Alternate Frame Rules Clarification

[wolf14]

Hello,
With an aim to reduce our chassis weight we have decided to go from alternate space frame this year. Upon reading the rulebook and maintaining the EI and the CSA we have narrowed down our choices to 26.5mm OD and 2 mm thickness for the roll hoops and 27.5mm and 1.2 mm thickness for the other members. The only doubts are about getting the proper documentation for the event. Is SES the only document to be submitted taking into account the testing of the H joint sample with the alternate tube joint and the baseline joint. What else is officially required under the sae international rules? Also what are some of the points that have to be taken into account for using the alternate tubing?

The other way for the alternate tubing is SRCF. If at all we decide to submit SRCF, do we need to attach SES with it? Is it necessary to attach the AFRN with SES too or its only a requirement of SRCF.
It is a risk we are taking considering the lack of knowledge on this matter. It would be really helpful if we were to get some experienced comments.

Thank You

Regards,
Sanath Salunkhe
Team Octane Racing
College of Engineering Pune, India
teamoctaneracing2015@gmail.com

[ChristianChalliner]

Having gone through this ourselves I'd like to help but there's a few things that need to be known first

Firstly, What event are you attending?

Secondly, Are you aiming to conform to the alternate rule set which requires JUST the SES and NO testing or are you going for the rule set which requires TESTING, Notice of Intent AND the other form I've forgotten the name of?

Thirdly, How did you get a roll hoop that is 26.5mm and 2mm thickness? we could only get a 30mm and 2mm thickness tube to meet the SES form?

[wolf14]

Having gone through this ourselves I'd like to help but there's a few things that need to be known first
Firstly, What event are you attending?

Secondly, Are you aiming to conform to the alternate rule set which requires JUST the SES and NO testing or are you going for the rule set which requires TESTING, Notice of Intent AND the other form I've forgotten the name of?

Thirdly, How did you get a roll hoop that is 26.5mm and 2mm thickness? we could only get a 30mm and 2mm thickness tube to meet the SES form?

Thank you for the reply
The real dilemma is what to present at the event. we are open to getting the H joint sample tested and we are aiming for the one which requires the physical testing. But if we go for that, will just the SES suffice or do we need something more with that. if yes, what exactly?

The basic idea we are trying to do is maintaining the EI of the pipes. but yes we are at the early stages of our iterations so these might change.

If you are open to discussing your personal information we might be able to discuss further.

Regards,
Sanath Salunkhe
Team Octane Racing
College of Engineering, Pune, India

[Adam Farabaugh]

There is essentially no way to use the AFR rule set to reduce weight of the car if you're doing a spaceframe.

[wolf14]

According to our calculations we have seen significant reduction that being up to 6 kgs for 20m of tube if we were to use AISI 4130 or ST 52. Since the rules create and internal loop of facts there is no way to clarify which documentation is actually required.

[ChristianChalliner]

Adam I disagree, We have managed to reduce the weight of the car whilst increasing stiffness considerably due to the larger diameter tubing we can use.

Sanath, I'm still not sure exactly what you plan to do. Substitution of tubes whilst meeting the SES is easy and is what we have done. Building a car to the more extensive alternate frame rules is not and requires fairly substantial testing of the frame and joints, there is also a notice of intent which you must file. When we enquired about this to the FSUK organisers we were told we would not be able to build a car to the more extensive rule set unless we had the car approved for an SAE competition in the USA.

Also, I got our chassis designer to look over this and we spotted something which we believe is the biggest issue. You have used an alloy steel in order to maintain your EI value, please check the rules, it is categorically stated that all steel is treated as basic mild steel, there is no allowance for higher grades. We believe your tube diameters are too small when this is taken into account, please check the rules again.

We also think that you need to check with your competition organizers since for the US competitions the deadline for the extensive alternative frame rules was months ago, as such, if you were intending to go to either FSUK or the US competitions you will not be able to build to this rule set.

Please check your information and get back to us, we can certainly help with suggesting tube sizes for the SES ruleset but again, the deadline for SES for FSUK was at the start of this month, as such your car would not be eligible here. For reference, our roll hoop is 30mm x 2mm and our side impact members are 31.75 x 1.22, in all other places we have used the base 25.4 x 1.2 standard tube. Take note that our 31.75 tube only JUST meets the SES spreadsheet in terms of tensile strength and area, in fact, we were 97.5% of the required strength but the SES note said: "Providing your nominal tube diameter is equal to or exceeds this value this is OK" since for the EI value we are far beyond the requirements of the base tube.

Hope this helps

[Adam Farabaugh]

Christian,

You can increase EI with larger diameter tubing, but that does nothing for A. I repeat: the only gain you get with larger diameter tubing/smaller wall is bending modulus! Are significant portions of your frame in bending?? So unless you have large cantilevers in your chassis the stiffness effect will be negligible.

EDIT: I'm not saying that larger OD / smaller wall is a bad choice. Increased bending stiffness may increase stiffness of a good frame by 1-5% (assuming no massive cantilevered sections). As well as making it easier to weld better by matching wall thicknesses at joints better. Or making manufacturing easier because some of the nodes are easier to grind.

Also my point is that any weight reduction you plan to do via SRCF could be done with SES as well. The fact of the matter is that 95+% of default frame rules frames (even with larger OD/ smaller wall adjustments) would fail the off-axis front bulkhead load case. To pass that, you need to add tubes i.e. weight, or go to some sort of sheet metal monocoque hybrid).


Sanath,

If you are assuming that you can use properties of 4130 and not the baseline steel properties listed in the rules, have you asked the rules committee about that? I was under the impression that you had to use baseline steel properties for the SRCF. If you can actually use 4130 properties I wonder if they would require you to develop your own as-welded allowables?

Also if you are reducing weight by using 4130 that means you are using strength as your design index... so you are sacrificing stiffness. In which case 1) frame design is I think primarily stiffness-limited (I am not smart enough to prove why) and 2) this may cause you to have trouble passing deformation allowables.. in which case you would again add material and weight.


I forget who said this first but I think that it's not possible to go below 45-50 lbs frame weight with standard rules (and no cheating). Good luck surpassing that with AFR. If you manage to do it, seriously, post your results - I think you would be the first. On the other hand, if you are one of the typical far eastern teams with a 100 lb frame, then yes of course there is weight to be lost, but you don't need to use the AFR to do so. If you post your chassis we can point out which tubes will be functionally useless to you (and can be deleted).

[wolf14]

Considering the section of alternate tubing, is it necessary to maintain the EI (buckling Strength) OR the Cross-Sectional Area OR both?

[ChristianChalliner]

In short both!

[Adam Farabaugh]

Christian is correct.

Cross sectional area governs tensile strength of a member (given that E and sigma of the material are known).
EI governs buckling strength and bending stiffness, again given that E and sigma are known.

Obviously it's not within the rules to make your chassis weaker/less stiff by using different tubing!