Forks, damping, stiffness etc.

[acebars]

I was recently in a discussion with some moto/mechanic guys and I mentioned that I've bought 37mm forks (EX500/GPZ500) and a 37mm steering stem (Seca XJ650) to do a front end conversion on my TX650 with the aim of stiffening it up and improving damping etc. but keeping the looks old (so no modern big forks the EX500 can be retrofied somewhat with mounts welded for the fender brackets etc.)

They told me that the width of a fork does not improve the suspension action of the fork, and that I'd be better off taking the bike to a suspension specialist who will put the appropriate spring and emulator in as well as adjusting the damping etc. and for the effort of swapping to 37mm they'd go to modern forks with more modern systems inside (I cannot do this for aesthetic reasons).

So I wanted to ask you all why you go about doing front end conversions and is it worth me doing so? I got the above parts for peanuts so can sell them again, but it's got me thinking whether it is all worth the effort.

Perhaps drilling the 34mm trees to fit 35mm and just upgrading to that width will be more than adequate? This has been suggested to me.

P.S. I will be modifying for a Triumph T120 early TLS front drum brake, they weigh roughly the same as a GT550/GT750 4ls so I will need a decent front end to deal with the unsprung weight.

Your thoughts as always appreciated!

 

[yamimoto]

I was recently in a discussion with some moto/mechanic guys and I mentioned that I've bought 37mm forks (EX500/GPZ500) and a 37mm steering stem (Seca XJ650) to do a front end conversion on my TX650 with the aim of stiffening it up and improving damping etc. but keeping the looks old (so no modern big forks the EX500 can be retrofied somewhat with mounts welded for the fender brackets etc.)

They told me that the width of a fork does not improve the suspension action of the fork, and that I'd be better off taking the bike to a suspension specialist who will put the appropriate spring and emulator in as well as adjusting the damping etc. and for the effort of swapping to 37mm they'd go to modern forks with more modern systems inside (I cannot do this for aesthetic reasons).

So I wanted to ask you all why you go about doing front end conversions and is it worth me doing so? I got the above parts for peanuts so can sell them again, but it's got me thinking whether it is all worth the effort.

Perhaps drilling the 34mm trees to fit 35mm and just upgrading to that width will be more than adequate? This has been suggested to me.

P.S. I will be modifying for a Triumph T120 early TLS front drum brake, they weigh roughly the same as a GT550/GT750 4ls so I will need a decent front end to deal with the unsprung weight.

Your thoughts as always appreciated!

From the ranks of the "I'm no expert" and "my two cents".

I have read virtually everything I can find on here and other places with regard to front end swaps for the XS and the idea of keeping it looking originalish. I am just getting ready to swap a 35mm front end, (year unknown but assummed to later type because it has the hex nut style cap with adjustable preload etc.) onto my '75 complete with Racetech emulators etc. Also will be installing All Balls, Tkat fork brace(an excellent product and upgrade). Will replace the springs if necessary later but may modify them in the meantime and experiment with different weight oils etc. I'm doing this because of what I have read from others' experiences. It looks to be a fairly easy job, have the fork tubes off the trees and apart ready to go. I don't know about drilling/machining out the trees as you have mentioned?
One of the other things that has convinced to do this is because I have the same forks on my '83 and I installed a Tkat FB, shortened the springs used PVC pipe spacers etc and tried a few different fork oils, no emulators yet and the difference is night and day, akin to installing good swingarm bushings and decent shocks on the rear. If the emulators have a similiar and equal additional affect I will be very pleased.
Yamimoto

 

[XSLeo]

On our forks they are a damper type fork.
In a damper fork the springs just push the front wheel down. That's all the springs do.
Inside are damper rods. These are tubes that have holes in them. As the wheel moves up and down oil flows through these holes. The size of the holes and the thickness of the oil controls how fast the forks compress and rebound.
These are very basic forks.
swapping on new forks will give better performance because of better technology.
I have found that modifying the dampers and adjusting the volume and weight of oil as well as replacing the sagged out stock springs with better straight rate springs has vastly improved my fork action.
After you get your front end assembled with what you have experiment with fork oils.
With out altering the damper tubes try several weights of oil. Thinner oil will let the forks move quicker than thicker oil. It will flow through the holes in the damper easier. I also recommend setting the oil level to six inches down from the top of the tubes with the springs removed, forks completely compressed.
This gives a bit of an air preload. Works a bit like an air shock by helping prevent brake dive.
Leo

 

[grizld1]

Ace, I've used Race-Tech Gold Valve cartridge emulators for the last 10 years, and they're on all three of my bikes, so obviously I think they're worth it. If you want to have a suspension shop do the installation that's your call, but if you have a drill press and a rotary tool you can do the job.

Emulators do well what progressive rate springs do badly: they enable the forks to adapt their response to different surface irregularities. Emulators achieve this by varying compression damping in response to the surface; a big bump with a squarish profile will trigger more damping relief than a smaller bump with a rounded profile. Baseline compression damping is controlled by adjusting preload on the emulator valve spring. Rebound damping is controlled by fork oil viscosity.

Bottom line: Emulator valves will transform your front end--if you install a fork brace, proper fork oil, and straight rate springs selected for your weight and the bike's weight, if you take the trouble to read up and set laden sag correctly, and if you install a pair of good shocks in the rear--and no, the OE shocks are not good and weren't good when they were new.

A few caveats. Real fork oil isn't expensive and you're not going to be changing it every 1000 miles or even every year, so don't listen to all the guff about pouring motor oil, ATF, etc. in your forks. It's pointless. Next, the rate of the emulator spring is critical. If you buy from Race-Tech, you'll know the spring is right. If you buy a cheap Chinese knockoff, you'll know that it looks right. And finally, if you buy from Race-Tech you'll get full specs and instructions and world-class tech support.

 

[grizld1]

Dual post--deleted

 

[XSLeo]

I agree that the emulators are a good thing.
The things I mentioned are just a starting point. Try these things and you might get forks that work well enough.
The next step will be replacing the sagged out stock springs. As well as the Minton Mods. I have done the enlarged holes of the Minton Mods. Very pleased so far.
The Emulators is the next step.
Start with what cost the least, then start buying new parts.
Leo

 

[acebars]

Bottom line: Emulator valves will transform your front end--if you install a fork brace, proper fork oil, and straight rate springs selected for your weight and the bike's weight, if you take the trouble to read up and set laden sag correctly, and if you install a pair of good shocks in the rear--and no, the OE shocks are not good and weren't good when they were new.

Thanks everyone, yea I've read Race tech being mentioned several times, I will probably take your advice and end up going that route (depending on cost effectiveness of importing from the US)

On our forks they are a damper type fork.
In a damper fork the springs just push the front wheel down. That's all the springs do.
Inside are damper rods. These are tubes that have holes in them. As the wheel moves up and down oil flows through these holes. The size of the holes and the thickness of the oil controls how fast the forks compress and rebound.
These are very basic forks.

Cheers for the lesson, cleared it up slightly

My issue now would be selecting the right fork 37mm would be the max size I'd want, and I've got spare 98 EX500 37mm forks. They are a budget fork but I presume light as I've got a big heavy Triumph drum brake to match them to.

Other 37mm options include GL1000 forks (but they are the heaviest) and GS500.

For 35mm options I'd go for CB750 forks as they have the fender stay fittings etc. and would require very little modification (I'll have to weld these on any forks I go for).

 

[jd750ace]

The primary advantage of a larger tube diameter is more strength with regard to flex. A larger diameter fork setup, assuming the same distance from the axle to the lower clamp and identical telescopic engagement will flex less than a smaller diameter tube. I have Race Tech springs and emulators on both of my other bikes. It's the top of mods for your forks. Progressive springs are made to carry varying loads acceptably. As mentioned, straight rate springs of the correct rate for you and sag setup correctly paired with well tuned emulators/fork oil height and viscocity will yield the best ride you will ever achieve on telescopic forks. Pairing with well sorted rear suspension is mandatory. One end of the bike being correct accents the incorrectness of the other, and can result in an even less predictable ride than both ends being shot to hell.

 

[acebars]

Pairing with well sorted rear suspension is mandatory. One end of the bike being correct accents the incorrectness of the other, and can result in an even less predictable ride than both ends being shot to hell.

Im putting in a thicker TX750 swing arm as well as doing bushes etc. just putting on standard Wassell or other adjustable shock absorbers.

The primary advantage of a larger tube diameter is more strength with regard to flex. A larger diameter fork setup, assuming the same distance from the axle to the lower clamp and identical telescopic engagement will flex less than a smaller diameter tube.

Right so 37mm it is then!

 

[acebars]

Sorry to bring this up again.

Is there a noticeable difference between the stiffness of 35mm and 37mm that makes it worth the effort?

How do stiffer forks translate on the road? Does it make the bike more stable at higher speed?

 

[grizld1]

Depends on what you're doing at high speed. Problems with fork flex will show up most when the bike is heeled over deep, particularly if the curve/corner you're taking has some bumps in the pavement. Bear in mind that the more flex you remove from one part of the suspension, the more flex will show up in other areas that you haven't touched yet. That having been said, the OE 35 mm. forks can do a fine job when well tuned and braced with a tweak bar--T-Kat, Daytona, etc. Tubular braces will also get the job done.

 

[5twins]

Yes, the later 35mm forks, when set up properly, are very good and probably all you need. What finally fixed mine was the fork brace. But that was on top of the new springs, the Minton Mods, and one ounce more of fork oil.

 

[acebars]

Depends on what you're doing at high speed. Problems with fork flex will show up most when the bike is heeled over deep, particularly if the curve/corner you're taking has some bumps in the pavement. Bear in mind that the more flex you remove from one part of the suspension, the more flex will show up in other areas that you haven't touched yet. That having been said, the OE 35 mm. forks can do a fine job when well tuned and braced with a tweak bar--T-Kat, Daytona, etc. Tubular braces will also get the job done.

Just wouldn't mind a bike that can handle faster speeds. May want to take the bike on some track days for example. Just want to make the bike more solid all around.

Yes, the later 35mm forks, when set up properly, are very good and probably all you need. What finally fixed mine was the fork brace. But that was on top of the new springs, the Minton Mods, and one ounce more of fork oil.

Well I was thinking of making a fork brace out of a thick sheet of steel maybe 4-5mm thick that I'd make go under the front fender/mudguard between the forks so it wouldn't be so visible. Not sure if this will be enough for bracing.

I must add that I'd go with Cb750/4 35mm forks if I went with 35mm because they have the fender mounts at the bottom I'm after.

I suppose I should look for something with 36mm that has the fender mounts at the bottom, perhaps that would be better than 37mm which is a little bit of an odd size.