RT1 frame mods.

[Nathan S]

OK, so I'm going to do the modification where the engine is lowered in the frame by 50mm.

For the front half of the frame, no dramas - cut the front down tubes and the end of the back-bone where they're parallel to each other, and bob's yer uncle. This much is well documented and understood (by me).

What about the rear half? Once the front's been modified, the rear needs to be modded too. Nobody seems to talk about this bit.

My thoughts are: Add the 50mm to tubes that run from the swing-arm pivot to the top shock mounts, and then add in whatever length is needed to the horizontal tubes to make it all line up again (my guess is that it will be around 35mm of extra length). And then add in a similar length (~35mm) to the swing-arm.

Is there a better way?

The obvious disadvantages of this (need for longer shocks [not longer travel], seat fitment, etc) aren't really a problem for me because I'm fitting an alternative seat, and am getting new shocks, etc.

[Lozza]

I would put scales under the tyres and work out your weight distribution, a angle gauge on the headstock and check the relationship between the countershaft and swing arm pivot to begin with.So you have some hard data to work from.If your dropping the countershaft by that much(I'll have to check) you will be introducing a lot of squat or anti squat into the rear suspension.Which might have some unintended consequences.Is lowering the CoG that desirable anyway?Is this just a back in the day work around by product of shit suspension?Where is the CoG now?

[jimg1au]

http://ozvmx.com/community/index.php?topic=2432.0
this is from earlier posts on same subject
cheers
jim

[JC]

Nath,
I remembered an article in DirtBike (5/72) on those lowering kits & also found a webco add. I'll try & scan & post here.

The Webco ad says their kit "lowers the engine 1.75" & moves it back 7/8". The result is improved handling thro lower CG along w proper swingarm angle, plus improved relationship of seat to pegs."

The DirtBike article (they rode a DT1 w the kit fitted) said it made the swingarm parallel to the ground & they concluded it was lowered too much (footpegs, brake, gearshift dragged on ground) for MX; that 1" lowered would be better.

They also claimed wheelbase was extended "to a whopping 57 inches". I don't know how 53.6" wheelbase (std DT1) + 7/8" from kit = 57" (even if wheel adjustment was right back). Perhaps it also had swingarm extended, or they just didn't measure it properly.

Webco kit only included 3 spacers - 1 for backbone & 2 for downtubes. They installed kit by cutting rear frame uprights (to top shock mt) where they attach behind & below swingarm pivot, shortening & re-angling the ends to be re-welded when engine cradle is dropped. Back-bone is cut below seat & front downtubes cut above front engine mt, spacers installed. Its quite simple that way.

[JC]

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[vandy010]

Nathan,
i've just completed my RT1 frame mod and have lowered it by 2".
i used 2 frames, i cut the "top" section "long"
and the "bottom" section "long" then joined the 2 together.
in short, the motor has been lowered by 2".
i'll see if i can get some picks up for you soon but for now see if this little sketch makes any sense.
i won't be doing anything to my swing arm and will leave it stock.
another bike here in Brisbane has had the same mod and the owner loves it and said he'd love to try another with a 3" drop.
ground clearance shouldn't be an issue as they have way to much anyway even if you chose to run a down-pipe.

[vandy010]

here's my RT1 dropped to the ground {no fork springs, no shock springs} before the frame mod.
as you can see by the ruler, theres heaps of room to play with.
around 6 + inches of clearance, so a 2" drop should lower the C of G and reduce the steep angle of the swing arm allowing the shock to work a little better.
i've also retained the same steering angle and the bike won't be ready to ride for a few more weeks yet.

[mx250]

This is what a lowering kit looks like.  It's a project well down the list but I am thinking of lowering an RT/DT. I've been keeping at eye out looking on Ebay US for a kit on the off chance. I have a friend who would run off a kit if I'm unable to find one but I'm gunna keep that in reserve - I don't want to flog the willing horse .

Having spoken to Firko about this it appears the amount of lowering varied a little, 20-50mm. Firko wasn't certain but he thought the 50mm, 2 inch, was the best and most popular. I remember seeing one 'in the day' with a 4 inch drop - not the best for MX but a tear away success at DT  .

[vandy010]

and here it is, all ready for CD6...........
lowered engine in frame by 2". oddly enough, the pipe still bolted straight up with a minimum of fuss.
we'll see how she goes after this weekend and i'll let you know of the difference from the lowering to stock.

[mx250]

Looks very business like . Love the baby blue tank, very distinctive .

So, you machined up your own kit or did you buy them or did you lower the frame in some other way?

[David Lahey]

This is an RT3 with motor lowered 2" and new 3" longer-than-standard swingarm, widened tank and RM250A forks. Photo taken 1978.

[vandy010]

Looks very business like . Love the baby blue tank, very distinctive .

So, you machined up your own kit or did you buy them or did you lower the frame in some other way?

well, the baby blue tank peeled it's paint off at the first sight of petrol spilt on it. thats what you get for doing a paint job the day before when it's raining i guess.
as for the frame, no spacers involved on mine as i was lucky enough to have 2 frames. i cut one frame "long" for the "top" half, then cut the "bottom/engine cradle" "long" for the bottom half making for a 2" drop of the motor.
i thought my bike handled reasonably before the chop but it's definately way better now. more leg room is a big bonus too! it seems to steer quite well and i'm using the later model DT400 triple clamps.
the swing~arm is un altered.
i sleeved each join and added a small "cross tube/brace" between the front down tubes for extra strength.
i'm happy!
just need to fine tune the "donk" a wee bit and she'll be the goods.

[Freakshow]

What happens at the Swing arm pivot, does the chain run on the pivot now the motor is 2 " lower or an i missing something here ?