It would hardly be possible to design two more different suspension forks than those used on the F-frame Moultons and the TSR (or APB, or many others with the same design).
The F-frame fork is a pure telescopic design, where a long inner steerer slides within an outer steerer. Visually, it is very unobtrusive except for the rubber bellows which cover the splines of the fork. Offset is achieved in the usual way, by curved fork blades.
(Steering characteristics are determined by head angle and offset, which is how far the wheel hub is ahead of the steering axis. Increasing the head angle or increasing the offset both make the steering quicker, or more twitchy. Most bikes need some offset to avoid the steering being “supertanker-like”, where the bike wants to follow a wide curve around corners, and it is hard to dodge obstacles in the road.)
The TSR fork uses a leading link design, where more of the suspension is on show. The springing element is inside the head tube, as with the F-frame, but there are two short leading links connecting the bottom of the true fork to a separate “stirrup” which holds the wheel hub. The stirrup is connected to the spring via a sliding bush or “bobbin”, but the fork itself is rigid. Offset is achieved by the leading links themselves.
The TSR design eliminates the sliding splined bush used in the F-frame fork, a potential (although in practice, not serious) source of wear.
In terms of steering geometry, the TSR fork has the edge. A leading link design, if correctly dimensioned, compensates for the effect on steering geometry of hard braking (or a heavy load on a front rack). With a telescopic fork, the head angle steepens as the suspension is compressed, making the steering more twitchy under braking. With a leading link, the offset is not fixed. As the suspension compresses, the leading link pivots upwards and backwards and the offset reduces slightly. This counteracts the effect of the steepening head angle, and preserves the handling characteristics. For the same reason, it is important to set the preload properly on a TSR, with the leading links horizontal under a rider’s weight. Otherwise, the steering will become quicker under light braking. In the real world, most people will not notice the handling benefit of a leading link design; virtually all current mountain bikes and motorcycles use telescopic forks, despite their suboptimal braking characteristics.
Damping is handled very differently in the two designs. The F-frame relies mainly on the hysteresis of the main rubber spring for damping, with some additional effect from the metal spring rubbing against the inside of the inner steerer.
The TSR only uses a metal spring with very little hysteresis, so adds friction (or Coulomb) fibre discs inside the leading links. These are adjustable by small allen bolts. Most riders set them as loose as possible to reduce static friction, which is the main problem with friction damping.
(Friction damping was used on early car suspensions, too, but disappeared very quickly as it just doesn’t work very well. It is light and simple but static friction gives it a poor response to small road irregularities and it needs to be adjusted frequently as the friction elements wear out. Fluid damping, which does not suffer from static friction except in the seals, is universal now.)
It is also common practice, although not a factory recommendation, to regularly soak the friction discs with a friction-reducing lubricant like one of the “dry” PTFE sprays. Static friction is greater than sliding friction, so the TSR suspension lacks suppleness. I have tried adding more damping, e.g. when using the TSR as a fixed-gear bike where climbing in the saddle is not an option, but it does not significantly reduce the pogo effect on climbs and merely makes the suspension less effective overall.
In the F-frame design, the rubber boot is critical for excluding dirt from the splined connection. The TSR uses a wiper seal to protect the inside of the steerer, where the sliding bush and spring are located. Both work well enough to protect the internal parts but the leading links of the TSR are very exposed. Earlier versions of the leading link fork, such as on the ATB and APB, used a rubber bellows instead of a wiper seal but, as mountain bikers found, a wiper seal does a good enough job without acting as a potential moisture trap.
F-frame suspension is completely unadjustable, as designed. Heavier riders – bearing in mind that an F-frame was designed when people were smaller and less well fed – can add preload by inserting a short section of dowel (19mm diameter aluminium bar works well; wood can absorb moisture and expand) between the spring abutment and the main spring. About 1/2″-3/4″ is best. Add too much, and the rebound spring becomes compressed too, plus the bellows will be overstretched when the bike is parked. Lighter riders should not add preload, but can cut a small section out of the main spring and replace it with dowel of the same length. This is obviously irreversible unless you can source a spare spring. The damping effect of the rubber main spring cannot be adjusted but works well.
The TSR suspension is highly adjustable. It is best set for minimum damping, with the small bolts evenly just nipped up against the leading links. There is a preload adjuster, which should be wound up until the links are horizontal under the weight of the rider (and any luggage, if you are finicky). The video feature on a smartphone is useful here. There are also three different springs available for a TSR: soft, medium, and “race”. The “race” spring has progressive winding, with a close-wound section and a looser section. The idea is that the close-wound section becomes coil-bound under moderate compression, effectively leaving a shorter, stiffer spring for the remainder of the travel. I confess that I went from a medium to a “race” spring and didn’t notice any real difference.
The F-frame fork is relatively difficult to overhaul, and will defeat most bike shops. You need two very long screwdrivers, a lockring spanner and a lot of care (or experience). There are full instructions in my other blog entries. The TSR fork is rather easier; the main risk is of losing or mixing up the small leading link parts, such as the different-length bolts. It can also be tricky to change the bush, or “bobbin” that slides inside the steerer tube, since it uses a tight balljoint to attach to the preload adjuster. At some point, and certainly for a repaint, the shell bearings in the fork and stirrup will need replacement, and these are pressed-in. A small vice and suitably-sized sockets can be used to do this, as they are not large or particularly tight. I found one of mine was a loose fit anyway.
Overall, I don’t like the TSR fork much: it isn’t supple enough, a lot of moving parts are exposed to dirt, the fit of parts and the action of the friction discs are affected by the paint finish (which inevitably wears off and leads to unsightly rust), and spare parts, while available, are very expensive.
The F-frame fork always has a bit of play due to its prodigious length and the use of a splined connection, but all the moving parts are protected by the bellows and it generally just works with no adjustments. Spare parts, if and when you can get them through Moulton Preservation, are reasonably priced.
Moulton has other leading-link fork designs that use Flexitor rubber elements and/or fluid damping, and these are no doubt better. The price puts them out of the reach of mere mortals, though.
Moultons and other endangered species 