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trailing arm box section size
jon200 - 29/1/14 at 02:16 AM

Im needing to remake them as the bushes are odd sizes. In the past the bars have contacted either the axle or the brackets (possibly due to knackered shocks). I dont want this to happen again as it will be putting stresses on brackets and welds.

so my question is: Can i use 20mm box rather than 25mm? the extra clearance should be enough to hit the bump stop before a bracket.

Jon

jon200 - 9/2/14 at 12:11 PM

Anyone got any advice on this?

Minicooper - 9/2/14 at 12:51 PM

Caterham use 20mm round tube on the trialing arms on the rear and it's very thin walled around 1.5mm, I would have no problem using 20mm for the trailing arms, but for an extra degree of strength I would go up a wall thickness if possible. So if your original trailing arms are 25mm x 1.5mm use 20mm x 2mm and so on

I would not use 20mm on the panhard rod

Cheers
David

[Edited on 9/2/14 by Minicooper]

Sam_68 - 9/2/14 at 12:54 PM

quote:
Originally posted by jon200
Anyone got any advice on this?


Yes: fit appropriate bump rubbers to your dampers, to prevent the suspension deflecting far enough to bind the trailing arms!



You might well get away with 20mm (CDS tube rather than box section, surely?), but to be frank, it's not worth taking the risk of reducing your safety factor on something as critical as suspension components.

I've seen 750 Formula race cars go down to 3/4" or even 5/8" (though 7/8" x 18g is more typical) on much longer trailing arms than you get on a Locost, and obviously they're running much stickier tyres (albeit with a much lighter car), but they're not taking the hammering that a road car's suspension gets. Theoretically the trailing arms are in pure tension or compression and only have to cope with the maximum torque on the axle that can be generated by the grip of the tyres, but how lucky do you feel?

Apparently Colin Chapman used to be fond of saying that you could suspend a London bus from a 1/4" bolt (though Martin Ogilvie, Team Lotus Chief Designer, claims to have pissed him off one day by doing the calcs. to prove he was wrong), but then he had a habit of building cars than broke...

JoelP - 9/2/14 at 12:55 PM

No one will give you a definite answer as it's not clear cut. Id vote that it could be OK. It would be harder to attach your bush tubes though, so extra support may be needed. Trailing arms won't see much bending provided the bushes are compliant and free. PU bushes would put a good twist into it in roll, which I think the tubes would be fine with but the welded ends might not appreciate over time.

Sam_68 - 9/2/14 at 01:06 PM

quote:
Originally posted by JoelP
No one will give you a definite answer as it's not clear cut.


This is true.

But can anyone come up with a good reason that suitable bump rubbers wouldn't solve the problem in a safe manner?

Jon Ison - 9/2/14 at 05:43 PM

quote:
Originally posted by Sam_68
quote:
Originally posted by JoelP
No one will give you a definite answer as it's not clear cut.


This is true.

But can anyone come up with a good reason that suitable bump rubbers wouldn't solve the problem in a safe manner?



No, I don't see any one claiming they wouldn't either.

Sam_68 - 9/2/14 at 06:30 PM

quote:
Originally posted by Jon Ison
No, I don't see any one claiming they wouldn't either.


Cool. Then that's our OP's best solution, would you agree?

Why risk downsizing a safety-critical component on the basis of guesses that it will 'probably' be OK, when there's an alternative solution that doesn't require you to?

jon200 - 9/2/14 at 08:47 PM

If I hit the bump stop hard would or could the rebound be a problem?

Sam_68 - 9/2/14 at 09:08 PM

quote:
Originally posted by jon200
If I hit the bump stop hard would or could the rebound be a problem?


No.

Your dampers have rebound damping, but in any case, if you're running correct damper lengths, the damper should reach full extension before other suspension components reach the limit of their travel.

Not to mention that the tarmac will be in the way to stop the wheels rebounding too far (unless you've hit a bump large enough and fast enough to cause the car to become airborne, in which case the answer is to pay more attention to the road in front of you!).

Bump rubbers are specifically designed to solve the problem you have: their whole purpose is to limit suspension movement progressively, by offering steeply rising-rate resistance, before everything goes solid.

jon200 - 10/2/14 at 10:34 AM

its not the extension i had a problem with it is when comprerssed the arms have previously contacted the axle so i want to stop it happening again. I have fitted new suspension so it may be ok. I want to limit the possibility of failure and contact when its back on the road. I dont want to have to make the arms again.

coozer - 10/2/14 at 11:37 AM

The rear radius arm off a Land/Range Rover are nice and thick, I think they would be ideal to make ones for 7's....

Part number NTC8328

Sam_68 - 10/2/14 at 07:18 PM

quote:
Originally posted by jon200
its not the extension i had a problem with...


Then why are you worried about rebound?

Anyway...

If suspension arms are capable of contacting the axle at full deflection, whether bump or droop, then you have a problem, but it's very easy to check: remove the springs, then move the axle through it's full range (using a trolley jack while supporting the chassis on axle stands). If the trailing arms hit the axle tube (or anything else) before the dampers reach their fully compressed or fully extended lengths, then you have a problem.

If it's binding before it reaches fully compressed, you need to fit a bump rubber of suitable length.

If it's binding before the damper is fully extended, you need to buy shorter dampers.

It's as simple as that!

I'm within spitting distance of you if you live at Dursley, so if you need a hand figuring it out, U2U me.

Peteff - 11/2/14 at 10:25 AM

It sounds like you need to redesign your brackets and move the arms further apart not make them thinner to accommodate.

jon200 - 11/2/14 at 03:28 PM

I was worried about putting extra bump stop in affecting rebound.

I think I will try removing the spring like you suggest. I hadn't thought of that! When it's jacked up its fine anyway

jon200 - 12/2/14 at 07:19 PM

Removed the spring as suggested and the arm doesn't contact the axle so all is fine with the 1" box section. When it's jacked up the axle rests on the chassis bar that runs under the axle. Is this right? When sat normally there is a 2 1/2" gap with a full tank and nobody in the car.