will lowering improve handling?

[Number_Cruncher]

>>you need to make comparatively large changes in spring rate

Having spent an enjoyable 10 minutes working through the derivation of the spring rate vs wheel rate equation; I realise that the above snippet from my previous post was *utter* nonsense. Mea culpa. The perils of relying upon memory alone!

What I should have said was that the wheel rate is the spring rate multiplied by the suspension ratio squared. So, if you double the spring rate, you DO double the wheel rate. However, if you buy a spring with a given rate at the spring, that is much reduced when you consider wheel rate.

Consider the case of a typical strut suspension, where spring deflection is virtually the same as wheel deflection - the suspension ratio is very nearly one, and using spring rate gives relatively little error.

With the inboard spring of the front suspension of a MB, the suspension ratio is much smaller than one, and the spring needs to be very stiff to give a reasonable wheel rate.

In rare cases, manufacturer's suspension engineers have got this seriously wrong by forgetting that the appropriate ratio for spring rate sizing is suspension ratio squared, leaving the suspension much too soft.

 

[stevesey]

Ah - I was just about to ask if that meant that cutting a quarter of a turn of a spring would lower the car without affecting the wheel rate (much). Thought it seemed to be too good to be true.

 

[wheels-inmotion]

>>you need to make comparatively large changes in spring rate

Having spent an enjoyable 10 minutes working through the derivation of the spring rate vs wheel rate equation; I realise that the above snippet from my previous post was *utter* nonsense. Mea culpa. The perils of relying upon memory alone!

What I should have said was that the wheel rate is the spring rate multiplied by the suspension ratio squared. So, if you double the spring rate, you DO double the wheel rate. However, if you buy a spring with a given rate at the spring, that is much reduced when you consider wheel rate.

Consider the case of a typical strut suspension, where spring deflection is virtually the same as wheel deflection - the suspension ratio is very nearly one, and using spring rate gives relatively little error.

With the inboard spring of the front suspension of a MB, the suspension ratio is much smaller than one, and the spring needs to be very stiff to give a reasonable wheel rate.

In rare cases, manufacturer's suspension engineers have got this seriously wrong by forgetting that the appropriate ratio for spring rate sizing is suspension ratio squared, leaving the suspension much too soft.

Your logic albeit accurate is the nemesis for those on the floor.

I have found the design concepts and actual reality don't correlate in real life?..... Unsprung calculations are history once additional (pre-sale) wheel sizes are fitted.

Don't get me wrong i am not belittling your post but i find actual mathematics calculations in design miss the bull.... we could pants on for days with math but in the real world it's the results that matter.

In summery
I think the designers and manufacturers need to fill the void in the middle! It's the end results that matter to most people not how it got there.

 

[Dieselman]

Fortunately the US market is large so there is still a home for these poorly developed cars..

 

[wheels-inmotion]

Fortunately the US market is large so there is still a home for these poorly developed cars..

So there is hope after all

 

[Number_Cruncher]

>>i find actual mathematics calculations in design miss the bull

I couldn't disagree more! (Although I would say that! ). I've been in the lucky situation of the results of my calcs matching test results very closely - in fairness, I've also been in the more embarrasing situation where, until correction, they were a back yard out!

Car design without guidance from any mathematical modelling and simulation would have us all still driving cars like Morris Minors [the pre-war ones at that!]

Turning your back on mathematical modelling in vehicle design and development because of the odd setback is like giving up using spanners because you've rounded the odd bolt head! You're no longer using the full kit of tools.

You can spend quite a lot of time writing calcs - paper and pens are really cheap when compared with production tooling!

We are at an interesting time in the development of this approach. Simulation is now considered to be so good that the need for verification testing is beginning to be waived - even in more safety critical applications than motor cars.

Where simulation currently suffers its biggest failing is the difficult element in vehicle dynamics - the driver. In terms of maximum steady state forces transmitted between vehicle and road, the difference between production cars (using comparable tyre types), isn't that great. Where there is a difference is on how this feeling of grip is transmitted to the driver, and whether the driver is, or isn't confident in the vehicle.

Philosophically speaking, using mathematical models is a truly strong method for engineering. Where you get good correlation between model and test, you can have some confidence in your understanding of the underlying physics, and you can make small changes to your design with only a small likeliehood of there being a problem. Where model and test don't match is actually where you get the best value from modelling!, in this case, you are being given a hint that you don't fully understand the problem, and there's a chance both to learn something new, and have the right tool in place to check that you really have gained the understanding you needed.

In short, despite the short term pain involved in programming simulations and writing calcs, you gain the long term advantage of reducing reliance on hunches, luck, guesswork, and blind faith.

 

[wheels-inmotion]

Superb post NC but don't you find the work done from mathematical modelling then mathematics calculations is totally lost in final production?

From my side of the fence i see the final production taking precedence and often destroying many previous model modelling or otherwise.

I re-wrote the chassis positions for the Lexus IS200/300/sc because the final production decided to use a one-size-fits-all front tie rod that controls the castor position, although the mathematical modelling showed a need for three versions, Asia, Europe and GB.

 

[c240yaz]

I re-wrote the chassis positions for the Lexus IS200/300/sc because the final production decided to use a one-size-fits-all front tie rod that controls the castor position, although the mathematical modelling showed a need for three versions, Asia, Europe and GB.

Did you do this for Lexus Motor Corporation directly or as an independent project ?

Just curious.

 

[Dieselman]

.

I re-wrote the chassis positions for the Lexus IS200/300/sc because the final production decided to use a one-size-fits-all front tie rod that controls the castor position, although the mathematical modelling showed a need for three versions, Asia, Europe and GB.

Why the differences, ride height and wheel widths.?

 

[Number_Cruncher]

>>is totally lost in final production?

For the sectors in which I now work [nuclear & aerospace] the analysis and safety case is paramount, and so, things can't get changed without engineering sign off, so some degree of control is kept.

Having said that, the parts I work on day to day don't have anything as nasty, difficult, non-linear, and unpredictable as a tyre attached, so, there's much less need for post proto-type tweaking!

 

[Dieselman]

Having said that, the parts I work on day to day don't have anything as nasty, difficult, non-linear, and unpredictable as a tyre attached,

Just a nuclear reactor.......

 

[wheels-inmotion]

Did you do this for Lexus Motor Corporation directly or as an independent project ?

Just curious.

As an independent specialist after a plea for help from the owners club.