Funny how close the total weight loss is to 1 lb.
FF: Forks Girdraulic machining specs for Lightning/Grey Flash
- Thread starter craig
- Start date
In that case the weight saving is only 1.7% of the front unsprung weight!
Maybe it wasn't so much about reducing total weight, but rather total mass moment of inertia. This changes the steering dynamic response (wobble, tank slapper, etc. ..) Here the effect of mass reduction is increased by the square of the distance from the axis of rotation so the fork blades are a logical choice.
Saving weight costs money........replacing steel parts in alloy or titanium.........The wheels are by far the heaviest items of unsprung weight, very hard to loose on these items.......I'm not so sure about the weight reduction from steel to alloy rims, the last ones I checked the difference was very minimal, the main advantage of alloy is they are stronger and run a bit more true........My Norvin single racer weighs in at 130 Kg's, the Godet Flash racer I had here weighed in at 124.5 Kg's..........With some extra alloy parts we could get that figure down to 120 Kg's without too much difficulty. On the front end, do away with the spring cases all together and run a simple coilover shocker in there, plus Titanium spindles and replace most all studs and bolts with same........Use alloy cylinder head brackets, if possible go to an alloy head stock and steering column........It's all doable it just depends on how much money and time you want to throw at it.
I have just calculated the reduction in the moment of inertia (of the steered assembly) resulting from milling 225 grams out of the fork blade and it varies between 0.5% at full extension and 0.7% at full bump, which, I suggest, is so insignificant that it would not have a detectable effect on steering response nor the propensity of the Girdraulic fork to escalate steering wobbles into tank slappers.
Converting the front end to full Lightning specs will decrease the risk of tank slappers occurring. It will be recalled from my article that the standard Series C Girdraulic forks suffer a 32% reduction in the moment of inertia of the steered assembly when the fork extends from full compression (M of I = 21.3lb/ft^2) to full extension (M of I = 14.5lb/ft^2) and it is this reduction in M of I which can be instrumental in causing a tank-slapper. Not only are the actual values for Moment of Inertia of Lightning spec fork (i.e. no headlamp nor brackets, no speedo, shortened mudguard, only one stay, ribbed drums, alloy backplates, alloy rim etc) reduced (i.e. full compression M of I = 15.3lb/ft^2, full extension M of I = 11.3lb/ft^2), which will give lighter steering, but the relative reduction in value on extension from full bump to full extension is decreased by 6% to 26% . Most of this reduction comes from the reduced weight of the (alloy) rim.
I had not calculated the Moment of Inertia figures for the Lightning set-up when I wrote my article and this reduction therefore comes as a surprise to me. If I had seen these figures, I would have included fitting an alloy front rim as one of the ways to tame the Girdraulic fork.
Still, it didn't help John Surtees!
Converting the front end to full Lightning specs will decrease the risk of tank slappers occurring. It will be recalled from my article that the standard Series C Girdraulic forks suffer a 32% reduction in the moment of inertia of the steered assembly when the fork extends from full compression (M of I = 21.3lb/ft^2) to full extension (M of I = 14.5lb/ft^2) and it is this reduction in M of I which can be instrumental in causing a tank-slapper. Not only are the actual values for Moment of Inertia of Lightning spec fork (i.e. no headlamp nor brackets, no speedo, shortened mudguard, only one stay, ribbed drums, alloy backplates, alloy rim etc) reduced (i.e. full compression M of I = 15.3lb/ft^2, full extension M of I = 11.3lb/ft^2), which will give lighter steering, but the relative reduction in value on extension from full bump to full extension is decreased by 6% to 26% . Most of this reduction comes from the reduced weight of the (alloy) rim.
I had not calculated the Moment of Inertia figures for the Lightning set-up when I wrote my article and this reduction therefore comes as a surprise to me. If I had seen these figures, I would have included fitting an alloy front rim as one of the ways to tame the Girdraulic fork.
Still, it didn't help John Surtees!
I'd seen that video during the build of my twin racer back in 2013/2014 and it was that incident that made me want to carry out some changes to the bike so that might not happen to me. I had experience with the tank slapping of my road bike, on a straight piece of road, no braking, just hit some ruts or similar accelerating from about 70 plus MPH....... I did not come off, but held the bars firmly until it settled again, it was seen by others who later comented what they had seen........My bike had the duel front springs, which to me is way over sprung.........This is a common find on these bikes.........There are loads of reasons for their bad behaviour, but you have to do something to help the problem. John Surtees learnt a lot when he observed the Horner brothers "Goodwood bike" back in 2014, with all the front end changes they had made to that bike........I remember checking it over myself at the Vincent gathering at Broadford in Victoria back in 2018 and their bike was still very stiff in the front end whilst still using the duel front spring cases to keep the bike looking stock........by comparison.....The front end on my racer was much softer and smoother. I intend on building another set up for my next road bike (or possible racer) to copy it as close as possible, it is the best handling Vincent I have ever ridden.