I'm enjoying this - excellent stuff! A great exchange of info!
If it is of any use, here is my take on the whole "black art" of double front trucks, it is very simple, and very effective (I have only personally tested this set up on standard Randal Luge Trucks, but the theory should apply to other similar designs):-
1. I mount both of my front trucks dead flat, and dead level with each other. I mount them as close together as possible without clashing wheels in the turns. I have all six wheels in contact with the ground from the top of the hill to the bottom.
2. I run both front trucks as loose as they will go. Neither is any tighter or looser than the other. I would like to run them even looser if possible, but there isn't enough length left on the kingpin. I am even considering longer front kingpins for next season...
3. I run the stock Randal bushings.
Important: The above only works if your luge is stiff, straight, and true. If you have any excessive flex in your luge, then the thing will lift one of the front trucks off the road under compression, and once that happens, everything turns to sh*t].
If your luge is suitable, and you follow the simple ABCs above - your luge will be balanced and predictable both in the turns and on the straights. You will never experience speed wobble, and you can enjoy the competitive racing without the distraction of constantly fighting your own vehicle.If you want to know why - Here is the theory:"The definition of a good mechanism is that the whole should be greater than the sum of its parts".Double front trucks actually yield
four-fold benefits to the rider, thus fulfilling the definition above:-
(1 + 1 = 4)
If you have two front trucks, you can run them half as loose as you would for a single front truck. This gives you:-
(1) Double the grip.
(2) Double the stability.
(3) Double the cornering ability.
(4) Greater speed both through the corners
AND on the straights.
Some Frequently asked questions:
Q - So why dont you get speed wobble on the straights if you are running your trucks so loose?A - By running an identically set pair of front trucks, they work as a team, and stabilise each other. It is like the difference between balancing on one leg, or having two feed on the ground. This simple set up defeats the harmonic frequency that generates a speed wobble. If you have only one truck in contact with the ground on the high speed straights, you will not benefit from any of this stability - and your luge will feel twitchy and on edge, thus distracting you from the important matter of getting past the guy in front. (Note: There is no speed disadvantage to running both trucks in contact with the road on a straight, this I will address as a separate point below).
If your luge is nice and loose you can turn easily, thus you don't have to sit up and hang off it so hard to get round the corners. This means you can ride flat, and keep a nice fast flat tuck.
If your luge is nice and loose you wont have to fight it, or be forced scrub off speed by sliding wide as you reach "maximum lock" through the corners.
If your luge is nice and loose you can out-turn the guy in front of you, and take the inside line to gain a position.
1 + 1 = 4
This only really works if both front trucks are set up identically : Dead level, Dead flat, and both tightened to the same amount. As soon as you start to tighten them to significantly different amounts, or mount them on different levels - you will break the equilibrium of the system and the trucks will fight each other through the corners, or lose contact with the road on the straights.
Q - So why not mount one truck higher than the other, so that it only kicks in when you turn - giving you extra grip when you need it, and less rolling resistance on the straights?A - Mounting one truck higher than the other upsets the equilibrium of the system, and instead of the
(1 + 1 = 4), you get (1 + 1 = 1).
Here is why:
In the corners : If you have mounted one of your front trucks higher than the other, it will only come into contact with the road in a hard turn. But crucially, that contact will only be on
one wheel on the
inside of the turn, where whatever marginal axtra grip that one extra wheel may give is not actually needed.
When cornering any vehicle, the vast majority of the gripping force is done by the wheels on the
outside of the turn.
Furthermore, that one extra wheel working on the inside is actually exerting a reaction force that acts
against the direction of the turn - and so actually pushes your luge further out wide, off the racing line.
On the Straights:There is no need to mount one of the two trucks higher than the other. Some people think that this will lift the second truck off the road on the straights, thus reducing the overall rolling resistance. This is not true, by riding on only one front truck on the straights, you
double the
bearing load on that one truck, thus
increasing its rolling resistance at the bearings. This means that there is negligible net gain in straight line speed when compared to having all four wheels in contact with the road, and both sets of bearings sharing the load.
When running both trucks set flat and level - whatever tiny increase in drag that the second set of wheels may cause at the road surface, is far outweighed by the
massive increase in straight line stability that having both trucks working together achieves. There is no advantage to riding fast, if the luge is uncontrollable at high speed.
Q - I heard that if you run two front trucks, you should alter the angle of the kingpin of the front truck by shimming it with washers under the baseplate at one end?A - The
theory behind this is that if you had a
car with six wheels at the front, the second pair of front wheels need to turn a sharper bend than the front two, as the front swings out wider in a turn due to the differing distances from each set of front wheels relative to the centre of rotation (the back axle). This can be seen in many six wheeled vehicles on the road today - usually utility lorries, or perhaps a vintage Tyrrel F1 car if you are particularly lucky.
Luge riders sometimes try to fiddle around with this by inserting extra washers under one end of the base plate to pitch the truck up ever so slightly, thus altering its steering angle, and so on....
Well, I'm afraid that this is all complete rubbish, as the steering mechanism of a car, is a fixed mechanical ratio, and is totally different to that of a Luge, and it cannot be compared or replicated in terms of how a luge steers. Road vehicles have a fixed mechanical steering mechanism, which makes these critical steering angles (toe in / toe out) necessary to avoid scrubbing tyres, and to allow for the finite variations of geometry that are necessary when turning corners
On a luge, there is no steering linkage locking the front four wheels together - thus the front truck can steer however much it needs to, totally independently of the second truck, and vice-versa. In fact, all three trucks can steer completely independently of each other, as they are free floating on a rubber bush. There are infinite variations in luge steering that are influenced by both the rider positioning their body-weight to achieve a turn, as well as the high frequency oscillations that the truck makes as it adapts to the differing road surfaces and conditions. It does not matter what tiny minute changes you make with little washers here and there, because the flex in the bush will soak it all up anyway.
The two front trucks are essentially self-adjusting, as the front truck will automatically steer slightly differently to the second truck, as it is located further away from the dynamic forces of the rider's body weight.
There is a lot of mis-information on the net regarding this business of inserting washers, and minute changes in steering angles. If an individual rider feels any conceivable benefit to the performance of his luge by doing so - then I'm afraid that the benefits are purely psychological (Basically, it is the placebo effect).
Footnote:If you are setting up a new luge, or trying to improve the handling of an existing ride - I would recommend going back to basics and setting both trucks up the same, and beginning from a neutral start point.
Another problem is that we don't really get enough hill-time here in the U.K. to test any of our set-ups properly, and I know it is tempting to go with some radical set up in the hope that it will compensate, or provide some hidden advantage on the hill. But I would advise you to keep it simple, and learn how your ride handles before you start fiddling around with it.
Don't be distracted by what other people are doing with their truck set ups - many of the differing combinations of front truck heights and steering angles that you see about at the races, are more often than not - riders trying to compensate for unwanted flex in the rest of the luge. If you find yourself having to do this too, then try and cure the flex problem first of all, because unless the luge is stiff enough - you will never get the best out of your trucks.
I have to say, that a prime example of a poor truck front truck combination is that featured on the early MK1 Len Luges (Sorry Len, Sorry JasonA!
) The hole drilling on the luge chassis does not always allow for both front trucks to be mounted close enough together to work as a pair, the trucks need stacks of spacers above them to fit, and the chassis has way too much flex to make the trucks work properly at all. I know JasonA was really battling with his set-up in Switzerland last summer! However, the good news is that the new generation Len Luges are some of the cleverest designs I have ever seen, and the truck positioning and chassis stiffness are now perfect! I'm looking forwards to seeing how they handle at Cadwell Park!
Well, this has gone on for way longer than I had intended - All of the above is simply my theory as I see it. But it is based on years of building and riding many many different luges, on a number of different truck combinations, at races all over the world. Even if it only helps one person, then it will have been worth all the typing!
Ride Hard, Stay Safe.
Tom W.
) The hole drilling on the luge chassis does not always allow for both front trucks to be mounted close enough together to work as a pair, the trucks need stacks of spacers above them to fit, and the chassis has way too much flex to make the trucks work properly at all. I know JasonA was really battling with his set-up in Switzerland last summer! However, the good news is that the new generation Len Luges are some of the cleverest designs I have ever seen, and the truck positioning and chassis stiffness are now perfect! I'm looking forwards to seeing how they handle at Cadwell Park!
) handles of yours, and join them to the top of the chassis rail somewhere near the front truck? (The link would pass under your legs). When I say 'link' I'm talking about a piece of bent tube lashed to the chassis rail at the middle, and plugged into the handle tubes at each end. (Could be something dead simple - like a bicycle handlebar, bent / modified to fit?). You could use U-bolts to hold it in the middle, and use some imagination to join it to the handle tubes at each end? (And I dont mean holding it on with stickers.... ;D)