Lotus Elan

'Reconditioned' steering racks

PostPost by: Foxie » Mon Aug 28, 2017 7:33 pm

Many years ago I replaced the steering rack with a 'reconditioned' unit from a well-known supplier (whose website doesn't seem to be working at the moment)

Although the ball-joints seemed new, I was never convinced that the rack itself had been re-machined to compensate for wear in the central position, so I set the pinion thrust with the rack off-centre

Checking it today by pushing it through full travel left to right, with the wheels off, I can distictly feel each tooth as it passes under the pinion.

On the road there is almost no detectable play, but any wheel imbalance seems to contribute to a slight vibration.

Any opinions ? :)
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PostPost by: Donels » Tue Aug 29, 2017 8:09 pm

Having recently rebuilt my steering rack in accordance with the workshop manual I can confirm that you can feel the teeth as the rack passes through the pinion. So feeling this with the wheels off is not a surprise and I suspect there is nothing wrong with your rack.
A wheel imbalance can create a shimmy through the steering. Balance weight are often added on the inside and outside rims of the same wheel to correct imbalance. Consider the case where one of these is removed. You then have an imbalance not only in a radial plane but also side to side. This will try to move the wheel side to side being felt through the steering as a vibration.

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PostPost by: StressCraxx » Tue Aug 29, 2017 8:18 pm

Donels wrote:Having recently rebuilt my steering rack in accordance with the workshop manual I can confirm that you can feel the teeth as the rack passes through the pinion. So feeling this with the wheels off is not a surprise and I suspect there is nothing wrong with your rack.
A wheel imbalance can create a shimmy through the steering. Balance weight are often added on the inside and outside rims of the same wheel to correct imbalance. Consider the case where one of these is removed. You then have an imbalance not only in a radial plane but also side to side. This will try to move the wheel side to side being felt through the steering as a vibration.

Dave


Not only will side to side (dynamic couple unbalnce) but also wheel lateral and radial mechanical runout will also be felt in the steering.

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Dan
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PostPost by: Foxie » Tue Aug 29, 2017 10:45 pm

Donels wrote:Having recently rebuilt my steering rack in accordance with the workshop manual I can confirm that you can feel the teeth as the rack passes through the pinion. So feeling this with the wheels off is not a surprise and I suspect there is nothing wrong with your rack.

A wheel imbalance can create a shimmy through the steering. Balance weight are often added on the inside and outside rims of the same wheel to correct imbalance. Consider the case where one of these is removed. You then have an imbalance not only in a radial plane but also side to side. This will try to move the wheel side to side being felt through the steering as a vibration.
Dave


Thanks for that info about feeling the teeth, it's good to know it's normal :)

I am familiar with multiple plane rotating body balance theory. The old double plane system (inner and outer rims) on car wheels has recently been superseded by a single weight system strategically positioned at a point across the width of the inside of the wheel well by computer calculation. Modern wider wheels facilitate this.

I am fairly meticulous about wheel balance and go to the best shop in town, and make a note of before and after figures. However, imbalance will always develop in time as tyres wear or worse, are flat spotted. A closely-toleranced steering system will help to damp small imbalances, but a loose one will knock like a freight train backing up :)
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PostPost by: patrics » Tue Sep 05, 2017 7:50 pm

Hi
As Dan pointed out you have the radial force run-out and the lateral force run-out.
As a general rule in after market only the lateral force run-out is ever checked - in production both are done.
On a new tyre there is normally a red dot and this is the point of the tyre with max runout or heavy spot - can't remember which, so on an aluminium wheel the red dot is alway mounted opposite the valve stem which is heaviest point of the wheel.
On a steel wheel there might be a dimple marking the lowest weight part of the wheel so the red dot should be aligned with that but that is probably way to new for our old steel wheels which are now not round anyway

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Steve
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PostPost by: Foxie » Tue Sep 05, 2017 11:51 pm

That's a very valid concept. The wheel/tyre combination could be in perfect dynamic balance, but any significant radial or lateral rim run-out would contribute an alternate source of vibration. I found this out when the protruding head of a 10mm disc- to-hub securing bolt on a Sierra brake conversion contributed to a very palpable wobble. :)
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