Improving the 2005+ Montesa 4RT

I am loving this thread, not sure what to try first but am considering options and when I might have the time to try them. :d

Not to de-rail more but... FWIW: I have a BMW with there twin spark (dual-plug layout) and find the things it seems to do for me are...
1) runs smother (less pulsing)
2) better fuel economy than the single spark version
3) deceleration is less abrupt
4) and when one of the two plugs started running bad the bike ran like crap... But it still ran. (Turned out I had a failed pin coil)

Bene,
When are you going to tell us about the suspension mods????

OK here is the suspension part. Most of it, anyway. I will not tell all because I give away too much information instead of profiting from it. I am a statistic these days, being an engineer out of the labor force. I'm spending my accumulated wealth building a ranch. If Buffalo Dream Services doesn't make some money, I'll not be able to be charitable. If anyone wishes me to do the following, PM me.

Problem: The 4RT suspension settings are old world. They are too slow on the front and way too slow on the rear. You can make 4RTs fully modern without compromising compliance control, and the quality of the suspension will then more fully compliment the bike. You will have among the best suspension of ANY available bike up to 2014! Lively, responsive, and certainly less heavy feeling.

SUSPENSION

The 4RT is blessed with excellent quality Showa suspension. The forks on the 2005+ models are a bit dated in design, being early 2000s, but they work well. I'd bet on Bou's and Fugigas' you will find sophisticated dual close-cartridge forks. Perhaps on the 2014 models for the rest of us are just tweaked versions of the 2005 forks:

Left-Side (Brake Side):

A low-tech damper rod for fixed compression damping. This side has the one and only compression spring. It also has the only top-out spring. (The other side will extend down free when you remove the front wheel axle). When there is only one spring it is in the leg with high-speed compression damping to reduce the force imbalance during fork compression. (I'll add the spring rate when I get around to calculating it).

The original oil was the stuff from the factory. At 120 hours it was not as dirty as I expected, but it was grey (metallic particles), and about the same in the left and right forks.

The only adjuster-based adjustment is a minor one, a few turns of added spring preload from the blue adjuster beyond the ~17mm of spring preload due to fork cap compression. Ride height and steering characteristics with respect to preload are minor in the adjuster, and much more significant through fork position in the triple clamps. We have been positioning the forks low in the triple clamps, from stanchion rod tops level to the top surface of the top triple clamps to sticking up no more than 4mm. This helps reduce over steer and hyper active handling.

The fix to slow compression is within the range of available oils. KrAzy's bike now has Red Line Like Water in it, and that is working well in colder temps. We can now make use of the adjusters! I'd bump up oil in this leg to 100% Red Line Extra Light 2.5 for a year-round oil. Keep in mind there is a compression adjuster in the other leg, so you have some range of adjustment beyond oil.

Oil level should be close to 3.5", spring out, fork fully collapsed.

Right Side:

No spring (other than the secondary or air spring). Inside is an older-type so-called 'open' cartridge system. Open means the oil pumps through on each stroke and ejaculates out a little hole in the top on fork extension. Closed cartridge systems are a pain in the ass to bleed. I prefer the older style open cartridge units regarding servicing.

The cartridge rod is attached to the fork cap, with fork compression and extension providing the pumping action to create resistance or damping (BTW 'dampening' is to wet). There are two adjusters, the top knob that controls a portion of the rebound resistance beyond that which is set by cartridge valving, and a compression adjuster, a flat-head screw in the center of the head of the damper rod retainer bolt in the bottom of the fork. Having two adjusters is a very good thing because you have a wider selection of oils you can use and still tune the compression and extension responses inside the more optimal window.

The cartridge valving spec is old world, so if you use the recommended Showa SS5 (a medium-light oil) you'll be wondering who turned up the gravity?

We were able to bring the rebound side into modernity without re valving buy using the very low viscosity Redline Like Water oil. The fork now responds quickly and the adjuster is useable instead of having to be set at full minimum. At last adjustment we were half way into the adjustment range with the Like Water oil. At a second iteration I'd use 50% Redline Like Water and 50% Redline 2.5 Extra Light.

I'm not religious and dogmatic about oils and spark plugs. But this is a case where I'd recommend a specific oil because you can't go by what oil sellers say on the bottle. One brand's light suspension fluid is another's medium. You can try what you want, but to repeat these results, I suggest sticking with the Redline. It is a full synthetic so you're getting a very high quality oil. Then once you know what faster feels like, you can try other oils, but keep in mind most brands start at SS5 thickness and go up from there.

Oil level, cartridge pumped through cleanly, is reported by KrAzy to be spec'd at only 1-1/2" or 38mm! with cartridge rod fully collapsed. Given there is no spring there, the level can be higher without over pressuring. We set this side at 3-1/2" not knowing better, and that softened the overall spring resistance fully compressed a bit.

You can speed up the forks nicely, but proportionally the bigger problem is the slow shock. To really transform this bike you need to re valve the shock.

SHOCK

This Showa shock is very large compared to many brand's shocks! It uses a larger-diameter body. Nertz! I forgot to measure the piston diameter. It's substantially larger than Ohlins and especially Reiger pistons. Here is Little Miss Piggie:



I'll provide the spring rate when calculated. Note the spring is long with respect to the preload nuts. Not enough adjustment to easily remove the spring collar clip! It reassembles easy enough, but getting the spring off requires some extra hands and tools.

That bulge at the bottom end (they run he shock with the body and bulge down) is the cavity for the reservoir, a rubber bladder accumulator hidden from view when disassembling the shock. The body threads into the cast aluminum bulged end, and shuts off against a bore inside. The only thing connecting the body to the reservoir cavity is one small hole in the side of the body, way down in there.

The shock is a conventional Japanese affair. That is, held together with one wire clip and gas pressure, which is around 200 psi (1380 kPa) of nitrogen. Air is 78% nitrogen, but it is common to use pure nitrogen in shock reservoirs, whether DeCarbon (piston) style or by rubber bladder. Not having ~18% oxygen in air at higher pressure may be the reason.

If you were to lose gas pressure, the seal head would back off and the shock would begin a process of falling apart. Thankfully there is no weird pressurization port. Showa used a conventional Schrader valve to charging the shock with nitrogen.

The Showa shock is oriented rod-side up. The positive is the body and reservoir are not closer to the radiant and convective heat from the exhaust. The negative is water and corrosives will get past the gap between the shock rod and the cap over the seal head and be trapped instead of running out under gravity, as with the flipped orientation. Bikes with lots of hours and run in wet and corrosive environments can have lots of dirt and corrosion in the space under the cap, right where the retainer clip for the seal head lives. If the corrosion is severe enough, the shock may not be able to be disassembled. After de gassing, the seal head has to be pushed about 10mm into the shock body to get access to the retainer clip to remove it. Oftentimes a good cleaning and oil soak will free things up, but sometimes not. But as long as you don't stroke a shock de gassed, you will not get air in it and you can re charge it and just keep using it.

Inside is a conventional powder metal or PM piston with low-friction sealing band, and a conventional set of shim valve stacks. One set of shims for rebound damping, and another for compression damping. The lawyers unfortunately got to the valve stack nut and rod interface, as they swaged the end of the rod over the nut. It takes careful work to remove rod material such that the nut can be removed without destroying the red threads. Here is what the bits from the inside look like after I did the necessary work to remove the M12X1.5 valve stack nut:



In this photo, the rigid rebound stack washer is at top left (the nut clamps the stack together there), then follows the rebound stack to the right. The round thing with ports is the piston, then right of that is the compression stack. I altered these parts to re valve the shock.

I used the Redline Like Water in this rebuild, and we ended up about mid way in the rebound adjustment range, so we have more adjustment to dial in when it gets hot again. But the next iteration I'd use Extra Light 2.5 to thicken the oil slightly. That should put the adjuster closer to mid range in hot weather, leaving plenty of room to back off in cold weather.

A fellow could simply change the shock oil for Redline Like Water and speed the shock up some, but the real magic is in the revalving, and I'll not be disclosing that.

I will say this about this shock. It is quite large and the rising rate linkage is apparently well thought out. This shock revalved on KrAzy's bike has produced the most plush rear end I've ever felt on a trials bike, and it is very controlled and responsive. When I switch back to my Reiger GasGas it feels really good, but not as plush.

We have fairly little preload on this shock spring. The rear end squat significantly under a 185-pound rider, but if you use too much preload you get oversteer. The 4RT will get hyper unless the overall ride attitude or bike pitch is a little head high and butt low. The preloaded shock length we are using was 5.06" (128,5mm) before the rebuild and we lowered it to 5.08" (129mm) after the rebuild. The fork position after the rebuild ended up with stanchions sticking up about 8mm.

SUPPLEMENT

As part of the overall handling related to bike pitch and suspension preload, it is VERY important on the 4RT to rotate the bars more forward than one might think should be. The cross bar on the 2005 will end up forward of and with clearance given a 6mm hex key stuck into the front handlebar clamp screw. While inclining forward is going toward oversteer, the bike just won't handle as well if the bars are back. KrAzy, we need photo from the side showing bar rotation, or getting from you what the distance between a 6mm hex and the cross bar is.

Rider above ~190 pounds (86kg) can up the shock preload, but don't overdo it!

We still suspect that master cylinder piston grew in size, uses same levers as my bike, dad took them to this one. Havent investigated, I assume the clutch cover is same part#, and specs. I dont know that.

Bene, I don't even own a 4RT, but I am fascinated with the level of care and documentation that you use in these threads. Good look, and keep sharing, there are a WHOLE lot of guys in our club that love these bikes and will be eager to apply the results i bet.

What about that Mitani stuff? Are those plates? If kits include springs take care as many after-market kits make the 'heavy duty' error on too-stiff springs. Wouldn't hurt to have other choices to play with though, especially when using 3 or 4 springs.

What makes a spring stiffer? You can tell by eyeballing them if you know what to look for. Certainly by measuring them with calipers.

First and foremost (to the 4th power in the equation for spring rate) is wire diameter. Second is the mean diameter (to the third power). Mean diameter is simply the outside diameter minus the wire diameter. Then there are the number of active coils. Active coils are simply those that are not coil bound during operation. Sometimes forks springs, for example, have a few closed-up coils at the ends.

No other variables factor in other than material shear modulus. Given springs are almost always made of spring steel, shear modulus just a another constant in the equation. Spring length doesn't factor in other than you often have more coils in a longer spring. So stiffer springs tend to have fatter wire, be smaller in diameter, and have fewer coils.

Well thanks, Beef! I do appreciate you saying that.
When I go looking for info on whatever, I generally find the web to be frustrating scattered and un detailed. My aim is to gather the bits together and through long experience have them make good sense.
Attention to detail is a madness that does come with benefits

I've had several people approach me at events recently and say they read and appreciate my work. Some have said they've been fans for decades, but they never pipe up... just lurk. That made me realize perhaps only a minority percent actually post or respond, but a lot of folks benefit from list serves and forums.

Do you have a formula for converting spring dimensions (fork in this case) to a rate that can be compared to available springs?
On a different bike I want to find softer fork springs but I don't know the rate of the current springs (they are straight rate) and don't have access to other springs for comparison.

The spring constant k is found by inverting Hooke's Law: K = F/(length free-Length deformed)

It's easy to measure shock springs cause there short and wide, fork springs can be tricky and really need a jig, (You can use a 1 1/2" PVC sleeve to support the spring (maybe 85-% of it anyway - depending on the weight you use) from sliding out from under the weight as long as the weight doesnt bottom against it) although I have done it free hand and only smashed my fingers half a dozen times with the 40 lb hand weight I was using. Probably wont get a NIST Calibration award, but usually good enough to differentiate the selection of available springs.

I wish I lived closer I would consider bring the hole bike over and have you help me rework it.

Would have cash in hand... if only.

But sense I can't bring the hole bike without having an extended trip out. I figure I will have to do the front but a rear shock re-valve by you is a great second option. PM sent...

The Mitani clutch is an assembly, plates and steels. 3 different fiber materials, and they have to be installed in a specific order. I'm not sure, but I believe the steels are dimpled for oil retention. I don't think the Mitani kit comes with springs. I did get the chance to ride an '07 Repsol with the Mitani clutch a few years ago, and WOW what a difference over stock. VERY progressive, not grabby at all. The Sureflex I installed in Miss Bitter's 315R comes very close to mimicking the Mitani, and it is just a fiber kit.

I did inherit the OE clutch (fibers and steels) out of that same Repsol recently. The original owner replaced it with the Mitani clutch when the bike only had about 10 hours on it. I'm thinking that the Sureflex fibers would go nice on those barely-used steels, since I'm not so sure that Jitsie sells their dimpled 4RT steels anymore.

Jitsie sell 4rt Steels still - but no info on them other than price. Cant tell from the photo if there dimpled or not. I'll ask them if you think you want some. Looks like there about $50.

Shweet!!!

Message sent to Jitsie. Probably hear back tomorrow.

On a different note - (But still 4rt related)

I've seen several FB posts regarding US riders planning on the new 4rt's coming to the USA. Anyone hear about this or are they referring to gray market bikes. I haven't seen any announcements regarding a change of heart from Honda. (or there lawyers)

I keep hearing rumors of Montesa coming back to the U.S. From fairly reliable sources too. We'll see.....

I would hope they started all the cert paperwork if thats the case. I hear that alone can take a year. Given the crack efficiency of our state and fed government - I'd say that's probably realistic. (hopefully is NOT a website based cert process...)

This rumor continues to float around. 4rt's to Honda dealers as in the UK. It appears that some US Honda dealers are considering wholesaling to trials only dealers as opposed to selling retail if they actually are imported. Still...only rumors and no time frame yet.

Probably won't happen but given the choice, I would love to see them sold again as Hondas (in Honda shops) even if not all dealers bring them in, just for the access, convenience and better brand recognition that would return.

Back to the clutch master cylinder topic, I'm probably going to replace my clutch M/C very soon (05 4RT) so if one was going to be replacing it anyway, would it be better to get the 315R master cylinder or just get the stock one for the 05 4RT? And mineral oil or DOT 4 fluid? Pro's or con's? Just curious...

I always thought HRC/SHOWA setup the suspension on Bou's bike, but upon careful inspection I found this decal.