Porsche 911 UK Enthusiasts Online Community Discussion Forum GB

Welcome to the @Porsche911UK website. Register a free account today to become a member! Sign up is quick and easy, then you can view, participate in topics and posts across the site that covers all things Porsche.

Already registered and looking to recovery your account, select 'login in' and then the 'forget your password' option.

Help needed understanding dynamic strain energy losses.


20 May 2009
I am trying to do some calculations about the loads on an engine during acceleration against a mass from different starting points and acceleration rates and am struggling to find anything helpful.

You see I have long held the view that relatively powerful engines driving in 1st gear loose a lot of their torque through strain energy losses caused by dynamic elasticity within the yield point of the transmission – but have never been able to find any physics to cover it. Most reports state that strain energy is negligible in transmissions – which I can understand under constant speed but not under acceleration - since acceleration is proportional to torque/resistance (or inertia or whatever you want to call the sum of all the resistances).

I am sure modern education systems have covered this subject (which we hardly touched on in my time) but going back to then (50 years ago) I remember when we were towing a trailer with two racing bikes on it with a

Ford Consul Classic and if we booted it too much in first gear the drive shafts would snap in half (we carried a spare just in case).

I occurred to me this can only be because the torque input trying to accelerate the mass of the car against an initial static resistance, (trailer and bikes) exceeded the torsional yield point and therefore the strain energy exceeded it as well. Once moving - the resistance to acceleration was less so as long as you accelerated from a standstill moderately – the drive shafts survived and you could continue on your journey OK but to snap a driveshaft with a torque wrench would require huge torque and torque is force * radius – so a huge force as well. It seemed to me that a huge amount of force was being absorbed twisting the drive shafts that was not available to drive the wheels.

Looked at slightly differently - I mean of you apply a torque to the head of a bolt for 1 minute or alternatively for I hour – you must have used more energy for the hour – yet at the end of it everything returns to normal in both cases – so where did that extra energy used disappear to?
Similarly - if a horse tried to pull a barge tied to a bollard for 1 minute or 1 hour – more horse force was used over the hour yet the outcome is no different.

I accept that in both examples above there is no movement (once the strain extension has been reached - so there is no rate of doing work measured over a distance or revs etc) but strain energy is a form of potential energy so there is still power used and a force must be being applied and as I understand Newton an object stays at rest unless acted on by a net external force?

The first law of thermodynamics is also that energy cannot be created or destroyed but there seems something odd about energy used to strain something over different periods of time but the outcome being the same?
The internal combustion of an engine is different depending on the load the engine is driving against (and requires different ignition timing to avoid knock). I am trying to work out what those differences are in relation to different rates of acceleration but so far it has proved to be beyond me.

OK I can work out the torque (and therefore the BHP) at any given rev point while Accelerating our rolling road dyno – but that is at a fixed resistance rate and usually at full throttle (WOT).

But if the mass being accelerated was altered or the power generated – the internal strain energy losses would be different.

If anyone can help I would be very grateful indeed especially if they can explain it in a tangible way (and not via mathematics I might struggle with).

Although not of direct interest to me right now – I once built a twin cylinder two stroke racing engine with identical everything except one fired two cylinders together and the other alternately (at 180 deg). They were as different a chalk and cheese – one had a very narrow power band from about 8,500 to 10,500 rpm while the other had progressive power from 2000 rpm gradually increasing and revved on to 14,000 rpm. I have had it explained to me that this is all to do with flywheel stored and released energy but I cannot help thinking that there must have been some strain energy differences involved?

So I get the gut feeling that there is more to strain energy losses than many give it credit for and current research would benefit if I understood it better.
I guess this might be of no interest to anyone else so a direct reply to me might be more acceptable to other readers?

I might very well be getting my sciences muddled up - but any advice would be welcome.

Your example of the snapped driveshafts reminded me of something I've never understood about my motorbike. On the road the clutch never slips regardless of what gear it's in or what speed I'm going. The bike is a zx9r which will do an indicated 175 but sometimes I've been into a head wind and it won't go over 160 and the clutch still doesn't slip.

The thing I don't understand is twice it's been on a rolling road and both times the clutch slipped preventing a proper run. Why does it slip on the dyno but not on the road?


New Threads

Forum statistics

Latest member