3 replies to this topic

[Captain Vimes]

http://www.evo.co.uk...t_transfer.html

 

Weight transfer again and somewhere you can experience its effects a lot: in the Lotus Elise

 

....Weight a long way from the middle though, is a design problem almost impossible to overcome satisfactorily, ..... So, apart from progressively reducing the size of the front tyres to reduce the bite, the Lotus designers have deliberately introduced bump steer at both ends. This they will admit, doesn’t cure the probem – any more than do the various attempts Porsche have tried over the years – but it helps to slow down the incipient phase and gives the driver a chance to recover the situation before it goes out of control.

 

Bump steer is not part of weight transfer, but having mentioned the Elise, it is worth a moment’s discussion. Lotus have arranged the Elise’s geometry such that on say, a right hand bend, the left front wheel steers slightly more to the left as the car rolls. This effectively tugs the car’s nose away from the turn and makes the steering less effective, they hope just at the critical point when the weight transfer combined with cornering effort is dipping the nose to one side. Better known as the Elise twitch... Meanwhile at the back end, they use bump steer geometry to turn the laden wheel slightly in towards the turn. Because this effect is taking place aft of the point about which the car is yawing (somewhere about the middle of the car), a rear wheel pointing in tries to turn the car away from the arc it is transcribing and makes the back less willing to follow the front into a spin.  

 

The rate at which the whole process unfolds is also influenced by the dampers – the technology and application of which could easily fill several volumes – but the inescapable downside of a geometric arrangement which relies on suspension deflection is that the ride height from which it starts, is critical – something I also touched upon last time. When people lower their Elises, unless they check all the results of their modification and almost more important, know what they might have to do to optimise it, they will almost certainly be introducing characteristics which they didn’t want. Could of course be the other way, but without the specific knowledge it’s hard to know which of your fiddling did what. I have driven two standard Elises on the same day at the same circuit and their handling was so completely different they could have been different models. None of it is irreversible, but if you suspect a problem – even with a standard car - then an investigation followed by a proper setup by someone who really does know their stuff is never a bad idea.  

Edited by Captain Vimes, 06 June 2015 - 09:37 AM.

[D-DAWG83]

Yeah i read that a few weeks back a good article by people that know their stuff.

[Rosssco]

Where is Scuffers to tell us this article is wrong?

[Scuffers]

Where is Scuffers to tell us this article is wrong?

LOL!

 

it's not wrong per say, but it's wildly misleading.

 

All cars have a degree of bumpsteer designed into them, some actually have active rear steer to do the same job.

 

 

Front bump is there to moderate the way the car (1) turns in and (2) holds the line on corner exit.

 

worth mentioning that an ideal setup will change with whatever tyres your running, depending on their carcase construction etc.

 

If you get it horribly wrong, (and it can be a very fine line) as you turn in, the rear will want to come round instantly, like you have castors on the rear, or wrong the other way leads to terminal understeer.

 

 

reading the other parts of the article, he's placing too much emphasis on the way the weight is laid out for it's handling traits, which is a little unfair as it;s perfectly possible to set one up not to act like he describes without changing the weight distro (compare an S1 with an S2 in std trim).