Reality check

Idle chat about any thing Sevens & Clubman cars.

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7ish
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Re: Reality check

Post by 7ish » Wed Feb 01, 2017 4:05 pm

LVVTA rejected my top wishbone design because it used a Ford Transit drag link ball joint at the upright end. I had 2 rod ends at the chassis end. The transit drag link is the most common top ball joint used on the lotus 7 replicas

https://www.rallydesign.co.uk/images/P7070021.JPG

But they have decided that it has a threaded shaft in bending load. Mark has replaced it with a captive spherical bearing, and kept the 2 rod ends at the chassis end... they approved that.
7ish

Mauntinbyka
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Re: Reality check

Post by Mauntinbyka » Wed Feb 01, 2017 8:53 pm

7ish wrote:But they have decided that it has a threaded shaft in bending load. Mark has replaced it with a captive spherical bearing, and kept the 2 rod ends at the chassis end... they approved that.


So transit tie rod ends might not be suitable.

I see Low Volume Vehicle Standard 195-00(02) under "suspension joints" 2.2(34) states:

(g) where the joint is a screw-in ball-type, have thread engagement of at least one-and-a-half times the thread diameter; and
(h) where the joint is a screw in/out custom spherical bearing rod-end, only be loaded in compression or tension through its longitudinal axis.

NOTE 1:
A spherical bearing rod-end joint is also commonly referred to within the automotive hobby and industry as a ‘rose-joint’ or ‘heim-joint’.

I guess there would be some non longitudinal loading under braking but this restriction seems to only apply to rod ends/rose joints at the outer end of the wishbone 2.2(34)(h). Not to screw in ball types 2.2(34)(g). I don't understand why your transit tie rod end was rejected.

It makes sense to me that a rose joint shouldn't be subject to sideways loading however a ball joint itself (as opposed to the threaded connecting shaft) Is designed to take loads from all angles. I must be missing something. I'll have to get to the bottom of this.

What do you mean by "captive spherical bearing"?

Terry

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Not-a-Number
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Re: Reality check

Post by Not-a-Number » Thu Feb 02, 2017 7:49 am

Every second 7 has a "Ford Transit drag link ball joint" on the top upright. Youd think it would be fine.

This is the bit that broke and wrote my 7 off 20 years ago! Broke at the thread!

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techtoys
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Re: Reality check

Post by techtoys » Thu Feb 02, 2017 11:01 am

Most single-seaters and sports-racing cars would use a rod-end at the outer end of the front wishbones. Rear wishbones too. Usually these would be at least partially exposed to bending loads - hard to see how they couldn't be. All you need to do is use a big enough rod-end for all the expected loads, and to consider the fatigue cycles before replacing them.

Yes, a rod-end is more likely to break at the threaded portion than at the ball/seat when subjected to enough bending load. But then you'd design the wishbone with a bigger rod-end with a threaded portion big enough to handle the bending load. I can't see any reason from an engineering perspective to impose a blanket ban on rod-ends in this situation, and thankfully in motorsport they don't.

In 30 years of motor-racing I've only seen one rod-end break catastrophically, and it was an old rod-end that the owner (and builder of the vehicle) already knew was marginally sized and overdue for replacement.

It seems to me that some of the committees who make rules or perform inspections for road-going vehicles are too stupid to look at a proper engineering analysis, so they dumb it down by creating generalised rules. Plus they need to protect the rest of the road-going public from the other clowns who might not maintain their vehicles properly.

On a dedicated motorsport vehicle I would ALWAYS use rod-ends. And I wouldn't want to even try designing a low-volume road-going car - it's supposed to be fun, not frustrating.

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7ish
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Re: Reality check

Post by 7ish » Thu Feb 02, 2017 11:53 am

The only way to find out what lvvta will accept is to submit your design application then talk to your certifier. There is a certain lack of consistency with the low volume certification. ...

See my build diary 7ish new build, page 5 for pics of my front wishbones with a captive bearing...
7ish

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heavyseven
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Re: Reality check

Post by heavyseven » Thu Feb 02, 2017 1:13 pm

7ish wrote:The only way to find out what lvvta will accept is to submit your design application then talk to your certifier. There is a certain lack of consistency with the low volume certification. ...

See my build diary 7ish new build, page 5 for pics of my front wishbones with a captive bearing...
I quite like the look of that :)

Mauntinbyka
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Re: Reality check

Post by Mauntinbyka » Thu Feb 02, 2017 8:01 pm

Yes, neat solution and nice build. I see the upper wishbone takes the spring and shock loading so that might have had something to do with the rejection. Yes I'll need to submit my design to know for sure.

Does anyone know of an alternative free CAD programme which will produce pdfs? Sketch Up doesn't creat pdfs anymore.

Terry

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scottie84
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Re: Reality check

Post by scottie84 » Thu Feb 02, 2017 8:36 pm

If it lets you print, download cutepdf and you'll be able to select it as your 'printer' which lets you save a pdf copy.


Sent from my iPad using Tapatalk

SR Components
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Re: Reality check

Post by SR Components » Thu Feb 02, 2017 9:41 pm

techtoys wrote:Most single-seaters and sports-racing cars would use a rod-end at the outer end of the front wishbones. Rear wishbones too. Usually these would be at least partially exposed to bending loads - hard to see how they couldn't be. All you need to do is use a big enough rod-end for all the expected loads, and to consider the fatigue cycles before replacing them.

Yes, a rod-end is more likely to break at the threaded portion than at the ball/seat when subjected to enough bending load. But then you'd design the wishbone with a bigger rod-end with a threaded portion big enough to handle the bending load. I can't see any reason from an engineering perspective to impose a blanket ban on rod-ends in this situation, and thankfully in motorsport they don't.

In 30 years of motor-racing I've only seen one rod-end break catastrophically, and it was an old rod-end that the owner (and builder of the vehicle) already knew was marginally sized and overdue for replacement.

It seems to me that some of the committees who make rules or perform inspections for road-going vehicles are too stupid to look at a proper engineering analysis, so they dumb it down by creating generalised rules. Plus they need to protect the rest of the road-going public from the other clowns who might not maintain their vehicles properly.

On a dedicated motorsport vehicle I would ALWAYS use rod-ends. And I wouldn't want to even try designing a low-volume road-going car - it's supposed to be fun, not frustrating.
The difference is that a Rod end / rose joint etc are manufactured out of a rated material with documented stress loads etc , where as a tie rod end etc / transit drag links etc are not rated.

7dreamer
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Re: Reality check

Post by 7dreamer » Thu Feb 02, 2017 10:16 pm

Were lvvta happy with the press fit spherical bearing as the loaded ball joint. I would like to use them in place of standard ball joint on another project I'm working on.

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