Why do you need long shocks on long travel suspension ?

[TMcRally]

I'm considering building a light weight competition off road buggy with around 560mm of travel. The buggy's I've seen have correspondingly long travel shocks (sometimes 3 per side) (Why 3 per side?).

Why can't, or maybe I should say why don't they design in-board shocks with a push rod activated shock with a ratio of say 5:1 for example.

 

[NormPeterson]

3 per side - I would think that shock fade would have something to do with that. Large amounts of energy are being converted into heat (fluid viscosity) and there are aeration/cavitation effects as well.

With respect to the shock motion ratio - the required shock forces would be proportionately higher for the same level of control, and damping for small wheel displacements might suffer when shock piston displacements and velocities are very small.


Norm

 

[WolfHR]

My guess is because when one has 5:1 motion ratio, forces will have to be 5x bigger than wheel loads- and with off-road vehicles one should consider scenarios like landing on one or two wheels and hitting largeish rocks at speed...

I did some 'iffy' work for a even considerably more 'iffy' off-road project, and was in favour of inboard coil-overs at the front with substantially lower motion ratio (below 2:1), but without pushrods (a concept I later saw on Pagani Zonda rear suspension). (unfortunately, the project was stillborn so I never found out how bad my concept was)

 

[GregLocock]

A motion ratio of 0.5 shock/wheel is entirely conventional, there are millions of cars that were built with that.

Going to 0.2 seems a bit tricky.

Bear in mind that off road shocks tend to use remote valving (as opposed to piston and based valving), so the cooling issues occur there, not in the ram itself.

I'd say that it should be possible to engineer such a solution, but it would be a substantial redesign as Norm indicates.

Cheers

Greg Locock


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[DanEE]

From what I have read, seen, experienced with off road vehicles you need shocks for some fairly important reasons..

As mentioned earlier if the vehicle is "jumped" (sand buggies are typically into this).. you need compression resistance to keep from bottoming out the suspension. I have seldom seen rock crawlers and serious terrain off roaders doing much of this...

A more important and overlooked reason for some fairly strong shock rebound control the limit the speed at which the suspension can rebound..

I have witnessed serveral times where a vehicle hits a not very big boulder on one side.. all the impact and compression energy is stored in the impacted wheel spring and if the spring is allowed to rebound at a high rate, it can literally throw an already high CG vehicle onto its side.. What was surprising was to see the vehicle body not react all that much during the compression phase of the impact, but once the spring started rebounding, it put enough roll momentum onto the body to carry the whole vehicle over on its side.. Obviously having NO roll resistance (bars) on these vehicles so as to not interfere at the maximum suspension articulation point, makes this more likely to happen.

Lastly every time we've broke drive train parts (typically drive shaft u-joints, crushed an axle spring perch mount one time) in an early Bronco, it was when the vehicle body started bouncing cyclicly as the tires gripped and let go while trying to get up a steep boulder.. Even dual shocks never seemed to stop this much, but can't imagine what it would be like without any.

 

[TMcRally]

Thanks all

If 2:1 IS OK and 5:1 over the top is there room for somewhere in the middle or am I just diluting the precision the further I move from 1:1.

Quick question on heat. Will I create more heat or just have a smaller volume to disperse it. What I'm thinking is if I use a heat sink on the canister or use a larger canister would that resolve the heat issue.

 

[GregLocock]

You'll have the same heat problem, so consider using an oil to air heat exchanger.

I suspect most of what the guys with 3 shocks are doing is increasing their passive surface area for cooling, compared with 1 big shock

Cheers

Greg Locock


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[patprimmer]

I think the traditional layout is to do with packaging, available equipent and cooling.

On the traditional VW beetle IRS type suspension, there is a long trailing arm that is a very convenient place to mount shocks. As they are line astern they have less effect on aero than a bigger mono shock and are easier to fit so as to not rub on a tyre.

Maximizing cooling, travel and effective damping are all critical to off road racing.

Regards
Pat
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[CRF339]

Don't forget about the buckling characteristics associated with a slender pushroad with long travel suspension systems.

 

[izzmus]

There is an interesting thread on the Special Stage forum regarding the restoration of a Toyota factory stadium racing truck.

While there were three shocks at each corner, in the rear the shocks were attached to a somewhat bizarre hockey-stick shaped link that attached to the rearend housing with rollers. The net result was that each individual shock had its own motion ratio, and no motion ratio was linear. One pair of shocks did absolutely nothing for the first five inches of travel, for example, and then started to ramp in. The front suspension was even more bizarre, with progressive torsion bars.

So, while you are looking at the number of shocks, pay close attention to how they are mounted...