Are all deflections bad?

Posted on May 5th, 2009 in electric,pedal power,Suspension,testing by Julian Edgar

One of the automotive ideas that seems to be taken as gospel is that the chassis and suspension arms should be stiff – that is, neither should deflect when subject to load. In fact, if I’d had a dollar for every time I’ve read that ‘good handling depends on a stiff chassis’ I’d be richer than I am.

But I think that, especially for ultra-light vehicles, this notion is simplistic.

Firstly, every structure deflects under load. That deflection may be small, but it occurs. Even the Sydney Harbour Bridge has an allowable deflection under maximum load of 4.5 inches (114mm) in the centre of its span.

Secondly – and more importantly – chasing reduced deflection will add substantially to weight. The corollary of that – the lightest possible vehicle will always have deflections.

Finally, not all deflections are bad.

Let’s start off with the last. Most cars use rubber bushes that are designed to have differing stiffnesses in differing planes. One reason for this is so that wheels can move fractionally backwards when they meet a bump, reducing harshness. Another reason is that in some (many?) suspensions, if the bushes didn’t have ‘give’, the suspension would lock up solid during travel.

Passive steering suspension systems – the first well publicised was the Porsche ‘Weissach’ axle of the 928 – often use bushes that deflect, or links that give an effectively ‘non-stiff’ suspension in some planes.

Going backwards to the second point, getting rid of measurable deflections in chassis and suspension arms will result in a major increase in weight. In ultra-light vehicles (eg those powered by human legs, a small petrol motor or an electric motor), and especially those made from chrome moly steel tube, deflections under major loadings are often able to be seen by eye.

For example, the peripheral torsional wind-up of a front suspension arm might be 5mm or more under maximum braking, and under max cornering there might be 3 or 4mm of bending in wheel supports. In a human powered vehicle (HPV) with a recumbent seat and front pedals, boom flex under maximum pedalling force can often be 10mm or more.

So does all this matter? In some cases (like boom flex, that subtracts from the power available from the rider), yes it does.

But in other cases – not necessarily.

What is required is that the structure is never stressed to the point of failure, and that the vehicle dynamics remain consistent.

I have been musing over these ideas in the context of the HPV I have been building.

I know that under brakes the beam front axle will torsionally wind-up, reducing the static castor of the front, steering wheels. That might lead to steering dartiness under brakes – but for the fact that when the front brakes are in action, the vehicle has some dive, that in turn causes a rapid increase in castor.

On my previous recumbent trike design (called the Air 150), I had difficulties in getting rid of steering twitchiness. The problem felt all the world like toe-in bump steer, where I’d put on some steering lock, the machine would roll slightly – and the outer wheel would toe-in, giving a sharper steering response than requested. That was the theory – but I found this odd when on the workshop floor, toe-in on bump was small or non-existent.

But I now wonder if the outer semi-leading suspension arm wasn’t flexing sideways a little with the sudden application of the lateral force, which in turn caused “turn-in steer” as the suspension arm and the steering tie-rod flexed through different arcs.

Certainly, at the very early stage of testing I am at with my current HPV ‘Chalky’, there’s no steering twitchiness on turn-in – and the front suspension is laterally much stiffer than the previous design.

(I fixed the Air 150’s twitchiness by setting the suspension up with either static toe-out, or toe-out on bump – but the problem returned when carrying really big loads. If the arms were bending laterally, perhaps it just needed even more static or bump toe-out to compensate?)

And I guess that’s the point. In a vehicle – any vehicle – there will be dynamic variations that don’t match the static settings.

(Many years ago, I remember having a wheel alignment done on my Daihatsu Mira Turbo. I was happy with the alignment machine’s read-outs – but then the mechanic got me to sit in the driver’s seat. On that simple car, the suspension settings immediately changed!)

If the weight of the vehicle has been has to be kept to an absolute minimum, and so major deflections occur in the suspension and frame, the trick is to optimise the direction of those deflections so that they don’t subtract from – and possibly even add to – the on-road experience.

That’s a very different notion to ‘keep everything as stiff as possible’.

9 Responses to 'Are all deflections bad?'

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  1. Matt King said,

    on May 5th, 2009 at 10:14 am

    I know you’re not a fan of bikes in general, but they make an interesting case of chassis flex being useful, and all-out stiffness being bad (read up about the carbon chassis Ducati Moto-GP bikes of this year, or of Rossi’s development of the Yamaha chassis a couple of years ago).

    On the other hand, I can’t think of any situation where the _chassis_ of a passenger type car (as opposed to the suspension) isn’t improved by being stiffer (all else being equal of course).

  2. doctorpat said,

    on May 5th, 2009 at 4:58 pm

    Off roaders claim that one of the big advantages of the Toyota landcruisers and similar vehicles is that the separate chassis have lots of flex to add to the wheel travel as they crawl over rocks, cliffs, dead roos, etc.

    Not exactly the autospeed target market, but clearly a case of deflection being desired.

  3. on May 5th, 2009 at 6:17 pm

    I never could understand why the builders of top fueler dragsters allowed their chassis to bend in such an apparently ridiculous way. Can it be that there is actually a benefit?

  4. Geoff said,

    on May 5th, 2009 at 10:25 pm

    A few years ago I read about how a Vauxhal Vectra racecar had the rear cross member designed to flex and offer some steering and handling improvements. It was designed to meet the minimum requirements of the racing series and yet managed to offer such massive improvements that the series was theirs for a number of seasons. A spectacular piece of intelligent engineering in a racing series rewarding innovation, unlike the local taxi racing farce where a team was accused of cheating because they had a flexible arrangement of mounting the nose-cone, so saving money by not making the cone destroy itself and allowing faster repairs and replacements during racing. Fortunately, after modifying the system to be like everyone else they still took pole and won the event. Even though I have worked in the pits for one team and admire the crews, engineers and some drivers I am over the procession and lack of real innovation. More catering to the lowest common denominator at the cost of progress.

  5. Ben Garside said,

    on May 6th, 2009 at 12:29 am

    Could it be that chassis engineers prefer to keep chassis components as stiff as possible so that deformations and geometry changes are concentrated in a few areas, i.e: bushes, springs, tyres and dampers, to make the dynamic behaviour easier to understand and control? I guess NVH could also be an issue given that flexing of body parts / chassis could lead to noise and/or wear between flexing parts. In contrast, a rigid-framed bicycle relies on frame, fork, seatpost and handlebar flex to ensure tolerable comfort.

  6. doctorpat said,

    on May 6th, 2009 at 9:41 am

    Good point Ben. It is clearly much easier to analyze and predict movement around a bearing joint than flex in a complex structure.

    The ever increasing abilities of computer modeling means that designed in flex is easier and easier to predict, so should be increasing.

    On the other hand, the old school vehicles had heaps of flex just because they couldn’t make them stiff if they tried (given the contemporary restrictions of materials, costs, weight etc.)

  7. wayne said,

    on May 10th, 2009 at 9:08 am

    A while ago I made a motorscooter and made the main chasis such that it flexed significantly under where I stood giving very simple suspension. The downside to having deflection in components other than springs is that while that flex is ‘sprung’ there is little to no ‘damping’. If you can make up for that lack of damping in the components which flex by keeping it to a percentage of the total deflection, increase the damping in the suspension ( percentage of the initial shock taken by flex, then as the damper allows, that shock transfers to the spring and unloads the flex somewhat ), a suficient compromise may well be achieved.
    In my case with the scooter, as i was the most significant mass, the chasis flex was the spring, and my legs became the damper, and by overcompensating with damping could effectively control the spring.

  8. tim said,

    on November 12th, 2009 at 3:04 pm

    wayne, you could have a look at how multi leaf spring works. Flex is taken care of by deflection of the spring, while the “drag” between the leafs has a dampening effect.

  9. Au said,

    on January 5th, 2010 at 7:58 am

    Obviously some parts need to be stiff some need to be flexible. Obviously springs are parts that are very flexible. But there is no such thing as absolutely unmoveably stiff, like in the text something like non-measurable deflection is mentioned, I believe no one searches for that, and it is not possible even if you have a infinitely heavy car. Engineers need a stiff chasis so that the suspension designed can function properly. If you can predict the deflection of the body well, than flexibility may very well be useful (like springs, we know how they will deflect so we can use them) Probably making the chasis stiff makes the suspension design simpler to control. And probably a very flexible chasis would require some kind of active suspension to be fully useful. And again you do not have to make a vehicle much heavier to make it stiffer. You don’t even have to use exotic materials. Just by changing the shape you can have very dramatical stiffness improvements. Like I beam vs. full cross section.