NX250 AX1 spoke wheel conversion

[scrambler]

The AX1 has a cast rear wheel that lacks a cush drive. There are only little rubber bushes which can’t be replaced and thus were never available as spare parts. The rear wheel of my AX1 was ruined by the previous owner when trying to replace the bushes and a spare wheel was nowhere to find http://forum.nx250.de/viewtopic.php?f=11&t=915

So I thought of using a NX650 rear wheel, which has a 220mm brake disc (same as AX1). However it turned out that the AX1 sprocket, lacking a cush drive, runs very closely (ca 10mm) to the swing arm.



The NX650 cush drive needs more space, that’s why the NX650 swing arm is wider than the AX1. One could grind the cush drive and abandon the circlip and the seal – but I don’t think that this is a permanent solution.



After some searching I found a rear wheel of a KTM 620/640 with cush drive - there are also some without the later. Those wheels can be recognized by the silver hub – older versions have red hubs. The ktm also has a 220mm brake disc and the distance brake disc-sprocket (158mm) fits too.



The only downsize – KTM has a 20mm axle, AX1 17mm.



So you need three new spacer - I had them made in a metal shop, because I can’t do something like this myself.



Fits



The only downsize – there isn’t much space in the AX1 swing arm for a much larger tire size. A 4.00-18 won’t fit, a 3.50-18 is tight as well as a 110/80-18. A 17’’ or even 16’’ rim would fit better.

[jonathan]

Scrambler, you are a star for posting this up! Thankyou. I'm about to do a spoked conversion on my AX1 (as you know) but I'm looking to go with 17 Rear, 19 Front, to retain the original geometry as far as possible. I'm on the look out for a KTM 620 cush hub (not easy to come by here in the UK) and will use a new, undrilled rim (with spokes to suit). For the front wheel, I'm thinking to use an XL125R hub ('87/88) or XL 250/350 (similar vintage) plus matching rim. Whether the XL speedo drive/spacers/axle etc. are compatible with the AX1 remains to be seen. Plenty of options for rim widths in both diameters so the next job is to trawl through the online tyre catalogues and see what's readily available (preferably in matched pairs). In short, I'd like to set the bike up as close to stock as possible, in terms of handling characteristics, but with greater tyre choice. Seems like an involved and potentially expensive way to go about it, but needs must...finding a good set of those cast wheels is akin to finding the Ark of the covenant and probably as priceless

I'll post up my progress as and when. I suspect it will be slow going...pics soon!

[jonathan]

Quick question: do you happen to remember what year the KTM wheel was from? I had a look on the KTM parts fiche and they changed the hub configuration over the years...

Kurze Frage: Erinnerst du dich zufällig an das Jahr, in dem das KTM-Rad gebaut wurde? Ich habe mir das KTM-Teiledokument angesehen und im Laufe der Jahre die Nabenkonfiguration geändert ...

[scrambler]

Quick question: do you happen to remember what year the KTM wheel was from?

1996, but I'm not sure. I was told by someone who knows more about KTM than me that the rear wheels of 620/640 are the same and can be
distinguished from older wheels (red hubs) by the grey hubs.

Affordable rear KTM wheels are rare, even around here. If possible try to get a complete set with the cush drive and all spacer, that’s usually cheaper than buying it separately.

without the cush drive
https://www.ebay.de/itm/KTM-LC4-620-EGS ... GTR=1#shId

https://www.ebay.de/itm/KTM500-1992-REA ... SwJhtddpn3

That’s the cheapest complete set I can find at the moment
https://www.ebay.de/itm/Hinterrad-Brems ... GTR=1#shId

[jonathan]

Thanks for taking the time Scrambler. Much appreciated. I'm trawling the internet as we speak. I think all the cush hubs have been pilfered by EXC and DRZ owners

So the KTM 500 hub in the link is the same as the 620/640? Germany seems to be the best source, based on my search. UK sellers tend to omit the details

[scrambler]

So the KTM 500 hub in the link is the same as the 620/640? Germany seems to be the best source, based on my search.

I think so - there have been different engines with 500 and 400cc (LC4) in the same chassis as far as I know. I let you know if I can find something more affordable.

[scrambler]

The only downsize – there isn’t much space in the AX1 swing arm for a much larger tire size. A 4.00-18 won’t fit, a 3.50-18 is tight as well as a 110/80-18.

ok, I’ve been rethinking this – according to this tire manual a 110/80-18 (or 4.10-18) has a smaller diameter than a 3.50-18, 120/90-17 or 130/80-17. So a 110/80-18 should be possible in the AX1 swingarm, but I haven’t tried it so far.

[jonathan]

Thanks for the chart Scrambler, another useful resource! I've been using a generic online tyre comparison calculator to compare radii between different tyres on different rims, but these don't take into account the actual tyre under consideration. Effective width is going to be as critical as diameter on the AX1, and one of the issues is that tyre manufacturers don't always state effective width (based on tread pattern and type), but rather the width of rim that the tyre can be fitted to. This will obviously affect the tyres' profile, flexure etc.
So, wall height, ply rating etc. will have to be considered also...nothing is ever as simple as it first appears

[scrambler]

Thanks for the chart Scrambler, another useful resource! I've been using a generic online tyre comparison calculator to compare radii between different tyres on different rims, but these don't take into account the actual tyre under consideration. Effective width is going to be as critical as diameter on the AX1, and one of the issues is that tyre manufacturers don't always state effective width (based on tread pattern and type), but rather the width of rim that the tyre can be fitted to. This will obviously affect the tyres' profile, flexure etc.
So, wall height, ply rating etc. will have to be considered also...nothing is ever as simple as it first appears

With that kind of conversion it needs lots of testing - I haven’t done any wheel size experiments with the AX1, but the NX250 geometry is identical. Here the 18/21 combination won’t change geometry and is superior offroad, so if seat height isn’t a matter I would choose this. Stability onroad is then a matter of the chosen tire brand.

A road oriented Michelin Sirac will do very well on-road, I guess you won’t notice any large differences to the stock tires.
On the other end of the spectrum, an offroad competition tire like the Metzeler extreme will perform spectacular offroad, but poorly onroad. Meaning, it will become pretty nervous at higher speed and in curves.

Something in between are tires like Michelin Anakee Wild (pic below on the rear 110/80-18) or Heidenau K60 (in front 90/90-21). Driveable onroad without too much “getting used it” and still decent offroad capabilities.



The second option would be 17’’ or 18’’ in the rear and the stock 19’’ in front. That changes the geometry quite a bit, which could lead to high speed instability, depending on the tire brand (see above). I would recommend this option with a shorter rear shock to balance the geometry. I had this on my NX for many years.

And the third option – a stock 19'' in front and in the rear a NX250 16’’ rim on the KTM hub. This should be possible but I haven’t tried it so far. Seat height and geometry won’t change and you can use the stock front fender. The NX250 front wheel will of course fit plug and play into the AX1 fork, and NX250 wheels are affordable around here .

The main disadvantage is the very limited supply of 16’’ offroad tires. But Bridgestone will bring the Adventurecross AX41 in Jan2020 viewtopic.php?f=6&t=1299

With that tire offroad only the driver sets the limit



So tough decision

[jonathan]

Hi Scrambler...I'm going to rename you The Oracle I've been on to Michelin about the Siracs...available in 130/80/17 Rear
Compared to stock (theoretical):

calc.PNG

Looking at the tyre itself, I don't think clearance will be an issue. The B/stone Adventure cross look interesting, but I reckon the SIracs would suit my intended riding. Rim width is the next thing to ponder, as it affects how the tyre actually sits...all good fun!