More travel is better, of course. But will more travel fit in my frame? Older bikes in particular often still have dropper posts with 150 mm travel or even less. If the desire for a longer dropper post arises, you should first ask yourself: Will a longer dropper post fit in my frame? Using a simple example and the Fox Transfer with 200 mm stroke, we show you how to determine the ideal length.
1 - Diameter seat tube
The correct dimension for the telescopic seat post is usually printed on the seat post currently fitted. If this is not the case, you can simply measure the inner diameter of the seat tube with a folding rule or calipers. This is often 30.9 or 31.6 mm. Most seat posts are also available with these dimensions. Seat tubes with a diameter of 34.9 mm are rarer. Many race bikes are fitted with slim seat posts measuring 27.2 mm. Manufacturers are also increasingly offering particularly lightweight seat posts with a small diameter.
We recently tested the new Fox Transfer dropper post. Our detailed report on the stylish support with Kashima coating can also be found on our website.
2 - The maximum stroke
To find out the maximum amount of travel you can use on your bike, you need to take a few measurements. To do this, adjust the saddle to the optimum seat height for pedalling and measure the Distance between the top point of the saddle clamp and the centre of the saddle frame (1). This value indicates the ride height, in our example 258 mm.
The second important value is the Stack height (2) of the desired seat post. This indicates how far the post protrudes beyond the frame when fully lowered. It is measured from the underside of the clamping ring to the centre of the saddle frame (2). The value is often also specified by the manufacturer.
The maximum possible stroke of the new support is calculated as follows:
Driving height - stack height = maximum possible lift
258 - 40 = 218 mm
3 - The maximum insertion depth
The third step is to determine whether the frame also offers space for the desired stroke. Kinks in the seat tube or pivot points in the chassis can reduce the insertion depth. The best way to determine the maximum insertion depth is to use a long standard seatpost. Insert this as far as possible into the frame and mark the post at the height of the seat clamp. Then measure the Distance from the marking to the lower end of the support and obtains the maximum insertion depth (255 mm in our case). Of course, you can also do this with a folding rule - but it may be a little less accurate.
Our desired post offers 200 mm travel and a maximum insertion depth of 292 mm - the manufacturer provides this information, which should be checked before purchase. This makes it 37 mm longer than the space in our frame, so it would protrude 37 mm beyond the seat clamp.
How to check whether the desired support would fit
Length of stroke + the stack of the new model + the height of the exposed part of the post above the seat clamp
200 + 40 + 37 = 272 mm
The result: with a projection of 277 mm, the desired support is too long for our ride height of 258 mm.
However, the slightly shorter version with 175 mm stroke and 260 mm insertion depth would fit.
175 + 40 + 2 = 217 mm
Important: In many cases, manufacturers specify the maximum insertion depth without the release units at the lower end of the supports. In addition, you should always leave some space in the frame for clean and kink-free cable routing.
Stefan Frey
Editor
