So help me understand this "Drop seat"

[member3480]

I have plenty of room for battery and even fake oil tank. I am building one right now and i will post pictures so you can see. I use the turn out manifolds for the carbs and that looks good. I just welded one up and going to spray three frames tomorrow so i will shoot a picture and show you. The bike in the picture the guy use a small beer keg for his electronics and it had plenty of room. Brat bikes you are dealing with swing Arm that has to move so you are limited so i just make a battery box on the bottom of the swing arm and Jimmy is right just use a smaller battery if you are kick only.

 

[projectxs650]

It is low because there the tail was built with a drop, the neck is raked to 38 degrees, and also a lower profile rear tire. Special models come with a 16 rear.

How much drop you think is in that tail?

 

[XSLeo]

Project, The wheel size doesn't have much to do with ride height. The height of the tire is what effects the ride height.
The Specials had a 16 inch wheel with a 130/90-16 tire. The figure the height of the tire you divide the width, 130 by 25.4. There are 25.4 mm to the inch. That gives you the width in inches. 5.1 inches wide. Next multiply by the aspect ratio. This is the 90 in the tire size. 5.1 x 90% = 4.59. This is the distance from the bead to the tread. There is tire above and below the wheel so multiply by 2. 4.59 x 2 = 9.18. Add in the wheel size. 9.18 + 16 = 25.18.
The same way with the 18 nch tire. 120divided by 25.4 + 4.72 x 90% = 4.25 x 2 = 8.5 + 18 = 26.5.
Now the axle sets at 1/2 the tire height. 25.18 divided by 2 = 12.59.
The 18 inch is 26.5 divided by 2 = 13.25.
The difference in axle height is 13.25 - 12.59 = .66 inches.
So changing from a 13/90-16 to a 120/90-18 raises the rear axle .66 inches. Under the engine the difference in ride height is half of the rear axle change. .66 divided by 2 is .33 inches difference in ride height. If you figure out the difference between the 100/90-19 front and the 300-21 the difference is less. 3.00 x 90% = 2.7 x 2 = 5.4 + 21 = 26.4.
100 divided by 25.4 = 3.9 x 90% = 3.5 x 2 = 7 + 19 = 26. 26.4 - 26 = .4 divided by 2 = .2.
So you can see that the ride height depending on which way you swap only changes at most 1/2 inch. Going from the 16 inch rear to the 18, the 19 front to the 21 adds the 1/2 inch. Going from the 18 inch rear to the 16 inch rear, going from the 19 to 21 front will lower the bike .1 inches.
To all you hard tailers put what ever tires and wheels you want, it won't effect the stance much if any.
If the bike has 2.5 inches of ground clearance the drop is closer to 3.4 inches. Stock ride hieght, 5.9 - the drop, 3.4 gives the ride height, 2.5. If you have the ride height subtract that from the stock ride hieght gives you the drop.
As James said the rake changing from 27* to 38* will lower the ride height. I don't know the math on that. That gets into geometry.

 

[projectxs650]

Alot of numbers, but for the most part I understood that (to my own amazement lol). So with those calculations.....a stock rake of 27 degrees and a drop in the tail of 3.4'' would give that bike its 2.5 ground clearance......did i interpret that correctly? yay or nay?

So then the math would be thrown off when you factor in that 38degree rake.....which would lower it even more than right? and more so because he has 21'' front.

I would say that its being compensated by the large wheel, but by your math that wouldn't make much of a difference (if i understood it right)....so then is it possible, that that additional 11 degree rake (27 to 38) could be responsible for the low ground clearance of 2.5''? and that the hardtail could actually have no drop in the tail?

 

[XSLeo]

Yes, on the 3.4 giving the 2.5 ground clearance.
Yes, the increase in the rake will lower the bike more than just the drop of the hardtail.
The 21 inch front won't make enough difference to count. Less than 1/2 inch. As I said the tire combo's only effect the ride height a small amount. Raking or shortening the forks has a much greater effect than the tire size.
I doubt the hard tail is a zero drop. Many are a two inch drop. With the drop of the hardtail and the increase in rake is why the bike sets so low.
You have the basics down. Just think about it and you will understand it better.

 

[projectxs650]

Thanks a ton, time to figure this out. I'm sure they'res some program out there that could calculate the change when plugin the number and the diffrences in rake degree

 

[member3480]

OK you young guys lost me in the numbers HAHA
What are you trying to end up with????
If you are looking for a certain hight i would build everything on a jig and tack it then take it off and mock it up to see the ride hight. All hard tails will change the rake by some and if you wanted zero rake you are better off with a universal kit . There are many different DROP SEAT bike pictures out there and look very different. David Bird makes one and its short looks good. Mine are long and low because i don't like that short look (JUST ME). I just finished up one yesterday and got it as a roller and i will measure rake and hight for you and post it with picture. It has 19 front 16 year NO MOTOR IN IT so it will drop a little.

 

[XSLeo]

Project posted a pic and was asking how it got a 2.5 inch ride height. I was just explaining a bit of the math on how much the hardtails drop effects the ride height. He also asked about wheels. 19 vs 21 fronts and 16 vs 18 rears. With a bit of calculations to determine tire heights you can see that the wheel and tire size won't effect the ride height much. The rake will effect the ride height much more.
The drop and tire sizes are easy to figure. Changing the rake takes a bit more than simple math to calculate.
I don't know just how to figure that.

 

[54inches]

Soooo, to make a long story longer what is about the lowest you can get one of these bikes using the stock rake and forks(dropped) and the hardtail offerings on the market?

Can you get one of these babies about 2" off the ground?