Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 ohlins

+1 @JVS

Hi Calum my top hat's are same size as yours! I'm now thinking my springs are larger diameter than yours. I turned the shocks round 5mm clear off belt but touching tank... I have put originals back on for now, hopefully out for a run next sat..

Hi Kevin, will have a look in the morning at the spring dimensions and what gap I've got from the tank to the swing arm and belt etc. It's a bit strange all told. I did run for a bit with one Ohlins and one stock shock on the back till Ben had machined up the offset top hats for me; worked well, just not quite as good as with both Ohlins.

Calum nearly left one on! but no. took them in to work today and faced off 2mm off ,managed to find 2mm washers exact size needed... so just case of rebuild and measure and test drive. will be a few weeks before done, off to William Medcalf this weekend. I post when done with good news i hope....Thanks for all your help again Calum. Have a grand new year

Had a look around the drive belt side shock again and have some measurements from my car;

Outside of shock bracket on swingarm to fuel tank; 21mm

Belt edge to tank immediately aft of shock; 74mm

Belt edge to tank at top of sprocket (straight above wheel spindle); 77.5mm

Ohlins rear springs are marked as; 00180-65/62 L5115 and their OD is 66mm.

Could the issue actually be that the tank isn't mounted as far away from the centre line as it could be?. Haven't had a proper look at this so could be worth a look at it. Apparently Morgan's have sometimes been known to have minor variations between seemingly identical models; normally the differences are of course much bigger!
Doing my best at this New Year business.

While I was away I had plugged in some guesstimated dimensions and weights into the calculator and found that I was getting spring rates roughly half the rate of the supplied Ohlins springs as it would appear have others. Now I'm home and have found some time I've just been down in the garage with an assortment of tapes, rulers, angle finders, a spirit level and two identical digital scales for getting the corner weights.
As I understand it, the shock angle is the angle between the shock and the plane of the lower wishbone; this I measured at 36 degrees with the suspension static loaded. The instructions on the calculator could be read to imply that the shock angle is between the shock and horizontal; this I measured at 44 degrees. The two screenshots are for these two angles, everything else remaining the same.
Conclusions?
Could be that the reason the front ends feels so supple is because it is actually softly sprung, not over sprung.
More likely is that the particular geometry of the M3W places itself outside of the bounds of what the calculator will reasonably calculate! A combination of a relatively large difference between Dimensions A & B, a very low corner weight to unsprung weight ratio and I suspect other factors take things out of the calculator's comfort zone. In addition I would suspect that it is aimed at racing rather than street use so could be expected to give high spring rates.
Adjusting all my actual measurements by 10 percent gives almost 4000lb/in in one direction or if they all go the other way it suggests about 900lb/in if all dimensions go in the direction that reduces the final calculated figure (this also assumes that it is the 44 degree angle to horizontal that should be used and that it was really 48 degrees!) I don't claim to have measured anything to a particularly high level of precision but I would be surprised if any of my measurements were anywhere near 10 percent out.
Real conclusion; if you think that the front end is over sprung then get hold of softer springs, fit them and see what happens. You might also want to consider springs of different free lengths with the preload adjusted to suit. If you are racing/hill climbing competitively then a big box of springs a stopwatch and a notebook would seem like a good idea but best get the springs on sale or return - you might need to try a fair few before drawing any conclusions!
Time to go back down to the garage, I've still got the tracking to do.

A great place to start would be the original spring rate - which is calculable with just 4 numbers: Wire Diameter, Outer spring diameter, Free length, and the Number of active coils. Make sure you use the right units of measure, and give it a go with this on-line calculator I quickly found: http://www.acxesspring.com/spring-stiffness-calculator.html

Unless there is a strong reason to change the spring rate from the original (ie, suspension bottoms out, too much suspension travel for some reason (perhaps bad geometry you want to avoid) or you want to increase carry capacity or roll stiffness at that end of the car, there should be no reason to mess with the OEM spring rates if they generally work well.

The calculations to find spring rates from spring dimensions do work up to a point but do assume that you know exactly what material you are dealing with and what heat treatment it was subjected to. Real way of finding actual spring rate is to put it in a rig that will measure its rate or just believe the spec sheet from the spring manufacturer!

Certainly agree - the spring material does have an effect - but the calcs will get close, and it would be interesting to hear or see what the OEM springs come in at. Or someone could ask Morgan I suppose...

Based on your fine detail work, I would suggest that the shock angle is relative to the lower a-arm in the calculator.

I know in most streetrod setups with IFS, the lower a-arm is usually considered to be lever with the road in normal standing setup.

I believe that some have let their shocks extend out a bit and lowering the outer lower a-arm at the spindle. I think some did this in the old days to help the adverse bump steer issues the M3W had.