 | Home > Technical > One Radius arm with one bush |
 
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| markb110 Member Since: 22 May 2010 Location: Guildford Posts: 2635   
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As will all things I would like to see it working to get my head around it.
Slow speed rock crawler, yes I get that, slow speed and squeezing every bit of articulation out a a build. But on a road going vehicle capable of doing 90 mph and then in turn expecting to brake at that velocity I am thinking that the strain on one bolt might be a little too much if repeated a number of times. Even driving on the roads around Surrey, with pot holes and speed humps (surprised they don’t just call them UCR’s instead) that repeated unsprung mass will still put a lot of wear on that single bolt. Interesting, for the right application, asking for trouble if you gave it to me to test…….  |
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28th Apr 2022 1:29pm |
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| Dinnu Member Since: 24 Dec 2019 Location: Lija Posts: 3414  
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Ok, so I made some rough calculation.
Assuming a Defender that weights 3500kg, and a braking g force of 0.5, all braking on the front axle. The force parallel to the road on the front axle is 17.2kN Assuming 32" tyres, the moments on that one radius arms just from the braking effect is 53.5kN on each bolt. Assuming that all the stopping force, the whole 17.2kN are on that one radius arm, then each bolt will see an additional 8.6kN each. So the total forces on each bolt is 62.1kN (it is actually less due to the vectors or direction these forces are acting). An grade 8.8 M16 bolt, in double shear is able to handle 118kN. Although the bolt is also under tensile stress, but that is also helping to reduce the shear stress as it is clamping the inner bush pipe. So mathematically (barring mistakes in my calculations), the bolts are adequate (perhaps need some finite element analysis for a better understanding -anyone?). In practice, when a Defender is involved in a collision, it either bends the radius arm, or rips the radius arm off the chassis. Yet to see the M16 (or 5/8UNF depending on year) on an axle shear off. 1988 90 Hard Top, 19J Diesel Turbo, Shire Blue - Restoration ongoing 2012 90 CSW, 2.2TDCI, Santorini Black |
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28th Apr 2022 2:11pm |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17375  
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I wouldn't expect the bolt to fail although I don't think that your calculations take into account the rotational force around the centreline of the axle under breaking.
However I don't think that it would offer the slightest benefit in terms of articulation since the axle will probably hang up on something long before the missing fixing would have the slightest impact. Think of all the long-travel suspension set-ups that have been built with two bushes on each side which do not appear to be limited by the presence of two bushes. I imagine also that it will negate the type approval on the vehicle, invalidate the insurance on the vehicle, and could cause lifelong wailing and gnashing of teeth in the event of a serious accident. To some up in one word, "why"? |
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28th Apr 2022 2:37pm |
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| DSC-off Member Since: 16 Oct 2014 Location: North East Posts: 1403 
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A friend of mine used to compete in trial events, with standard suspension arms, driving to and from the event with all 4 bolts in the radius arm/axle bushes.
Before the trial he would remove 1 of the bolts, in effect leaving the same configuration as the picture above, with only 3 bushes being used. It certainly did help with the axle articulation and never gave any problems with slow speed driving, off road, in a lightly loaded vehicle. I don’t know if he ever forgot to put the bolt back in, or what it drove like on the road. I imagine with 3 bushes they would wear very quickly. |
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28th Apr 2022 5:09pm |
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| Dinnu Member Since: 24 Dec 2019 Location: Lija Posts: 3414
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Thank you for sharing your views.
Blackwolf, actually my calculation is that 53.5kN force on each radius arm bolt is from the torsional stress from braking only. Thats 5.5 tons of force on each bushes if one radius arm is fitted. But indeed that is not very accurate as it is assuming a static load. No factoring on how dynamic the load is, for example braking while going over a bump or pothole. As DSC mentioned, maybe they will wear prematurely. Although to keep in mind that the radius arms would not be working against each other, so maybe less forces on the bushes if the vehicle is constantly being articulated. Yes, removing one of the 4 is an option. Assuming he was removing the front nearside bolt (I guess want to keep the diff pinion pointing towards the transfer box as much as possible), the axle would have more droop on the passenger side than the driver side, as there is not much clearance between the radius arm and the axle bracket, and would bind up before can droop on the offside wheel. The 'why' is more for fun.. and agree, not worth the risks for a road going vehicle. 1988 90 Hard Top, 19J Diesel Turbo, Shire Blue - Restoration ongoing 2012 90 CSW, 2.2TDCI, Santorini Black |
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28th Apr 2022 7:24pm |
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| DSC-off Member Since: 16 Oct 2014 Location: North East Posts: 1403
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maybe they will wear prematurely. Although to keep in mind that the radius arms would not be working against each other, so maybe less forces on the bushes if the vehicle is constantly being articulated.
When trialling with 3 bushes they lasted longer, as you say, they were not fighting each other under extreme articulation. It is on the road where they would wear out quicker. With only 3 bushes to locate the axle and counteract the braking and acceleration loads, it will move about more, wearing the bushes and adversely affecting the handling. Yes, removing one of the 4 is an option. Assuming he was removing the front nearside bolt (I guess want to keep the diff pinion pointing towards the transfer box as much as possible) Exactly that.  |
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28th Apr 2022 8:51pm |
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| Dinnu Member Since: 24 Dec 2019 Location: Lija Posts: 3414
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I can see that handing will be adversely affected, more during breaking as one side would be stiffer than the other. To counter that, perhaps can replace the single bush side with a harder polyurethane bush. 1988 90 Hard Top, 19J Diesel Turbo, Shire Blue - Restoration ongoing
2012 90 CSW, 2.2TDCI, Santorini Black |
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29th Apr 2022 5:30am |
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