Minton Mod's on a 34mm damper rod ?

[dave_a]

I did a search on Minton mods. I have 34mm forks. Minton's mods are for 35mm fork dampers which are decidedly different.
Any thoughts or experience on performing MM on these from a '74 TX650? Anyone ever try them on a 34mm?

The big drill bit in the lower hole is 15/64".
Minton says drill a second 1/4" hole in damper to the 35mm

Next, Minton says to drill out the ONE small hole to #54 number drill or .055". Then drill out through the other side.
My damper has TWO .0625" holes already. Here's a picture, definitely apples & oranges on the rods.

Any advice on this from the brain trust? TIA

 

[dave_a]

Anyone have anything on improving 34mm forks?
Oil capacity and or weight advice?
TIA

 

[drgonzo]

I don't have any real tips but don't be afraid to experiment with different weight oils, aftermarket springs, preload spacers, a fork brace.
While not XS650 specific, I found it interesting that the old AMA Harley 883 spec road racing class racers used 30 weight or higher motor oil in there non cartridge forks.

 

[bosco659]

Anyone have anything on improving 34mm forks?
Oil capacity and or weight advice?
TIA

I’m interested in this and will be watching. I think a fork rebuild will be a winter project for me. My forks are stock with progressive springs. I was planning on 20W oil - no specific reason.

 

[dave_a]

I enlarged all 3 of the holes by 1/64".
Pretty brave, huh? I was telling 5twins that the rear suspension was so bad I can't remember what the front was doing, other than it hurt to hit bumps!
I'll report back when I have something.

I read that there are valves for 34mm forks (Mikes?), that might be the quick fix.

 

[jetmechmarty]

Anyone have anything on improving 34mm forks?
Oil capacity and or weight advice?
TIA

Fork Oil Level
Bottoming Control - sets final firmness in the last 1/3 of the fork travel. Tune in 10mm increments.
Measured from the top of the fork tube to the top of the oil with the Fork Springs out, Emulator in, and forks completely bottomed.
Do not exceed the maximum oil level as the fork will run out of compressible air space and will inhibit full travel.
100-180mm Sets Final Firmness in the last 1/3 of Fork Travel
Fork Spring Preload
3-30mm Fine Tunes Ride Height, Sag, Fork Travel, Firmness of Fork

The above came from Race Tech's emulator tuning guide. I use it myself. My fork oil level is 130 mm.

You may also mess with fork oil viscosity. Less viscosity speeds up high speed dampening and rebound, which is great if you ride on a lot of broken pavement. With stock forks there's a lot of give and take and no perfect solution. I have a lot of poor condition roads to ride on. 15W is not ideal. 10W is better. The weight is inconsistent between brands.

 

[Its been a long time]

I enlarged all 3 of the holes by 1/64".
Pretty brave, huh? I was telling 5twins that the rear suspension was so bad I can't remember what the front was doing, other than it hurt to hit bumps!
I'll report back when I have something.

I read that there are valves for 34mm forks (Mikes?), that might be the quick fix.

I find myself wondering about a simple and easily reversable alternative to making damper holes bigger is to use a thinner fork oil. I have recently stripped and cleaned my forks and fitted new seals. I have rebuilt them with 5w oil. Seems a sensible alternative to making holes in dampers bigger. And if I don't like it, it's easily changed, unlike holes made bigger.

(As an aside, I was shocked at just how much dirt there was in the forks when I stripped them. Very glad that I did so).

 

[5twins]

Yes, that's why I like to install gaiters. I think they do a much better job of keeping the dirt out, along with protecting the tubes. I also like the "old school" look, lol. And the fact that cheap truck shock boots fit the 650 forks perfectly is another bonus.

 

[jetmechmarty]

aside, I was shocked at just how much dirt

Most of that dirt is actually aluminum from wearing away at the fork lower. As the grit in the oil increases, the rate of wear also increases. I heard Race Tech’s Matt Wiley say that the fork oil should be replaced with every third engine oil change.

Yes, I would also do more experimenting with oil viscosity rather than drilling bigger holes.

 

[dave_a]

I have the 10w oil set at 6 inches or 150mm. A 1/64" increase didn't change much imho, but it's in the direction/spirit of Minton.
I said to 5twins: the rear was so bad I couldn't remember what the front was doing. So my leaky OE shocks get replaced by Hagons. We'll reassess.

 

[jetmechmarty]

I have the 10w oil set at 6 inches or 150mm. A 1/64" increase didn't change much imho, but it's in the direction/spirit of Minton.
I said to 5twins: the rear was so bad I couldn't remember what the front was doing. So my leaky OE shocks get replaced by Hagons. We'll reassess.

If you get too much dive when you brake hard, just raise the oil level another 10 mm. I have 15W Bel-Ray in my XS650SH (because that's all I could get when I needed it) and on a bumpy road it's pretty awful. I have no damper rod mod to the forks. I now have 10W on the shelf, so...

 

[bosco659]

If you get too much dive when you brake hard, just raise the oil level another 10 mm. I have 15W Bel-Ray in my XS650SH (because that's all I could get when I needed it) and on a bumpy road it's pretty awful. I have no damper rod mod to the forks. I now have 10W on the shelf, so...

Now your post has me thinking. I plan to rebuild my forks and replace the oil (obviously). I don’t know what weight of oil is in the forks now but it is quite mushy. I had planned to use 20W fork oil but after reading your comments I think it may be too heavy a grade. Maybe I should stick with the 10W that I too have on the shelf and add a bit extra. One problem for me is the big brakes I have on the bike. They provide tremendous braking power but also cause lots of front end diving.

 

[TX650A Van Islander]

A few years ago I rebuilt the forks on my TX650A and kept everything stock. I think I went with 10W. It's OK but I do avoid super bumpy roads of possible.
I just rebuilt the forks on my Fj1200. It is absolutely shocking how dirty the oil gets and the sludge buildup which occurs. I think almost no one replaces fork oil regularly. My recent experience tells me it's necessary, often.

 

[bosco659]

A few years ago I rebuilt the forks on my TX650A and kept everything stock. I think I went with 10W. It's OK but I do avoid super bumpy roads of possible.
I just rebuilt the forks on my Fj1200. It is absolutely shocking how dirty the oil gets and the sludge buildup which occurs. I think almost no one replaces fork oil regularly. My recent experience tells me it's necessary, often.

Are you saying the 10W doesn’t provide adequate damping or it’s too harsh? I agree that most people wouldn’t change fork oil regularly - me for one.

 

[Its been a long time]

FWIW, I have a history of preferring lighter rather than heavier fork oil. On my last bike, 5w gave me the best feel, for others it was 10w. I put 5w in the forks I recently rebuilt. Thing is, on these old forks there's even the luxury of a drain screw provided. You just don't get that on modern bikes. Now that I know my forks are nice and clean inside, a two yearly fork oil change should keep them that way.

 

[Guy]

Probably best to drill only the bottom hole or add another bottom hole and leave the top one alone and experiment with different weight oil . .. IMO. Remember one large ( 1/4'' ) hole will flow much more oil than two small ( 1/8'' ) holes.

 

[TX650A Van Islander]

Are you saying the 10W doesn’t provide adequate damping or it’s too harsh? I agree that most people wouldn’t change fork oil regularly - me for one.

I think the 10W is adequate for our bikes (same weight called for in the FJ1200) just in general these really old forks and their old technology doesn't do well on super bumpy roads. My bike rides fine on decent roads. We have one here on the island called "the loop" which may have been why my original Hagons blew out (see my thread). I don't ride the old bike on that road anymore.
I will be replacing that fork oil in the TX650A this winter.

 

[jetmechmarty]

One problem for me is the big brakes I have on the bike. They provide tremendous braking power but also cause lots of front end diving.

We are playing a game of compromises. Even the stock brake with 45 year old rubber brake lines will give fork dive. Raising the fork oil level will help a lot with that. Maybe start at 150 mm and increase until you're comfortable. Fill them all the way up and you get no dive at all until the seals blow out. (No don't try that!)

Are you saying the 10W doesn’t provide adequate damping or it’s too harsh?

Here's the deal. The holes in the damper rod are fixed. So, to alter the speed that the forks move, you alter the viscosity of the oil. Theoretically, when you hit a pothole, the forks will suck it up better if the forks can move faster. So, hitting the pothole you should get less of a jolt with the 10W than 20W fork oil.
HOWEVER, once those forks compress, they have to return to the extended position. You have 20 lbs of brakes and whatever else is below the springs. When all that mass returns to extended you are going to feel it. That's rebound. So, 20W oil will slow the rebound and you feel it less than with 10W.
Now what?
I fitted my Eleven Special with Race Tech emulators, straight rate springs and a fork brace. The 20 lbs of brake came stock. Setting all this up myself was very fiddly and time consuming. It was also a lot more expensive than I had expected. My cousin took his forks to Traxxion Dynamics in Woodstock, GA and they set them up. 10W in the left fork and dialed in for maximum compression dampening. 15W in the right fork and dialed in for maximum rebound dampening. It works. The emulators eliminate the fixed holes and replace with adjustable valves. He also installed 2 lb brake rotors. It was probably $1200 all said and done. It's still a heavy old motorcycle with skinny forks.
So, you can experiment and see how it goes. There's no rule that I know of that says each fork has to have the same oil.

I hope I've explained in a way that doesn't add too much confusion.

 

[atom4488]

I have found that Motul 10 W oil, 6" down the tube, stock springs, 7/8" preload spacer, my version of the Minton-mods (qty 4 .250" lower holes and qty 2 #55 upper holes) on 35mm forks works very well for me in terms of wheel control over bumpy / uneven surfaces. Brake dive is not objectionable. BTW, my front brake is light; aftermarket skinny drilled disc + Brembo caliper.

 

[dave_a]

The 34mm (1974) internals are very different. 4" preload spacer, One 15/64" hole thru both sides of the rod at the bottom, and two small holes (.0625") not drilled thru to the other side.
Anyone know what the small holes control?