Date: Sun, 25 Jan 2004 14:07:57 -0800
From: Jeff Chan
Subject: Softer bushings on U.S. Elise
The question of the more compliant A-Arm bushing change for the U.S. Elise came up on the elisetalk forum, to which I responded:
Regarding the bushings, I think you may be misinterpreting the intentions. If the tires are upset by large disruptions they won't be in good, stable contact with the road. If the tires don't have a consistent interface with the road then what you feel will be scrambled. It's by building a little compliance into just one part of the suspension that the tires will be kept in better contact with the road and you will feel the important stuff, i.e. the real, usable surface, not the large gaps and large disruptions. What you'll feel is the tires working with better conact with the road surface that actually provides grip. In other words you'll be able to count the insect legs through the steering wheel as you run over it, even if the insect is just beyond a pothole. Before you would have felt the chassis, suspension and tire flailing madly to deal with the pothole. That's a benefit.
This should be especially noticeable during a bump in a turn, because as I remember Lotus Engineer Clive Roberts explaining about the M100 subframe raft, the longitudinal compliance allows the suspension to be set to smaller castor settings, which means the driver is presented a finer feel of the tire loading:
Absolutely right - the Something Else is pneumatic trail, a property of the tyre.
The effort you feel at the wheel comes from two sources (if we disregard friction in the rack and column - well worth checking on Triumph-based racks)
1. The moment about the kingpin axis, tending to return the tyre to straight-ahead, caused by the castor angle - don't need much more detail, except that it's virtually constant, and gets larger as the castor angle increases. The important point is that it doesn't vary as cornering effort changes - it's just there, a force you have to work to overcome. It only needs to be there, in some small amount, to ensure straight running.
2. The torque caused by the cornering force acting through the contact patch - this is the interesting item, because as cornering effort rises so the contact patch changes shape - thus the point at which the cornering force acts on the steering system (pneumatic trail) changes position. This means that the tyre is actually sending you signals to tell you how much work it's doing on your behalf - as it approaches the limit, the torque felt through the steering gets smaller - in other words, the steering gets light. This is a very good signal, AS LONG AS YOU CAN RECEIVE IT. If you have a large castor angle, you won't - it will be masked by the unnecessary and unhelpful grunt factor required to overcome the castor.
There is really no need for large castor angles on a well designed suspension - there is no inherent merit in having 6 degrees instead of 3 - it only needs to be just positive at all times. Old wishbone systems will tend to lose castor under braking (as the brakes try to rotate the upright), but more recent designs will have ways to compensate, allowing low static castor settings. (an extreme case being the M100, whose raft allowed as little as 2 degrees static castor, and kept it constant under load).
I suspect the slight added longitudinal compliance created by the new front bushing arrangement is meant to control castor change after a large bump or pothole, thus preserving the feel of pneumatic trail in the new Elise which Clive mentions in general terms. That would be an excelent question for Lotus regarding the new model.
I didn't mention it originally, but Kuah's article also mentioned that the Elise bushings are already 50-60% stiffer than most other cars, accoring to that same development engineer. I seriously doubt that Lotus has turned the Elise into a Cadillac Seville wannabe. Lotus undertstands how important steering feel is and have found a way to preserve it even with our less than perfect road paving.