Pre 81 MX class rules?

[Kwaka-Kid]

hey guys,
firstly is anyone racing in this class on here?

The biggest issue i have is my XR500 has a 23" front wheel. and its proving (online) hard to find any manufacturers that make any good rubber for it! i can only see that bridgestone make an enduro tyre for it but id need a proper MX style full knobbly as it will only be used in the sand and on the dirt.

so my main question is do the rules either let me drop a whole front end off a XR200 in or later model 250 or something, or can i at least just go get a 21" front wheel for it?

and if its the 21" wheel... anybody got a spare they are willing to sell?

any info on this class is much appreciated or links to sites which state the exact rules (though i know from circuit racing some of the rules are let slide like this for reasons like availability of parts etc)

cheers KB'ers!

[Firefight]

hey guys,
firstly is anyone racing in this class on here?

The biggest issue i have is my XR500 has a 23" front wheel. and its proving (online) hard to find any manufacturers that make any good rubber for it! i can only see that bridgestone make an enduro tyre for it but id need a proper MX style full knobbly as it will only be used in the sand and on the dirt.

so my main question is do the rules either let me drop a whole front end off a XR200 in or later model 250 or something, or can i at least just go get a 21" front wheel for it?

and if its the 21" wheel... anybody got a spare they are willing to sell?

when do you need the 21 incher by Logan ?I think I know where one is, also looking for my old MNZ rule book now can, t find it yet too much shit in shed of humble cottage, may pay to pm Oscar/ Motu or Merv I think they may be able to answer your question.Will be working at woodhill next saturday, pop out if you get a chance.

F/F

[Kwaka-Kid]

next saturday? im there. Surely wont have bike ready but ill come out on the streeter to remind myself where it is and say hi.

k, i dont need it in any big hurry, got a bald front tyre on the beast atm, good enough to get the bike going on and the engines still in getting a main mount welded back on. So sorry but i should have checked my book i have one somewhere too, completely forgot it covers dirt doesnt it? i have it on my PC somewhere too... damn just as disorganised

[NordieBoy]

Get your 23" hub laced into a 21" rim.
The 23" hub has better braking effect than the 21's (bigger diameter).
I think you can even get away with using for example a new Excel rim as it is a safety thing not performance but I just picked up a 21" from a kx125 off trademe and stripped the rim off it.

I have also made a new seat for my 250 that's 3cm taller and a lot firmer than the 25 year old foam in the original

Check out some of the rules at http://www.vmx.livewire.gen.nz/

Have fun

[Kwaka-Kid]

your a legend nordie!

thanks for the link and the tip with the wheel - my dad says he has relaced rims loads before, not hard at all, but i still have the problem of not having a 21" rim and a limited budget which crosses out buying new... but hopefully if FF's contact or whatever works out i could relace the 21" rim to my hub if i find braking is bad (otherwise ill be lazy and leave the 2 seperate) So tell us nordie when you planning on racing ur 250? i used to have one of those too when i was about 12! awesome machine eh.

[NordieBoy]

So tell us nordie when you planning on racing ur 250? i used to have one of those too when i was about 12! awesome machine eh.

I had my 1st ever race at the North V South on the 10th.
http://www.classicmx.nelson.geek.nz/...outhClassicMX/

I like the 250 as I can race in Smallbore, Pre 80 B, Pre 85

Unfortunatly I'm still using the 23" front and can't turn as tight as basically everyone else, that's my excuse and I'm sticking to it
I've got all the bits to go to 21" but money and time...

I'm going to remove at least the front balancer, get the head cleaned up, compression raised and another pipe on it.
It's a bit slow revving for MX but perfect for trail riding at the moment.

I'm building up 2 sets of wheels so I can have an off-road and an on-road set as I'm getting it road registered

I've already built up the seat to my 6' 2" spec - no more 25 year old foam and knees around ears

http://motorsport.nelson.geek.nz/mybikes/dirt/xr250/

[Kwaka-Kid]

hmm.. interesting.. what is the downside of not runnnig the balancer shaft etc ? aside from more vibes, does it not ruin the engine anywhere quicker?

also what tyre are u running on the 23" and is it suited for motorcross?
going to the links to checkout now dude, you the man tho! good to hear u just had a race! i cant wait just to toodle about and have some good old fun - did you find like everybody was really really good?

[NordieBoy]

hmm.. interesting.. what is the downside of not runnnig the balancer shaft etc ? aside from more vibes, does it not ruin the engine anywhere quicker?

Revs pick up/drop off quicker = quicker shifting and better acceleration.
More vibes = rubber mount the exhaust.

also what tyre are u running on the 23" and is it suited for motorcross?

Some good knobblys but I have no idea what brand or anything.
The bike is stored in my brothers shed so I can't check.
They were 2nd hand when I got the wheel and don't have much in the way of nice clean edges on the knobs.
Coming out of hairpins I couldn't keep in the ruts that had formed as the bike just wanted to push wide across the damn things.

The 23" is brilliant for trail riding as it goes over/through anything without any twitchyness at all but isn't sharp/responsive enough for MX really.

going to the links to checkout now dude, you the man tho! good to hear u just had a race! i cant wait just to toodle about and have some good old fun - did you find like everybody was really really good?

It was great fun. I had some good battles and didn't come last.
After the first race I didn't even get lapped
Really really good? That's why I entered Pre 80 B Grade
Leave the A Graders to their own racing...

[laRIKin]

The 23" hub has better braking effect than the 21's (bigger diameter).

Sorry but I think that you are wrong on this one.
The 23" wheel has more leverage on the drum so that means less braking.
Also has more mass to stop.

I know with my van has trouble with 14" wheels to pass a WOF (hand brake),
until I put the 13" rear wheels back on (thats what it can out with stock).

[laRIKin]

When to 23" wheels came out we found that they were good over ruts.
But were not so good following them.
They seamed to rail the bike and you could not get back out off them.
I can not tell you why this is, but when I was riding with a mate that a
XR500 with 23" front wheel. And I had a XT500, when I turned a corner in the
bush my mate would go straight ahead, unless he could see the ground.
Are the 23" tires narrower?

I think that you are alright to race as long as your motor is on the era.
And you have drum brakes and are twin shocked.
Well that was the rules down here a while ago, any way.
Could of changed how, because of the rule police.

[NordieBoy]

Sorry but I think that you are wrong on this one.
The 23" wheel has more leverage on the drum so that means less braking.
Also has more mass to stop.

Not when you match the hub up with a 21" rim
The 21" brake drums are a smaller diameter than the 23" ones.
The braking effect is about the same with the 21's and the 23's but stick the 23" hub in a 21" rim and it's even better and you havn't even had to get a twin leading shoe setup (and you also have about 15mm square more braking surface as well) :sly:

[Kwaka-Kid]

choice info thanks guys!

hmm so nordie where are you? in aucks where would i race and what class would i enter the 1980 in, you say pre 80 class, that means id be in the next step?
i was under the impression there was a pre 81?

Either way should be fun just getting the beast out there, and excellent a B class, how about a C ? im going to do loads of woodhill and general riding before i come close to racing but for sure it is the eventual goal.

[NordieBoy]

You'd have to find out your local club rules as to what classes they run.
Down here in Nelson it's Pre 75, Pre 80 A, Pre 80 B, Smallbore (<250 4st, < 125 2st), Mid Bore (<350cc 4st), Open 4st, Open 2 st and usually All In.

Some clubs have small bore as <350.

The 1980 XR 500 had no changes from the 1979 one except the tank sticker so you should get away with it.

[laRIKin]

Not when you match the hub up with a 21" rim
The 21" brake drums are a smaller diameter than the 23" ones.
The braking effect is about the same with the 21's and the 23's but stick the 23" hub in a 21" rim and it's even better and you havn't even had to get a twin leading shoe setup (and you also have about 15mm square more braking surface as well) :sly:

I stand corrected, I was not sure, or even thought that Honda up-ed the size
of the drum on the 23" wheel.

I was thinking after my post.
I can remember going to a Honda promo do, boy did I leave there with Honda
is the greatest, and the most innovative bike company for designing the 23" wheel ringing in my ears.
If they said it once. Hang on they didn't, they kept on saying every two
minute's until we all weakened and joined in, NOT.

The 23" front wheel stayed around long than the 20".
Boy that one did not last long at all.
Does any one know how long the 23" lasted for?

[NordieBoy]

XR's had 23's in 79 and 80 and the XL's had them in 80 and 81.
Then the XR went to 21" front and a 17" rear

The brake drum diameter on the 21" is about 124mm and on the 23" it's about 137mm.

The actual braking surface on the shoes is only 3mm longer for the 23 over the 21.

I need to get out more