Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 M3W rear unsprung weight reduction

So as I'm progressing through a rear tire change and various odd jobs around the rear of the car, I thought I'd take a moment to weigh the complete rear wheel/tire/brake/hub/sprocket assembly, basically everything except the spindle shaft and nut (they're at the platers).

Especially interesting as I'm changing from a steel rear sprocket (works fine, and available, btw) to a SuperMax upgrade on the original alloy sprocket. Turns out the steel sprocket is a full 10 lbs (~4.5kg) heavier than the SuperMax. Which on a total assembled weight of all the components (steel sprocket) of 73 lbs, means the sprocket change alone gives about a 14% reduction in rear wheel assembly weight.

On Lotus Elan's I've owned, reducing unsprung weight by just a few pounds per corner was noticeable in terms of impact harshness reduction and mid corner bump stability. Will be interesting to see if there is a noticeable change on the Morgan.

Are you still using the oem shocks and springs?

Yes. So far. OEM damping seems fine to me, especially front and just okay on rear. Rear spring rate seems high, though.

Reducing weight back there might make it a more rough ride.

Reducing unsprung weight won't hurt the ride.

Unsprung weight is just that so it cannot load the suspension. Reducing it can only increase ride compliance.

I remember taking a wrong turn in mine leaving me on a straight but undulating Norfolk fen road. This set up a quite severe low speed oscillation of the relatively soft rear suspension such that I couldn't exceed 45-50 mph without fear of being thrown off the road. I did wonder at the time if this was due to excessive unsprung weight.

Always good to reduce unsprung weight, but it does not "increase ride compliance", compliance as such is related to the wheel and spring rate, you are not changing that are you?
Unsprung weight reduction will help the suspension respond quicker to road inputs and make it easier to tune the damper to the primary suspension modes.
Reduction in unsprung weight will however change the secondary ride vibration in terms of magnitude (usually reduces) & frequency (usually increases) of the secondary modes; primary ride being the car on its springs typically at a much lower frequency than secondary ride freqs..
Secondary ride is concerned with the motion and vibration of parts that are sprung mounted to the sprung part of the car, so such as power train and suspension parts including wheels and tyres of course. Secondary ride from unsprung suspension parts can show up as a short duration 'judder' after a road event (pot hole) as the mass of for instance the unsprung suspension parts and wheel resonates between the primary suspension spring and the tyre stiffness before being damped out until the next event. Note that many cars use separate damping devices on the unsprung parts of the car that are carefully tuned to do so, not Morgans though.
Cheers PJB. (SSL)

When my original alloy sprocket was changed to the now standard steel version I couldn't tell any difference in the ride or handling (on standard non adjustable shocks).

The biggest changes were switching tyres from Avon to Blockleys on the front and later replacing the standard shocks with the Spax Adjustables.

That''s what I was trying to infer by compliance , my armchair thoughts suggesting a lesser mass will allow quicker acceleration and deceleration such that wheel can better follow road surface. There is obviously action and reaction at play as well though

Richard, understood, it is only terminology - sorry if I was a bit/rather pedantic - Cheers PJB (SSL).