Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Shimmy theory!

Having sorted my +8 without resorting to Suplex, and yes I agree Suplex would probably provide a more compliant ride at lower speeds on bumpy roads, I have the following thoughts.

I have spoken with many owners on the front suspension and recently a +8 owner who uses his car a lot and races it, he has gone through the majority of front end mods and his car after a rebuild has 6 degrees castor and still suffered shimmy issues, this has been alleviated with a steering damper. So the question is what could be causing the shimmy in a car that has everything that should mean no shimmy?

Now I have another friend local to me who has a 2002 +8 which has had the usual steering issues and had new springs and bearings under the top springs and good shocks, 4 degrees of castor and the shimmy issue, he fitted an SLK steering damper and it made an immediate improvement and I also drove the car and was acceptable to me in fact very close with one exception, it was resistant to turn in to corners. Now all is set up correctly before you ask, especially toe in is correct. So is the same as my +8 but is not as responsive to turn in, so what is different ? This got me thinking and after a bit of head scratching the only variable that I can come up with is the tyres! I run Bridgestone that have done 2500 miles and are 2years old. He has 2/3 worn continentals that are a more modern type of tread design.

So this got me thinking, what if more modern tyre patterns a designed specifically to work with modern suspension designs and some worked better on a Morgan than others. In other words if the tread resisted being thrown off of its direction of travel on rough roads but transmitted this through to the Morgan suspension could it induce a shimmy? So the test to be done is to put my tyres from Rosie onto my friends car and see if it then drives the same. I will report on our findings in the near future.

Has any one a subjective view or experience of different tyre types affecting the Morgan setup?

Interesting....I wonder if the Assymetric tread pattern tyres are the reason? Are yours or his Assymetric?

I believe his are assymetric, with large blocks on outside edge, these are cut at and angle to the outside edge of the tyre withe small channel in the middle of each block, I reckon this is the offending tread, giving improved grip but resisting turning. Mine have straight cut blocks.

It's an interesting theory Adrian. Modern tyre tread patterns and particularly asymmetric ones are designed for modern suspension systems, not for crude 1930s systems.

The missus took the "shopping trolly" on a longish trip recently and came back complaining about vibration from the wheels at 130 km/h. She even stopped at a tyre dealer and had the wheel balance checked. The tyres were nearly due for replacement but were still legal, I had thought to change them after next winter when the winter tyres come off.

When she got back I took the car to the dealer. In the end they decided it was the tyres and changed them for new ones. Problem resolved.

Interesting theory. I had a slight shimmy|vibration at 60mph but then changed the Dunlops for Toyo Proxes and the vibe disappeared. I have all the usual suspension mods although not Suplex. There is a terrible stretch of Tarmac passing by my sons school so it might come down to whether they repair it or I go the Suplex route.

I've got Toyos and no problem either (on standard no bearings front suspension)......are we getting somewhere here I wonder?

Perhaps tyres could make some difference but basically I think 'the dance' is a result of several components not adjusted correctly or are worn.
The other day I talked to an experienced tyre/wirewheel expert who told me that wire wheels often needs remounting of the tyres until the rim and tyre becomes a good round 'unit'. He also said that ride height, tyre pressure, wheel balance, toe in/out, steering and joints, bushings as well as a cracked or bent frame all can contribute to the shake. Any of them can be the cause and/or contribute. Also, a low 'tyre' profile actually reduces the stabilizing effect of the camber angle.

In my case (-99 4/4) the rims were badly maintained and a replacement solved the shake problem. Balance looked good but despite spinning and balancing on the car several times the shake was still there. Perhaps rims can become unstable and flex under load...

I actually meant castor angle - sorry

I have Toyo tyres on my 2004 +4.
They are the best I have had on either a 4/4 or a +4, good feel, secure and no shimmy - despite replacing the rubber boot on the steering rack and disturbing everything - solid.
I would have no hesitation fitting them again.
All the best
Tony