Air Suspension upgrade for New Holland tractors.

Colliedog said:

I have had the last of the component prices in today so for anyone wanting a quote for an Air Suspension Kit please PM me your email address, along with your tractor model and year and I will be able to get a quote over to you in due course. It will take some time to get the kit together and sent out and there will be a limited number of kits to start with. If you are unable to fit the kit for what ever reason or you are unhappy with it there will be a returns policy. I will need full payment before dispatch and I am also more than happy to provide a reference if one is required.

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Colliedog said:

Air suspension kits for Case and New Holland tractors!

So, after a year of development and testing I am almost at the point where the first kits will be ready to order. The plan is to hopefully sell the kits though the CNH dealer network, but seeing as it all started here, I would like to get a few out to The farming forum members first so as to get some feed back. I'm going to write a bit about the development and how I got to the final kit, which is quite different to where I started last year!

As anyone following this thread will know, I started this post last summer as way of helping others being able to improve the ride in the CNH cab. What I wasn't expecting was lots of messages asking if I could make a bolt on kit to sell to other members. As I like to help people out I started to look into it, discovering there was a lot involved in putting a professional kit together. I started by talking to Mike at Erentec as he is one of the country's best Pneumatic experts, and he has also built a business on selling aftermarket kits. He couldn't have been more helpful with his advice and I learnt that all components had to meet ISO 9001 and TUV standards, but also needed to be of a high quality. He was also very helpful on the tech side of the plumbing and the kit now includes a pressure protection valve. This will keep the air brakes working if an airbag blew or a serious leak developed. The next thing was to find out about how it would affect the ROPS certification. I contacted the HSE and a major UK cab manufacturer & test centre and came to the conclusion that the components that were being changed were of the same approved standards, but also on this cab the rear suspension struts don't form a part of the ROPS structure.

By this time the original air strut had started to wear a hole in the air bag where it was rubbing on a hydraulic valve and therefore a rethink was needed. After testing this damper on various models I found it fitted nicely on a Lwb T7. It was close to a valve chest on the Swb T7 but rubbed on the T6. At this point I learnt there was also another issue with the original Volvo damper fitted in this situation. To put it in simple terms the spring force of an air spring is determined by two factors, the air pressure and the diameter of the piston at the bottom of the ram. A lot of air bags have tapered pistons so depending on which way they are tapered, they can either keep the force of the spring the same, as the air is compressed, or give a highly progressive spring rate as the air is compressed. Many cab air struts use hour glass shaped pistons so as to give a soft ride in the middle, but give a more progressive spring rate as the top and bottom of the spring travel is reached. So what this means is the spring force is rising as the spring approaches the bump stops at full travel. Most bumps felt in a tractor cab are when the bump stops are reached at the end of the suspension stroke. The hour glass taper also helps with lateral swaying of the cab as well.


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As you can see in this picture, the hour glass shape of the piston is sat below the ride point of the bottom of the air bag. What is happening with this original damper is as the piston approaches the bump stops the spring force reduces, hitting the bump stops harder. So it was back to looking for a new damper to try and rectify this. I went though the entire Monroe catalogue looking for an alternative with no luck. Therefore I thought I would build a fully adjustable test damper and look into having a bespoke damper made.

Due to the fact that tractors cabs are sat over an unsuspended axle being operated over rough terrain, it is almost impossible to build a cab suspension system that wont bottom out. The only thing that can be done is to try and soften the transition of impact on the bump stops. This means that whilst the Volvo damper works well eating up motorway miles on a large artic, as a damper for a tractor its not quite the perfect choice. Some of the mid range lorries fitted with steel suspension and designed for harsh environments are fitted with air cabin dampers with an airbag with a very aggressive tapper on the piston. This led me to think that this would rapidly increase the force of the spring as it approached the bump stops. So two of these air bags were purchased as well as two motorsport dampers that were independently adjustable in bump and rebound. I machined a few components on the lathe and fitted it all together to make an air damper. I used some hydraulic ram stops so as I could adjust the air bag ride height, together with the damping forces.

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My theory of the aggressive air bag taper proved to be correct. But the adjustable damping was making me realise another problem. When it was set to give that nice smooth ride you expect from air suspension, I found it would bottom out on larger bumps. When the dampers were firmed up (to stop the bottoming out), you started to lose the benefits of the air ride. By this time I was struggling with a manufacturer to make a bespoke damper for me. Monroe's minimum order quantity was 30,000 units and any of the smaller manufactures were not interested in helping with an agricultural tractor project. Having tried various brands of damper, Monroe's seemed to offer the best performance. The results from all my research led me to the next step of trying to find a damper of the correct specification and mounting it from a new bracket, so as not to be limited by the maximum eye length. There were a few I felt could work but one in particular stood out. When comparing the specs to other brands the Monroe had a travel of 2 inches where as the other brands for the same part number had a travel of 4 inches. Further investigation revealed it looked like the Monroe was fitted with internal progressive bump stops. If this was the case it looked like I could engineer the positioning of the damper to smooth the transition to the tractor bump stops. So two of these were purchased for testing and evaluation which turned out to be the ultimate choice.


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Here is a picture of the damper being tested. It is not quite a shock absorber dyno, but with the aid of an oil pressure gauge and air pressure regulator I was able to calculate some forces of the internal bump stops, in order to build some test mounting brackets and then fit them to my tractor.

These dampers, without a doubt, provided the smoothest most comfortable ride of everything I had tested!

The suspension has a total of 4 inches of movement. It is very soft and smooth over small bumps but as soon as a large bump is encountered the progressive nature of the dampers rises rapidly as they approach the tractors bump stops, reducing the shock load on the cab. The internal bump stops work at both ends of the dampers smoothing shock loads if the tractor is rocking from side to side over rough ground. There is a small amount of lateral movement in general use but overall this is as about as close to semi active suspension you could get without the use of electronics.

I have measured as many tractors as I possible could find and currently have mounting brackets in CAD format for most T6, T7 Maxum and Puma models.

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Having found the ultimate suspension solution, I then turned to the air management side of the system. My original levelling valve idea was a bit vulnerable in the place it was mounted and because of not having a straight push on the linkage, it needed constant adjustment. I thought I might be able to simplify it by using a either a pressure regulator or a pre-set pressure valve which pumped the system up to a pre-set pressure. This proved to be a failure as the pressure required to keep the system level varied 10psi just from weight transfer driving up or down a slope. This pressure variation could then be doubled from a light driver to a heavy driver with a passenger and toolbox. In simple terms the ride quality is maintained by keeping the ride height correct, therefore leading me to believe that a cabin levelling valve was still the best solution.

This was the biggest engineering challenge of the design of the kit so far, finding a mounting solution for various tractors models, despite them all having different chassis. The solution has been to mount the valve on an adjustable bracket under the cab, making the ride height easy to set. It took me two days to find the perfect angles and hole positions in order to get the linkage straight and avoid all obstacles.

I feel the kit has now had enough development to move forward with its production. I am very pleased with the performance of it and I feel it has a factory fit look. I am hoping to get the prices back for the laser cut brackets this week which will enable me to finally price the air suspension kit. As you can see a lot of research and development has gone into the final formula for this kit since my first attempt. The quality of the components is higher, as well as there being nearly twice as many components, resulting in a final kit that should represent excellent value for money for the performance you will gain from it.

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Colliedog said:

I

If you send me your shocks measurements l have nearly every cabin damper spec on a list on my desk. I could have a look for you.

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Chae1 said:

Did you read the first 4 words of what you quoted?

I know what decent cab suspensions like, my other tractors a fastrac.

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Hilly said:

Dose your fastrac have can suspension ?

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Colliedog said:

Thank you @fieldfarmer I enjoyed meeting you and getting the first opinion on the kit!

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Lakesdigger said:

Early days but air cab suspension is on and working. Only been on a couple of hours but initial impressions very pleased with it. Definitely a worthwhile upgrade imho. Just need the front suspension to go with it

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Lakesdigger said:

I can also add its possible to fit without removing back wheels but it will be easier with wheels off.

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Half Pipe said:

How long did it take you to fit it?

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