Dry Lakes Racers Australia

Hi all. Just joined up and first post. Commencing a build of a custom Yamaha 250 motorcycle for the 2017 event (bucket list!).

The bike is a 1980 Yam 250 modified frame, carb on worked motor. Targeting 100mph?

I've some queries hoping experienced members can assist so i don't make too many rookie mistakes:

* Tyres - what tread pattern is preferred for salt? Standard road tyre (H)'? Low profile?
* Sprocket gearing - 15T fron/48T rear is standard with a 5 speed gearbox. I imagine i will need to move to 16T+ front and 45T or less rear to maximise top speed. Any suggestions. (i'm running 21" front and rear wheels)
*Suspension - planning a hard tail, is this advisable or better to maintain rear shock/spring for stability on salt surface

Any advicer greatly appreciated.
See you all in Feb!

Regards

Darren R

Hi Darren

Welcome to the House of Fun ....

What model Yamaha 250 do you have?

Pete

Hi Pete,
It's a 1980 XT250 single cylinder 2 valve. Will be fully customised to race and look nothing like the original!. I normally build cafe racers (5 to date). Like the challenge of the salt - should be an experience looking at the You tube videos.
If all goes well in 2017, a turbo will follow.

Do you have a race bike? 250?

Cheers Darren

Hi Darren

First thing I found before starting a build

Have you downloaded the Rule Book .... http://www.dlra.org.au/rulebook.htm

do that and read it through, then read it again ....

Pete

PS oh yeah just in case I forgot make sure you read the Rule Book ...

I only say that because it does take a number of readings to get your head around it ...

Then if you need any rule clarifications best thing to do is contact/email the Chief Motorcycle Inspector Graham directly

http://www.dlra.org.au/contacts.htm

XT250 you say , that's a relief, I was gunna bring more aeroguard

Easy Stayt'ie
Those 2 strokes will blow the doors off any 4 stroke and they do sound good!!!
Darren
Why 21' at both ends?
Can you get a 21" rear tire?
By using a 21" front you are limiting your frontal area reduction. You need this bike to be as LOW as possible.
Use a bigger front sprocket. It takes more HP to turn a chain around a small sprocket. As for overall gearing, I recon if you calculate for 100
and take rear sprockets a couple of teeth either side.

Make the bike rigid and you will be able to have a seat height lower than the top of the wheel.
What class will you run in? Modified has limited wheelbase--- check the rule book
Is 'H' rated for 100mph --- check the rule book
Oh-- and what Pete said---read the rule book

If you are in Sydney I am happy to have a look.

cheers Bones

Hi Darren,
you are in good hands with the people you have commenting. To go 100mph without a fairing you will need 23-25 rear wheel HP. We ran 94.5mph with 21hp and then 106mph with 28hp unfaired.

Good luck.
PS remember to read the rule book.

JP Afflick
AAA Racing

Thanks all for comments and advice. It's really motivating as the clock is ticking on the build and i've got alot of late nites ahead of me.

Some responses to welcomed suggestions:

I'll reread the rule book as there is alot of detail i missed first time around.
I've contacted Graham (first thing i did) to get some initial helpful direction on the build.

The XT comes with a 21' front wheel as standard. I wanted to match the rear for balance and asthetics (and to be different) and hoping to get increased top speed through overall gearing - thanks Bones. I'm in Wollongong so would appreciate an experienced eye when the build progresses

Great suggestion JP re: fairing - i was looking on ebay today and taking measurments. I'm getting that wind resistance is going to be critical and also weight as i'm 6'2' and 105kg - so i guess i'll be holding the bike back!!

For clarity, the 250 is a 4 stroke not 2 stroke.

H rated tyres are good for 130 mph - i was interested in what tread pattern works best for salt - any suggestions? I'm considering Dunlop 404's

Bones mentioned sprocket size - the standard bike runs front 15T rear 47T. I've used 16/45 which gives 10km/h extra top speed. I can get 17/43 (max) but wondered if this was too tall. Any advice is appreciated i will have no way of testing prior to getting on the salt.

Thanks all.

Darren

Darren

Normal Road Tyres are the way to go ....

Smallest Frontal Area you can get will give you the Highest Speed ....

When you do your mock ups for Tuck/Seat/Footpeg position make sure you wear all of your Racing Gear, Leathers Boots Helmet including Back protector .... only then can you get a real indication about how low you are ....

As far as Gearing get those sizes and start with what you know .... there is time to change Sprockets over the week ......

Pete

Yeah the 2 Stroke thing is an on going thing .... Nothing better than Bleeding ears and the Smell of Castor Oil in the morning I say

Also be aware that your "shed tuck" will be always tighter than your actual "race tuck"

Thanks Guys for the words of wisdom.
The support is really appreciated and has resulted in the engine teardown commencing today! So i'm officially committed.

Pete/Stayt'ie - i would not have considered the finer points of the 'tuck position' in the design - many thanks.

A couple of questions re: suspension.

Is there an advantage of having rigid rear end over some form of suspension? I'm assuming the course will have some deviation however was thinking this is negligible to justify needing rear suspension. Therefore planning on rigid rear end. Also assists in reducing seat height. Am i correct?

I'm also planning shortened front forks with minimal travel. I see on the forum some suggestions of less than 50mm travel. Comments appreciated.

Cheers

Darren

When it comes to suspenshion there are two camps,, I support the "Without Suspenshion" team ,, have many arguments as too why, and those that support the "with" team have their reasons as well ,, My advice is too set the suspenshion as you think best ,, My strongest "proven" reason is that at speeds in the 200mph range my bike has seen a 3-4% speed difference(over all salt conditions) in favour of without,, obversely that percentage difference will diminish at lower speeds,, I'm thinking that at 100mph it would be two parts of stuff all
Yup, slam that front end, but keep a close eye on your overall geromity settings,

Thanks Staly'ie - i'm with you brother. Hard tail and low.

Some more question:

Remote fuel shut off. I s there a good local product available, or is it a matter of cabling a lever to an in line valve. I see the Bonneville guys use a spring loaded thumb lever on the bars. Any simple solution?

Front dampener - the production bike does not have. Are these mandatory?

Class?? I'm a little confused on what class the bike will be. It will have a production motor - o.s. piston and some head work. Production frame will have extended swing (+100-200mm) are and rear suspension mods (rigid). Front cowl added. I was thinking Modified production? Any advice here appreciated.

Thanks Guys.

Darren

Hi Darren,
Ran for the first time last year on a vfr400 with no fairing class Modified 500 G
Had a ball on standard bike less fairings
Fuel shut off for bikes is the tank tap and you need a tether type engine cut out
Steering damper is compulsory fairly easy to sort mount on front fork and other end to frame(other options possible)
Class will be Modified 250 gas or Modified Partial Streamlining 250
You can increase wheelbase by 10% over standard in these classes
Good Luck
Brad
#1273

One other thing is that if you rebuild the engine oversize and you are over 250cc then you will need to run in the 350 class.
Cheers
Brad