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Old 01-30-2014, 02:36 PM   #1
adrenal OP
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Airhead salt racer

Potential threadjack diverted here!
We're talking about gearing for a salt racer that I'm building.
On the salt, the front wheel may be going 200km/h while the back wheel is doing 220! There's that much slip. The number thrown around is 10%.


Quote:
Originally Posted by pommie john View Post
Please excuse the thread hijack,
Adrenal, my calculations tell me that with a 2.75 rear drive ( I think that's what you said you were fitting) and a 130/70 rear tyre you should be hitting 150MPH at 8300RPM. 10% wheelspin gives you 9130RPM.

I would think 150 MPH is a bit optimistic for an airhead on salt. Let's say you hit 145 at 7700RPM , the wheelspin would be running the engine up to 8450RPM.

IMHO you have the the gearing in the ballpark with the 2.75.
Agreed PJ, thats why I bought the R1100 swing arm with R1100S 2.75 bevel drive.

My numbers are a bit different:

Worked backwards from target speed. The speed record here in my category is 136MPH so target speed across ground= 137MPH (lets not get too greedy!)
Therefore target rear wheel speed 137 + 10% slip = 150.7MPH.

I went with a K100 rear wheel to reduce centerline offset. The wheel is 17" and with a 130x90 tyre is about 8mm less overall diameter than stock wheel.

In 5th gear (1.5) and 2.75 bevel drive, the target wheel speed is reached at 8700RPM.

So yes, the 2.75 drive is looking pretty good and has a bit in the bank to possibly go faster depending on drag and slip and how much power I can extract from the lump

More info in my salt racer blog here:
http://skrunkwerks.com/skrunk/category/saltracer/


.
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adrenal screwed with this post 03-10-2014 at 02:31 AM
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Old 01-30-2014, 02:47 PM   #2
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Old 01-30-2014, 03:34 PM   #3
Rob Farmer
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Serious question - what are you doing to stop the clutch frying or do you anticipate it will handle things?
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Old 01-30-2014, 03:48 PM   #4
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Think I'll keep an eye on this, I'd love one of my airheads to do the big 150

Keep up the good work
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Old 01-30-2014, 03:51 PM   #5
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Quote:
Originally Posted by Rob Farmer View Post
Serious question - what are you doing to stop the clutch frying or do you anticipate it will handle things?
Pommie John put me onto a possible solution for this which I've since found is not uncommon practice.
You use the heavy duty clutch plate that is commonly available with a pressure plate from a K100. The K100 plate increases engagement force over stock. It all bolts up without modification.

The problems is, the salt racer will be maxed out for a longer sustained period than the average racer so still a question mark hanging over this one.

There are other options including a number of aftermarket kits.
Would certainly welcome any suggestions..
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Old 01-30-2014, 03:56 PM   #6
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Quote:
Originally Posted by rambozo View Post


Think I'll keep an eye on this, I'd love one of my airheads to do the big 150

Keep up the good work
Yeah, would love to join the 150 club on an airhead! But only possible with a streamlined bike methinks.
There's a guy here in Aussieland, Josh Schuit, that made 148 on his faired airhead on the salt.

My bike is unfaired and the 150.7 mentioned in above post is rear wheel speed as opposed to target ground speed of 137
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adrenal screwed with this post 01-30-2014 at 04:19 PM
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Old 01-30-2014, 04:11 PM   #7
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Aside from his salt racer, Josh races an airhead dragbike.

Here's his salt racer at Lake Gairdner speed week in outback South Australia:

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adrenal screwed with this post 01-30-2014 at 04:20 PM
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Old 01-30-2014, 05:46 PM   #8
pommie john
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Quote:
Originally Posted by adrenal View Post
Pommie John put me onto a possible solution for this which I've since found is not uncommon practice.
You use the heavy duty clutch plate that is commonly available with a pressure plate from a K100. The K100 plate increases engagement force over stock. It all bolts up without modification.

The problems is, the salt racer will be maxed out for a longer sustained period than the average racer so still a question mark hanging over this one.

There are other options including a number of aftermarket kits.
Would certainly welcome any suggestions..


I'll just clarify that clutch.

Standard pressure ring,
Standard clutch plate,
Heavy duty spring,
K75 pressure plate
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Old 01-30-2014, 05:54 PM   #9
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Quote:
Originally Posted by pommie john View Post
I'll just clarify that clutch.

Standard pressure ring,
Standard clutch plate,
Heavy duty spring,
K75 pressure plate
Thanks PJ, I was relying on memory...not good call in my case...
I have the K75 pressure plate and heavy duty spring.
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adrenal screwed with this post 01-30-2014 at 08:14 PM
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Old 01-30-2014, 11:51 PM   #10
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Can you fit a larger diameter rear wheel in that? You'd be pulling a bit less RPM.

Here's my graph of speed vs RPM with a 130/70 - 18 wheel

It doesn't take into account your wheelspin, but might be a handy tool.

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Old 01-31-2014, 12:02 AM   #11
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Quote:
Originally Posted by pommie john View Post
Can you fit a larger diameter rear wheel in that? You'd be pulling a bit less RPM.

Here's my graph of speed vs RPM with a 130/70 - 18 wheel

It doesn't take into account your wheelspin, but might be a handy tool.
Thanks PJ. I've written a program in Mathcad which I can plug any numbers into so pretty sorted in that department.
Had to compromise with the 17" wheel unfortunately...due to other considerations such as wheel offset.
Did you have a look at my blog?

http://skrunkwerks.com/skrunk/category/saltracer/
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Old 01-31-2014, 12:52 PM   #12
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We ran a '55 R50 111.8mph at Bonneville. We ran up to 9600 rpm in 3rd gear and only managed maybe 7700 in 4th. Needed a bigger range of final drives, obviously. Pretty seriously build engine, but let's be honest, we would be lucky to be making horsepower in the low 40s.

I never detected any significant and certainly not sustained wheel spin. I just don't think the old bikes make enough horsepower to have that much problem with wheel spin. We were there is 2008 and 2009, both were "good salt" years.

Chris's R100 from San Jose BMW has an SCTA record at 164, so 150 can be done. I shudder to think of the time and money spend at that bike.

Good luck!

Eric

p.s by We, I mean my wife. I only went 109.5. It was LONG drive home. I am very proud of her.
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Old 01-31-2014, 02:09 PM   #13
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Quote:
Originally Posted by ericrat View Post
We ran a '55 R50 111.8mph at Bonneville. We ran up to 9600 rpm in 3rd gear and only managed maybe 7700 in 4th. Needed a bigger range of final drives, obviously. Pretty seriously build engine, but let's be honest, we would be lucky to be making horsepower in the low 40s.

I never detected any significant and certainly not sustained wheel spin. I just don't think the old bikes make enough horsepower to have that much problem with wheel spin. We were there is 2008 and 2009, both were "good salt" years.

Chris's R100 from San Jose BMW has an SCTA record at 164, so 150 can be done. I shudder to think of the time and money spend at that bike.

Good luck!

Eric

p.s by We, I mean my wife. I only went 109.5. It was LONG drive home. I am very proud of her.
I dips me lid to anyone who has a crack at the salt! Thanks for your post ericrat. I guess Chris' bike would have been faired...

Wheel spin also becomes an issue at higher speeds as you push against the elastic wall of air resistance, assuming you have power in the bank at those speeds.

Do you have any pics?
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adrenal screwed with this post 01-31-2014 at 07:51 PM
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Old 01-31-2014, 03:06 PM   #14
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Quote:
Originally Posted by adrenal View Post
Aside from his salt racer, Josh races an airhead dragbike.

Here's his salt racer at Lake Gairdner speed week in outback South Australia:

That's a purposeful looking machine

Just out of interest is there a reason you're going for unfaired?
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Old 01-31-2014, 03:22 PM   #15
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Quote:
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That's a purposeful looking machine

Just out of interest is there a reason you're going for unfaired?
Yes, less futzing about - time constraints. Version 1 will be unfaired and maybe faired by version 2..or 3.. if the salt fever is still with me that is!
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