Lotus Elan

Brace across rear turrets +2S

PostPost by: rgh0 » Fri Apr 03, 2020 12:19 am

I have thought about a front cross brace for the Elan towers but in the end decided the benefit would not be worth the effort to fit it in given the restrictions in my racing class of tyres and wheel width limits the actual amount of extra loads that can be fed into the front suspension. When I looked at it my idea was to run a thin tension rod from the lower suspension arm inner pivot to the upper arm inner pivot on the opposite side as the cornering loads try to move the top of the suspension tower out and these tension braces would help stop that if it was happening, however space to do this is very limited with the radiator and steering rack and engine nose all in the way.

Looking at the body I see little actual flexing taking place around where it bolts to the towers and I would have expected some cracking to develop there if the tower was moving much. So my conclusion is the tower moves very little even under competition loads with sticky tyres.

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PostPost by: alan.barker » Fri Apr 03, 2020 6:27 am

If you just put a horizontal bar that just makes a parrellelogram which can not resist side movement. If you make a brace you need triangles.
Another point you need to do it to a NEW Chassis before it's bent☺☺
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PostPost by: nmauduit » Fri Apr 03, 2020 9:20 am

rgh0 wrote: When I looked at it my idea was to run a thin tension rod from the lower suspension arm inner pivot to the upper arm inner pivot on the opposite side as the cornering loads try to move the top of the suspension tower out and these tension braces would help stop that if it was happening


I would have thought the cornering load pushing the top tower in rather... and on older chassis I've found the towers bent in, too.
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PostPost by: Foxie » Fri Apr 03, 2020 8:46 pm

alan.barker wrote:If you just put a horizontal bar that just makes a parrellelogram which can not resist side movement. If you make a brace you need triangles


You are correct if you are considering a parallelogram with 4 pin-joints.

However, in the case of the Lotus Elan front suspension, we are talking about two (almost) box section uprights substantially connect to a box section base. ( No doubt considered by Chapman to be strong enough to obviate any future claims for chassis failures ! )

Connecting the tops of these uprights with a pin-jointed cross brace will mean that any unequal load on one side, for instance caused by cornering, will be 50% shared with the opposite upright.

Additionally, in the case of normal equal sided suspension loadings, the brace will counter any inwards deformation of the uprights.

In the case of shock loadings caused by sudden road level changes ( ramps or yumps ) the suspension towers are protected.

In my opinion, it is poor road surfaces, hump backed bridges etc. that do the damage. (The bog roads and mountain roads of Ireland will test the best of suspensions :) )

My 1970 Plus 2 had the nearside front tower almost completely detached by 1987 as a result of fatigue cracking, before it was ever raced.

When I removed the body I discovered that both rear upper suspension ears were substantially bent. I very much doubt this was caused by excessive cornering forces. I was able to repair the damage and reinforce them with an oxyacetylene torch.


alan.barker wrote:Another point you need to do it to a NEW Chassis before it's bent☺☺
Alan


I replaced the original Lotus Chassis with a Spyder chassis in 1998.

In 2015 I did a body lift to fit a T9 gearbox. I discovered the much stronger rear tower ears on the Spyder chassis had bent up :shock:

Out with the welding torch again :)
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PostPost by: Foxie » Fri Apr 03, 2020 9:29 pm

HCA wrote:Kelvedon I see makes a brace for the two seater Elan. It goes through the inner wings and joins directly onto the turrets. I cannot find anything though on a brace for the +2!


I've been able to find this on the Kelvedon website.

It's basically a tube, with opposite threaded rose joints at each end. It seems the rose joints would be fitted into holes drilled in the forward faces of the front suspension turrets. The tube would then be sufficiently forward not to be obstructed by the water pump/fan pulley. The opposite-threaded rose joints would allow for minor adjustments in span of the front turrets drillings to pre-load the fitting.

In the short term it seems it would be effective enough.

However, using rose joints on the front face of the turrets would be exerting a varying torque on them in resisting any inwards deformation. Putting this through a shear load on the turret front face drillings as opposed to a direct compression loading between the inner faces might not be the optimum solution.

For GB£100 ex VAT it looks reasonably priced, and simple to fit.

And, as Rohan has commented, it will fit both the two-seater Elan and the Plus 2

:)
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PostPost by: rgh0 » Sat Apr 04, 2020 12:38 am

nmauduit wrote:
rgh0 wrote: When I looked at it my idea was to run a thin tension rod from the lower suspension arm inner pivot to the upper arm inner pivot on the opposite side as the cornering loads try to move the top of the suspension tower out and these tension braces would help stop that if it was happening


I would have thought the cornering load pushing the top tower in rather... and on older chassis I've found the towers bent in, too.


The vertical weight of the car is resisted by the suspension spring / shock unit and imposed upwards and at a slight angle to the vertical inwards on the tower ( a bigger angle in a Plus 2) . Hitting pot holes and speed humps would distort the towers in with those loads but there are not many of these on race tracks but towers bent in are common on road cars that have had a hard life on rough roads especially the heavier Plus 2 with its more angled front shocks.

In cornering the outer wheel has a horizontal force at the tyre contact point inwards and the suspension upright is trying to pivot around the lower wishbone and that is trying to pull the top wishbone outwards and this force is greater than the weight force at an angle that is pushing inwards. So if trying to maintain suspension geometry in race cornering a tension brace appears to be whats required though a compression strut between the towers may not hurt in addition even if it does add weight.

However as none of the movements seem significant in terms of chassis distortion or body cracking on my Elan I have not bothered. The bigger issue on both my Elan and Plus 2 has been cracking around the engine mounts and suspension mounts where concentrated loads are fed into the thin chassis panels and brackets

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PostPost by: nmauduit » Sat Apr 04, 2020 10:31 am

rgh0 wrote: In cornering the outer wheel has a horizontal force at the tyre contact point inwards and the suspension upright is trying to pivot around the lower wishbone and that is trying to pull the top wishbone outwards and this force is greater than the weight force at an angle that is pushing inwards. So if trying to maintain suspension geometry in race cornering a tension brace appears to be whats required though a compression strut between the towers may not hurt in addition even if it does add weight.

However as none of the movements seem significant in terms of chassis distortion or body cracking on my Elan I have not bothered. The bigger issue on both my Elan and Plus 2 has been cracking around the engine mounts and suspension mounts where concentrated loads are fed into the thin chassis panels and brackets


ok thank you Rohan, a bit counter intuitive but now that you've explained it it seems a lot more clear... This would be quite dependant of tire grip and lower suspension arm actual position under load (for which I think the bend version would be more favorable : the more lateral force taken up by the lower arm, the less torsion transfered to the upper and top tower which is not designed for huge lateral force).

I'll go by your approach, event though a cross brace (push and pull) would be easy enough (yet probably not FIA as we don't see any around here) a kg saved is a kg diet I won't have to endure...

It's good to think a bit about the future these days, and plan along the build to give some perspective to a daily routine that tends to get stuck when in lockout...
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