Std forks cartridge damper conversion

[DougB]

The only thing I really can't get on with on the KR is the front suspension. The roads around me are pretty rough and it's very harsh, then riding on a track day and I can't seem to get the sort of feel I want so decided it's time to change things. Problem is I want to keep everything as standard as poss so I reckon it should be possible to drop h***a NSR250SE (MC28) fork uppers into the KR1S legs. They're both 41mm, similar weight bikes. MC28 SE & SP are Showa forks with cartridge dampers with adjustable rebound, and generally much better quality than the standard KR forks, I've ridden plenty of h***a with them and they give a great ride and feel.

Anyway I've got the bits I need. Checking the original forks side by side the NSR forks are shorter, the initial movement is softer but the progression is harder so at the bottom of the travel they are stiffer than the KR. Travel length of both forks is only 8mm different so no problem with that.

So I've stripped down both forks. So far so good. The biggest differences I have to get around are that the NSR fork tubes are shorter than the KR. With the new setup assembled the visible fork length above the fork leg top is 13mm shorter. The second issue is the bottom damper bolt, on the KR it's a 14mm thread and on the h***a 8mm. How 2 manufacturers can come up with such a massive difference in the req'd size of that bolt I've no idea!



As I see it the way around both of these problems is to make a 13mm thick adapter plate to sit between the bottom of the NSR damper and the KR fork lower. The plate is easy, the bolt is a bit harder. I think I'm going to have to get a bolt made that looks identical to the KR std bolt, but steps down to 8mm to screw into the damper. I'd like to have been able to remachine the KR bolt but with the adapter plate in it'll need to be approx 13mm longer. That aside though I'm getting the plates made and as soon as I can sort the bolts they'll get built up for testing.

[maccas]

Regarding the extra length, the KR1S as standard has the forks pushed 11mm (i think) through the yokes anyway so at 13mm shorter all you need to do is mount them flush with the top yoke and you are pretty much spot on!

Then as you say all you'd need to do is manufacture a special bolt. You could turn the KR bolt down in a lathe and cut the 8mm thread on it surely? Then you are about there?

Is the footprint of the damper rod the same size as the the one in the KR forks? Could it be drilled out and tapped to accept the 14mm bolt or is it much smaller?

I like what you are doing here it sounds good to me!

Dan

[DougB]

Hi Dan, thanks for the suggestions. I did have a think about the fork length and putting the forks flush with the yolks, my worry is that the NSR forks are softer over the first 40mm or so of travel, so static sag (with me on top) might have the front end sitting lower. If I make the forks the same length and start with the forks pushed through as normal I can drop them through if I need to raise the front a bit without starting to use too much preload.

The bush at the bottom of the NSR forks is 1mm smaller od than the bush at the bottom of the KR damper so sadly I can't machine it at all to fit the leg, and the cartridge damper tube cant be re-drilled and threaded any larger. I'll have to make an adapter of sorts. It's surprising the difference when I got it all apart! I've found an "off the shelf" 14mm bolt at 55mm long but it's a Hex and the the bolt head flange might cause a problem. The KR bolt head is 21mm diameter and 8mm thick (anything thicker would hit the wheel spindle) so any new bolt would have to be the same od to seal properly. They're £5 a pair on Ebay so I might take a punt and see, if they won't work I'll just be back to having some made from scratch.

[JanBros]

my worry is that the NSR forks are softer over the first 40mm or so of trave

I'd say you need stiffer springs then, and that would solve your problem about the length.

[DougB]

The NSR forks are softer initially but stiffer than the KR towards the last 30mm of travel, they have better progression which I think is why they have so much better feel (plus the cartridge damper units). If I put stiffer springs in I'll lose some of that benefit so I'm going to try to get them rebuilt at the same length as the originals then go from there.

[James P]

With the new setup assembled the visible fork length above the fork leg top is 13mm shorter.

Would a pair of extensions (a bit like these: http://cyclehaven.co.uk/cyclehavenonlin ... AIRS-.html) solve the problem of the forks being shorter Doug? If you can't find ones with the correct dimensions, they should be fairly easily made by anyone with a lathe.

Regards,
James

[DougB]

It'd be a nice idea James

The cartridge dampers bolt up into the top cap of the fork so the total length of the upper fork leg stanction can't be altered (not sure of the tolerance really but don't think I'd want to find out!).

I picked up the 41mm alloy bar today though and sorted some time on my friends lathe so as soon as my new bottom bolts arrive (mail ordered some shouldered M14x2 bolts, fingers crossed they should be right enough) and I can get over to his workshop for a couple of hours I can make the bits up and see if it works..

[Mohawk]

See this new thread I just posted ! viewtopic.php?f=11&t=12908

[DougB]

Nice work.. Let us know how it rides, did you have to make adapter plates between the damper rod and the leg as well?

I spent a few hours machining today, one more bolt to machine down to M8 and I can look at the rebuild. Mocked up the "new" fork setup is now 2mm longer than a standard KR1S fork so I have a bit of adjustment to play with and get the ride height right.

[Mohawk]

I thought about an adaptor, but the fork bottom is flat @ the VTR cartridge foot fits inside the KR1-S one, so I think it will be fine.

[DougB]

DONE!!!!

First the adapter I had to make and the new bolt machined down:


The NSR cartridge damper and upper leg:


The NSR upper assy with the adapter and the KR lower leg:


The finished job


The rebuilt forks are the same length as the standard now. I had the old forks set flush to the yolks (it was like that when I got it and I just got used to it) to I've set these the same, the ride height with me on is exactly the same so looks like as soon as I get chance I can take it out and see if I've improved it at all!

[maccas]

Nice work

Dan

[dmac]

Lovely job =D>

That's really sweet,

wish I had seen this before I spent the cash getting my standard forks replated.

Oh well, maybe next year

[DougB]

Finally managed to get a ride out, the gale force winds dried the roads out nicely but didn't make it easy holding a straight line. Anyway they front end felt great, way less harsh on the crappy roads near home, ride height seemed good, had the tie wraps on the legs to check dive and didn't bottom out doing a couple of emergency stops. All round very happy with the result. When the weather's better and I'm not fighting 60mph crosswinds I'll hopefully get chance for a bit more set up, preload and rebound currently backed right off and I think it'll be getting a few turns in but the effort seemed worthwhile

[Mohawk]

Tried mine out the other night before the MoT this weekend, blimey what a difference, perfect now, love it

Great minds think alike & all that