Engineering Street Rod Suspensions & Steering

Technical Advisory Committee

Recommendations re Engineers Reports for Suspension and Steering Components Sept 2015

In the early days, when building a Street Rod we used suspension and steering from whatever was available, be it front or rear suspensions, differentials, steering, or brake parts because that was all we had. 

Usually we made sure the parts came off a heavier car and all went well.

In more recent times rodders have been availed of a gamut of custom built parts from a variety of sources, some 'well established' and some that seem to come and quickly go. The following is some advice on procedures to follow if buying anything safety related such as independent suspensions, beam and tube axles, hairpins, four bars, dropped stub axles, steering parts, or anything that could cause an accident if it fails.

When your selected Engineering Signatory (be it an ASRF TAC representative or a VASS Professional Automotive Engineer) comes to inspect your car he will want paperwork for all critical components that show they are designed and tested as acceptable for the application.

In Australia the established Hot Rod parts suppliers are used to being asked to supply test reports for the parts they sell. If the parts in question have been bought direct from overseas then there is a chance they company will not have any test reports and your signatory may reject the part. However there are some overseas companies that have done testing just for the Australian market and usually this information is available if you buy from a local distributor.

Big Question: 

How do you (or more importantly your Engineering Signatory) interpret the Engineering Reports that accompany the parts you have just purchased? 

You may ask "that’s not my problem. If it comes with paperwork it must be right?"  Well, not always, so let’s look into what you might get as 'certification paperwork' and what it’s really worth.

Material Certificate

The very minimum some companies will include with their parts might be what is called a Material Certificate

This confirms what grade of steel or whatever was used to make the part, and that’s about all. It is fine if you know what the correct grade should be, but if you don’t, then you really don’t have much to go on. That still doesn’t mean the part is designed right for the job.

Finite Element Analysis ( FEA )

Next for parts that are 'stressed' such as stub axles, suspension arms etc the designer might begin his product evaluation by doing some computer analysis of the proposed item, commonly called Finite Element Analysis or similar. 

Basically the dimensions and material properties are computed against the expected loads and safety factors to check that the design shouldn’t fail in service.

A word of warning about Computer Analysis. 

The information you get out is only as good as you put in, so if the input loads are less than real life then the computer will pass the product when on the road it might be less safe than expected. It is even possible to take a product that is obviously unsafe and make it appear acceptable by putting low enough loads in the formulas. ie: work backwards to get the answer you want.

So if using FEA alone to approve a product, take the report to someone who knows how to interpret it for them to make sure of the basis of the calculations.

Physical / Dynamic Testing

A good designer would not just do the theory but should do some physical testing as well. Vehicles are subject to continual fatigue from the weight of the car as it hits bumps in the road and as a starting point a safety factor of 4 times static load is used as a test load. So for a simple bracket a test rig might be built to simulate the greatest load expected in use, take it to at least 4 times that and it must not fail. If it takes 10 times the expected load then we all sleep well tonight.

If continual fatigue is expected then a dynamic test rig might be built that loads the product to well over the static load, then pulses and vibrates the whole assembly for a long test until either cracks appear or it proves OK. When a manufacturer does these test reports they should then include the parameters that the product is approved for, ie: a maximum static weight per wheel, max offset of the wheel or similar.

OEM’s do a huge amount of dynamic testing of their car and parts and base this off intensive Computer Designs backed by many years of practical experience ... but we still see the occasional recall ... so how confident are you in a suspension design knocked up by a guy who once built a couple of hot rods, thinks he knows how to weld and has just decided to go into business in his backyard shed?

Reproduction Bodies

Some products such as fibreglass bodies are required to be approved to ADR Standards for Full Reg (ICV) use, and whilst not strictly required by the National Street Rod Guidelines your Engineering Signatory will be pleased if this data exists for your project. The tests for replica bodies might include side intrusion tests; roll over, door locks and latches or seat belt loading. 

The body or doors are statically tested to check strength against ADR standards. If you have proof it’s all strong, then its so much easier to get it approved than it is for a floppy glass thing with a bit of 25 x 25mm RHS tube here and there.

Test Report Validation

It’s great that the manufacturer has gone to the trouble of having his stuff approved, but you need to be sure what they sent you is the same as what was tested. 

This can be an issue with Asian stuff where they get a top notch part tested, then cut corners on the production parts later on. 

Same for welded parts like control arms, where the maker needs to follow welding procedures, use the correct size and grade tube, etc.

So in summary when buying custom 'engineered' parts for your project make sure that Test Reports are provided, and have someone who knows what they looking at reads the report to validate that the results represent the real world. A good Professional Engineer should be consulted if its too hard for your primary school maths.

If they say Reports exist but they are 'company confidential' or similar diversions, then treat the product with suspicion. In fact, you have every right to ask for a report before you spend your money, as usually there is nothing in a Test Report that would help a pirate copy the product. But if they say “don’t worry, we will send the report later after you pay” be careful ... because often as not it doesn’t happen.

Keep the wheels turning.

Peter Koning


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