yesterday in Illinois. We took a 67 Camaro [3250 lbs.], an 05 Mustang [3650lbs.], a 71 Camaro [3580 lbs], and a 70 Buick GSX [3950 lbs]...not what you would consider prime autocross contenders. None of these cars were set up specifically for these events...they were run right off the street. They were equipped with double adjustable ShockWaves and/or DA shocks and airsprings. We typically stiffened up the shocks a few clicks from the optimum street settings and let it fly.
We did very well. Kevin Orr and David Gravely from Year One drove these cars in the autocross in Atlanta and were so impressed that they ordered 3 systems on the spot to be installed in time for the Power Tour. We couldn't run as well as the prepared lighter weight import cars like the RX7 or the WRX's, but could easily outrun most of the serious musclecars in attendance.
In the SCCA autocross yesterday, we chased the lightweight prepared cars and the "civilians" chased us. It was a long track...the REALLY fast cars were running 39-40 seconds...we were running 45-46 seconds...the civilians were running 48-50 seconds per lap.
I attribute most of this success to shock tuning, proper sized sway bars, and appropriate alignment settings...just like any traditional suspension. The one really helpful aspect of the air suspension is the ability to make small adjustments in air pressure to affect spring rate. These adjustments are akin to adjusting your line around the track...you can gather opinions and evaluate textbook information, but at the end of the day it is really a matter of trying different settings until your car is happy. With air you can make infinite adjustments right from the drivers seat instead of changing components. It moves your tuning curve ahead light years!
I don't want to make any secret of the fact that I own Air Ride Technologies...so consider my opinions biased. However, I have been a hotrodder much longer than I have owned this company so I will try to keep that shameless self promotion to a minumum. I typically dont quote a lot of textbook theories for the same reason I don't design parts [initially] on the computer...I just don't think that way. I try, I fail, I try again, I fail some more, I try again, I succeed [maybe].
Anyway...hopefully something in my rambling here will encourage your to try the air suspension to see if you get successful results.
As to your Lotus...if it is a coilover suspension there is likely a ShockWave that will drop right in place of the coilover for an easy installation. Go with the double adjustable version so you have independent rebound and compression valving. As far as a compressor kit...you can start out basic and go from there. The expensive compressor kits only serve to lift the car faster...likely not something you are interested in for this car. Good luck!

[hssss]

Basic Hot Rod is what this car is about. It's actualy intended to be my 93 Cobra with a larger eng and half the weight.

[bret]

After re-reading your post I do have a couple of things that you may want to think about...
The Mark VII Lincoln is considerably heavier than the Lotus. The airspring and strut package is geared toward that particular weight of vehicle and that particular suspension geometry. If I remember right, the Lincoln front suspension is an airtrut configuration which may be a bit difficult to adapt into a SLA/coilover style suspension. It is also likely that those units are non-adjustable for shock valving. An adjustable shock is a great tuning tool for a corner turner. While it is true that theroretically the air pressure can be adjusted to accommodate any load imposed on it, in reality if you are trying to work with an airpspring that is too large, you will be forced to run a very low air pressure. This very low air pressure [anything under, say, 30psi] is very finicky about small air pressure changes. A 3psi air pressure change is a 10% change at 30 psi...it is only 3% at 100psi. In the real world...tuning is easier with a "normal" air pressure range in the 50-120 psi neighborhood. Another advantage of an airspring that is appropriately sized is that it will be more appropriately dimensioned for the application...you won't have to attempt to fit an 8" diameter airspring into a suspension where a 3.5" coilspring was intended. You may very well need an airspring that is only 5" in diameter and therfore much easier to package.
If you have a general idea of the load capacity requirements of your Lotus suspension we can recommend an appropriate airspring size or ShockWave unit. The weight of the car is a start but you will also need to figure the motion ratio of the suspension to determine how much load the airspring itself will really see.
The beauty of an air suspension starts right here...if you were to use a traditional coilover unit you would need to calculate [or guess] much more accurately to get the correct spring under your car. An airspring accommodates a much wider latitude of load while still being effective.
In English...a 6.5" diameter double convoluted airspring would be a good place to start on the front. A 4"-5" diameter sleeve style airspring will likely be appropriate for the rear. The ShockWaves are usually a simpler installation, but an airspring with a separate shock [a good quality adjustable shock] is just as effective.

[hssss]

Bret
Here are the specs that I am building toward. The resulting car will probably have more kinship to a circle track car than a Lotus. I'm attempting to use as many production parts as possible both from a reliability and cost standpoint. The front will use the MII crossmember with longer arms, probably circle track. What I had thought was to use the rear bags front and rear.
The below link suggested a few things one of which is using a secondary chamber to tune the bags.

http://www.sportcompactcarweb.com/editors/technobabble/0311scc_techno/index.html

Front suspension Mustang II crossmember, Cobra 13" Brakes, lower arms lengthened to 15", upper arms lengthened an appropriate amount for both the lower arms and to lower the pivot point 1", 93 Cobra power rack, Ajustable Sway Bar, MII Konis, Weight Jackers
Wheels 95 Cobra R 17x9 with 255-40-17

Rear suspension 3 Link, UCA 15", LCA 20", Mumford link, Adjustable Sway Bar, 06 Mustang 8.8 , 3.55 , Torsen T2, Lincoln MKVII brakes, MII Konis, Weight Jackers
Wheels 95 Cobra R 17x10.5 with 315-35-17

Controls 92 Fox mustang, dash cluster, switches, steering colum, 12.5" Bertini wood wheel, wiring harness, pedal assy, 96 Cobra Hydroboost Fox Mustang clutch cable and firewall adjuster
Radiator Fox mustang 4 core and Blackjack electric fan
Engine
Initial 91 F150 351 1000 mi on factory rebuild, stock/steel heads, Edlebrock performer manifold, Holly 750 dp with choke milled, Lunati bracket master II cam and kit, HEI distributor, Crane "Cobra" roller rockers, Front sump pan modified to 7 to 8" depth with kick outs, SVO Billit 351 flywheel, King Cobra clutch, 350 to 400 hp

replaced by 96 351 roller block out of F250, Forged pistons, Stock truck rods, 3.85 stroke crank (forged?), Crower E-43603 278/286 108 cl .528/525 with 1.6 rockers (probably use 1.7), aluminum heads 2.02/1.60, Trick Flow EFI??, Mcleod 7.5 twin disk clutch and aluminum flywheel, Roush 4 pickup "Cup" drysump, 500 hp

Lakewood scattershield, Cobra T5, Kirnban shifter
Exhaust either JBA shorties or Headman MII long tubes, 2.5 out pasenger side rear, Supertrap mufflers
17 GAL RCI FUEL CELL 30"W X 7"D X 17"T, Custom?? oilbag in front of rear axle on rh side

Chassis
4" to 6" wider on 92" wb generaly following Ausi mods, Upper and lower rails, some vertical, and atleast part of trans tunnel 1x1.5 14 gauge, other vertical 1x1 16 gauge, cross pieces raised to top of trans and main rails on passenger side to run exhaust/fuel/oil, floor/lower firewall/trans tunnel sides/rear bulkhead 16 gauge steel full welded perimeter and .25 on 1" plug weld on other tubes, Aluminum/steel/ss ?? sides and boot
Linexed tub, Carbon fiber body pieces, 914 driver seat, padded floor and bulkhead to create seat for passenger (kinda like early 914s), cage modeled after Westfield racing cage but probably including the windshield frame

A +4 with 302 built by someone else came in at 1250lb with 50/50

engine rpm at rear wheel power peak =5200, maximum engine rpm (shift rpm) =6200, maximum rear wheel power =440, 506 flywheel horsepower, 3.34 lb/hp, effective tire radius =13, car weight =1400 dry
axle ratio =3.73, transmission gear ratio first =3.35/second =1.93/third =1.33/fourth =1/fifth =.68
0 to 60 time = 1.19, 60 foot time = 1.089 (calculated)
eighth mile speed = 127.75/elapsed time = 5.253 (calculated)
quarter mile speed = 150.39/elapsed time = 8.469 (calculated)

https://static1.pt-content.com/images/noimg.gif
A +4 with 302 built by someone else came in at 1250lb with 50/50

[bret]

I should have considered that! I looked at the link to the mag article...good info there...probably above most civilians, but you are obviously a bit beyond that level.
I usually avoid spitting out these kinds of formulas because they scare off the masses. the people they don't scare off end up taking them almost too literally...like trying to calculate how far to turn the steering wheel to go around a corner!
Like I said good info there...it is worthwhile taking the time to understand and implement.
One thing I would like to add to their statement about all airsprings being progressive. There is actually a way to make an airspring linear. On a rolling sleeve airspring the shape of the piston has tremendous influence over the dynamic spring rate of the airspring. A straight sleeve rolling over a straight piston will inherently have a slightly progressive spring rate because as the spring compresses the airspring volume is decreased. The progressiveness can be increased by building a tapered piston with the larger end at the bottom. You can also build an inversely tapered piston that will allow the airspring to maintain a constant volume as it compresses, therefore offering a linear springrate, or even a regressive spring rate. In the real world how does one make a specially tapered piston that suits their specific application? You may consider starting with a straight piston airspring and fabricating tapered sleeves that slide onto that piston to try different piston shapes. I have even used duct tape as a quick and dirty method to experiment with different shapes. Lots easier than winding your own coilsprings, or even swapping them out! Of course, this kind of experimentation is usually done after the nornal air pressure adjustments and shock valving adjustments are exhausted on your initial airspring selection.
You have obviously done some homework and planning here...you will succeed. I am looking forward to learning from your progress!
BTW...this is shaping up to be a badass ride...I want a ride when its done!

[hssss]

Me too. I believe the rear bag assys off the MK VII are sliding sleeve with at least 3 different piston assys. On the MK system the height is computer controlled but I believe I can tune by varying the mount height on the frame and using a variable chamber to increase volume. I hadn't thought about the duck tape but that is a good idea. Actually I have most of the components on hand. One thing I am trying to do is a proper job of planning without over analyzing. For the most part production or readily available parts are being used just being repackaged. At times I wish I was more of an engineer but OH WELL.