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SP: Spring Boxes (Rear) Thornton coil over spring rate.

Dave Hulstone

Well Known and Active Website User
VOC Member
Hello all. I am very interested to find out what the consensus of opinion is regarding the spring rate of the rear Thornton coil over spring on a Comet? Laney supplied mine with a 260lb when I told him I weighed around 80kg with my clobber on, rarely take a pillion or carry luggage and am running the standard series C setup with the friction dampers. I have had mine on now for about 4000klms with a small cable tie fitted to give me an indication of travel being used, I have had to put quite a bit of extra pre-load on to keep it from been buried in the bump stop. The result being my rear stand is no longer kicking up slightly when it’s parked, but level?
I don’t see this very often on a Comet. I am thinking off having my spring rate checked by a spring supplier and going up a rate and lessening the amount of pre-load but before I do I would like to hear your valued opinions.

Dave.
 

davidd

Well Known and Active Website User
VOC Member
Dave,

I don't think that AVO will be much help with the Works Performance springs, but you never know. I think it is important to know exactly what you have.

Rear Coil Over Springs.PNG

You are a pretty light rider (176 lbs)to be bottoming the heaviest spring. If you can measure the wire and count the coils, you should have an answer.

David
 

Dave Hulstone

Well Known and Active Website User
VOC Member
Hello David. I should have known you would be the guru with this. I’m in good hands. I have just measured my spring dimensions and it’s definitely a 260lb, which until now I thought to be a ‘soft Comet’ spring, although I know the great Greg Brillus runs a 260lb also on his Rapide so I should have been suspicious. I must be reading my marker wrongly. As you say, there’s no way me being a light weight with no passenger or luggage I could be bottoming out. I need to speak to my friend Martyn Goodwin who also rides a Comet with a coil over but weighs in at least 200lbs, before iso, to see what he’s running, although the last time I followed him the only time it was off the bump stop was as he crested a hill. (Not sure how he bent his top axle )
Thankyou David. You are a font of knowledge. I will get a 215lb from somewhere and start again. 8DB00876-60A9-47DA-9A0F-224EA0CA748D.jpeg
 

Dave Hulstone

Well Known and Active Website User
VOC Member
David, I’ve been thinking a lot about this. I definetly can’t go softer with my spring. There is so much travel at the minute I have to have my chain like a dirt bike chain or it restricts my travel. I’m going to back my preload right off and measure it in case it’s been creeping back.
 

davidd

Well Known and Active Website User
VOC Member
No, I did not think that you could go softer, You just needed to check where you were. If you are overwhelming the 260, you do need to go higher. I would not be afraid to use all the preload and I will see if there is anything higher.

"Dave" in the note is David Malloy.

David
 

davidd

Well Known and Active Website User
VOC Member
Thanks, David

The next step up in the Works Performance springs is 300 lbs. It is 7.2" free length, 0.341" wire diameter, 9.3 coils and the ID is 1.75". I was hoping there might be a 280 lbs, but there is not. I found a spring on line, but it is 6" free length, which is probably not of help.

David
 

Dave Hulstone

Well Known and Active Website User
VOC Member
Now I know I’m running the same as most twins out there I feel I can make it work. I have plenty of preload left to play with. I have my friction dampers completely wound off? David, on a completely different topic, which cams have you ended up running in your Grey Flash, Megacycle?
 

davidd

Well Known and Active Website User
VOC Member
I ran a large lift cam that Megacycle makes for me. On David Tompkins racer I had clearance problems after I built the engine. The big lift cam caused the rocker to kiss the valve guide under power.

David's first engine (not the one I built) started with the 105 cam which lowered his BHP to about 26. He went back to the MK2 and got the power back to about 30. I told him to build a Gold Star exhaust system with a megaphone and it went to 34 BHP. When he put my engine into the bike it went to 37 BHP. Recently, he tried a different primary pipe (1-3/4" instead of 1-5/8") on the bike and got 38 BHP. Using the megaphone pushed the red line to 6700 RPM, which was nice.

I have been very impressed with the Megacycle Mk2 as a result. It is difficult to get all of this power on the street, but it showed me that the Mk2 is still an excellent cam.

David
 

Comet Rider

Well Known and Active Website User
VOC Member
David D and Dave H
1.75" ID is quite a common size used by many manufacturers including Ohlins etc for twin shock setups.
There maybe someone in Aus who can help you with alternative springs.

On my AVO as I'm a big fat bloke I ended up with a 300lb/in spring for my Rapide, which is heavier than the normal spec'd by Timetraveller/Hadron who did much of the development work

A point to note is that the AVO uses a 2" ID spring

All the best
Neil
 

Comet Rider

Well Known and Active Website User
VOC Member
Hi Bill,
I'm no expert, but the way I understand it is that soft spring high damping will not do you any favours as it makes the shock too rigid.
You need the right spring rate for your weight + plus luggage and moderate damping so the spring takes the hit and the damper smooths it out, also restricting rebound
 

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