Understanding Drop & Stretch "Measurements"

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ryno

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Okay before I get flammed. I'm no fabricator or welder. Which is why I am having a friend weld my hardtail. Because he can't get it any time soon and I rarely see him, I would like to grasp a little bit of know-how on how this process is going to work.

At first I ignorantly bought a TC Bros hardtail (3"stretch 5"ground clearance). [I presume ground clearance and drop is the same?] That's another story.

Anyway I decided to go another route and purchase Travis's Universal Hardtail Kit (in the near future) because I wanted 0'' stretch. *I also presume 2" drop is most common/best look for 0'' of stretch?*

So how do you calculate stretch and drop?
This is just a guess I have no idea what I'm saying but, would "stretch" be the distance of bottom of backbone to axle nut? And "drop" be the distance from the bend on the backbone to the axle nut? - If that makes sense what so ever.:shrug:

This would also be a good write up for all your engineering smarty's out there.
 
Your over thinking this. Ground clearance is just that, the distance from the frame rail to the ground. Drop is how much the bike is lowered from STOCK height. Say for example stock ground clearance is 5", and you want a drop of 2", that would result in 3" of ground clearance when the hardtail is welded on. The distance of the axle from the ground is static in this case, and the frame is moving down.

Stretch is based on the stock wheelbase. If my memory serves me correctly, the xs650 has a wheelbase of 59"?? When welding a hardtail to a stock frame all the stretch is in the hardtail section... so you are essentially moving the axle further away from the engine.

Hope this helps... :laughing:
 
Thanks!
Now this is very insightful. I knew I couldn't be to far off.

Here's the tricky part. How are you supposed to know what "stock" measurements are when all you have is the frame (like myself) and weld on a hard tail with lets say the look i'm going for 0" stretch and 2" drop.
I know i've seen pictures with people it doing both ways with forks & front wheel still mounted, but most people just have the frame setup, I'm sure there's an easy calculation to all of this.
(I'm guess you are going to say manual)

And wouldn't drop depend on what kind of rear wheel size you are using. Like some of the earlier models having 18" rear wheels and later models having 16" or am I overlooking tire sizing aswell?

I think I am over thinking this to much :doh:

I'm more of a visual guy. :(

Thanks
 
For the drop, as I said, distance of the frame rails to the ground. You only need the frame for that, and yes you need to know what size TIRE you will run so you know the overall diameter, take that divide in half, and you know the distance of your axle from the ground.

For calculating stretch and building a frame from scratch you NEED wheels, having a swing arm will also help tremendously.

If I were you, I'd just buy a bird tail... but thats me. You know its made by a guy who knows what hes doing, and has done TONS of them, and its welded in a JIG...
 
Thanks SGJ.

I'm sure/hoping my friend knows how to do all this. lol
Might have to actually stick with my tc bros hardtail I have.. Would really like 0" stretch though.

I appreciate the insight.
I actually have a 79' frame with a motor out of it. And I purchased another 78' that WAS mostly complete, i took the wheels and tires off to get started on disassembly (I'm sure this would have helped a bit more)
http://www.xs650.com/forum/showthread.php?t=9202 to give you a better idea

I had already thought of the David Bird.. I just can't seem see past those gaudy triangular pieces. I know it's a great product, just not for me. And Elswick is out of the picture not really keen on their communication nightmares i've heard about.


Thanks
 
TC tail to me looks like a dog taking a shit... JMO. The triangle gussets on the bird nearly dissappear with the motor in, and foot controls on. You can't get away from those short rear tubes on the TC though.

Its your bike though... do it the way that suits you.
 
You don't need wheels to calculate stretch if you have a complete frame. As you can measure the existing rear axle position, or build a jig around that point.
Then keep it in place for existing 0Stretch ,0Drop. or move it in the required axis.

You can calculate the Drop if you get measurements of standard wheel vs you future wheels. Surely someone on the forum will have the wheels you want.

Just another way of looking at it.
 
i would think that modifying a TC hardtail would be pretty easy!
cut off 3" from the lower rails.
on the top rails - cut the inside of the bends almost all the way through and bend it in, cut the length and add gussets.. cut a tube in half and weld a brace over the inside of the bend

you could also adjust the drop..
 
AussieHard said:
You don't need wheels to calculate stretch if you have a complete frame. As you can measure the existing rear axle position, or build a jig around that point.
Then keep it in place for existing 0Stretch ,0Drop. or move it in the required axis.

You can calculate the Drop if you get measurements of standard wheel vs you future wheels. Surely someone on the forum will have the wheels you want.

Just another way of looking at it.
Click to expand...

Yeah yeah, ok the stretch can be worked out without an axle as well really... BUT, without having the wheels and spacers it would be silly to weld up axle plates. Unless you have a jig.

He has wheels though... hes covered. :D

bloodthirstysystem said:
i would think that modifying a TC hardtail would be pretty easy!
cut off 3" from the lower rails.
on the top rails - cut the inside of the bends almost all the way through and bend it in, cut the length and add gussets.. cut a tube in half and weld a brace over the inside of the bend

you could also adjust the drop..
Click to expand...

Easy, yes. Safe? Not safe enough to my wife on it. Plus if he doesn't like the little triangle gussets on the BIRD tail, hes gonna HATE all the extra gusseting to make that strong enough to be safe. :wink2:
 
Easy, yes. Safe? Not safe enough to my wife on it. Plus if he doesn't like the little triangle gussets on the BIRD tail, hes gonna HATE all the extra gusseting to make that strong enough to be safe.
Click to expand...

what makes a modified hardtail any less safe than the original?? just DONT cut up the top rails like that.

shorten the lower rails, replace the slugs. take an oxy-acetylene torch and heat up the bends on the upper rails till theyre almost cherry hot, just enough to get them to bend. make sure you measure out the length you heat to make both sides even and make an easy way to check how much they need to be bent. heat em up, bend em in, shorten, done.
 
bened said:
what makes a modified hardtail any less safe than the original?? just DONT cut up the top rails like that.

shorten the lower rails, replace the slugs. take an oxy-acetylene torch and heat up the bends on the upper rails till theyre almost cherry hot, just enough to get them to bend. make sure you measure out the length you heat to make both sides even and make an easy way to check how much they need to be bent. heat em up, bend em in, shorten, done.
Click to expand...

:doh: I'm sure you already knew, BUT a single piece of tubing from axle plate to the backbone is MUCH stronger than one that has pie cuts and gussets.

Why would you buy a TCB tail and modify it if you can accurately bend tubing to accomplish what you are describing? :thumbsup:
 
I'm sure you already knew, BUT a single piece of tubing from axle plate to the backbone is MUCH stronger than one that has pie cuts and gussets
Click to expand...

which is EXACTLY why i said NOT (note the word not) to cut it like that.

by simply increasing the radius on the upper rail bends and shortening the lower rails he can use the hardtail he already has. it makes things super simple...

Why would you buy a TCB tail and modify it if you can accurately bend tubing to accomplish what you are describing?
Click to expand...

Why?? you wouldnt... but if hes already got one, its definitely an easy way of getting a 0 stretch hardtail like he wants...
 
bened said:
which is EXACTLY why i said NOT (note the word not) to cut it like that.

by simply increasing the radius on the upper rail bends and shortening the lower rails he can use the hardtail he already has. it makes things super simple...



Why?? you wouldnt... but if hes already got one, its definitely an easy way of getting a 0 stretch hardtail like he wants...
Click to expand...

Im not here to argue... you say "simple" and "easy". Its not. Cutting and re-radiusing the top tubes enough to get them to fit right isn't going to look good either.

The only reason I'm even responding to this thread again is so that people reading it don't get the idea that making the modifications as described is a good idea.
 
Cutting and re-radiusing the top tubes enough to get them to fit right isn't going to look good either
Click to expand...

haha thats why i said NOT to cut them... wow. thats 3 times now....
 

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