KR250A track bike project.

[Gerrit]

I recently obtained various bits and pieces as the bare bones of a KR250A project, frame No. KR250A-009209 and engine no. KR250AE-009255. As frame and crankcase set came from the same source (a breaker's in France) and the numbers are close it's safe to assume they belong together.
The intention is to build a track bike so originality is NOT the goal. Front forks will either be KR-1/KR-1S or RGV250 upside down fork assembly. As for the weird standard rear suspension, this will have to go. Nobody makes a replacement shock so an alternative setup will have to be designed, perhaps a copy of the 1982 GPz550 setup if there is enough room to squeeze that in. If not, a copy of the Yamaha monoshock setup.
Wheels- 17" front and rear. KR250 race fairing and seat, if I can find a source for them.
The frame has been taken apart and first step will be to weigh the bits.

[Gerrit]

Weight of main frame: 6.16 kg.
Weight of rear subframe: 2.315 kg.
Weight of LH side plate: 1.33 kg.
Weight of RH side plate: 1.385 kg

Total weight of frame minus nuts and bolts is 11.19 kg.

[StrokerBoy]

Sounds great ! Looking forward to seeing it progress.

[Gerrit]

Surprisingly, the steering head of the KR250A is 5 mm longer than that of the KR-1(S), 195 vs 190 mm, which means the steering stem of the KR-1(S) lower fork yoke is 5 mm too short, requiring a longer stem to be made.

[Gerrit]

First pics.
The side plates have been modified by cutting away the footrest brackets:

Tabs will be welded on for the rearsets.

[Gerrit]

As a full fairing will be fitted the four lugs for the belly pan aren't needed and were removed:

[Gerrit]

First rough surgery on the rear subframe:

[Gerrit]

Next step is to carefully remove these two weird brackets, one at a time, and replace by 20 x 3 mm round tubing:

I might need them later for an altered rear suspension setup.

[maccas]

Fantastic! Keep the updates coming!

I'll be embarking on something similar in the future.

My plans were tz250 reverse cylinder forks , swingarm, shock and linkages with similar butchery to original frame. Here is q video of a bike that was modded in japan with tz rear shock and zxr400 swingarm, i think!

https://www.google.com/url?sa=t&source= ... 9GgKw7Lv43

I'm heavily basing mine on this video!

Dan

[maccas]

Regarding the steering stem and head, be careful here as the rake is quite relaxed, so the yokes have quite a lot of offset built in to bring the trail back to a reasonable number. Make sure to check the offset on the forks you intend to fit so you don't make it super lazy steering!

Dan

[maccas]

See pic from Tim's website of the same bike. Sorry attachment didn't work.

Dan

[Gerrit]

I've been looking at the same bike's pic and I believe the swingarm is a ZXR250 part. Fitting this will require some major surgery, however. The two curved tubes at the rear of the main frame would have to be removed to allow the ZXR swingarm to fit. Also, the ZXR swingarm's pivot bolt is 16 mm so the holes in the side plates would have to be increased in diameter to 16 mm. Then the shock mounts would have to be fabricated and welded. As I don't have a ZXR250 swingarm I don't know whether the width of the swingarm would have to be modified to fit between the side plates, and the bearing sleeve may also have to be reduced in length.

[maccas]

Yeah i couldn't decide between whether it was a 400 or 250 swingarm. Modding the side plates is no biggy as they can easily be mounted on a mill.

How are you getting on with the build?

Dan

[Gerrit]

The standard fork yokes have a 50 mm offset, the KR-1/1S yokes 40 mm so trail will increase somewhat. However, the wheelbase will decrease and the front wheel will move to the rear which will put more weight onto the front wheel. Regarding the ZXR250 swingarm, this will probably be longer than the standard KR250A item, possibly as long as a KR-1S swingarm which would increase the wheelbase. I have just received a (Japanese) ZXR250 parts catalogue so I can check the part numbers. Judging by pics the ZXR250 dogbones are very similar to those of the KR-1S, they have the same number cast into them (1085) so they may even be identical.
A 3MA swingarm will probably also be longer than the KR250A item so it will have a similar effect on the wheelbase as the ZXR part.

[Gerrit]

The suspension knuckle for the KR-1(S) and the ZXR250A, B, C and D share the same part number, 39007-1155. The dogbones for the KR-1(S) and the ZXR250A also share the same part number, 39111-1085. The dogbones for the ZXR250B, C and D have different part numbers, 39111-1108, 39111-1115 and 39111-1116 respectively. So depending on the ZXR250 swingarm type one could use the KR-1(S) suspension knuckle and the corresponding ZXR dogbones. And depending on the lengths of the dogbones the rear height can be adjusted by swapping the various dogbones.