# Thread: difference in diameter better 24inch(25-540) and 26inch(25-590) wheels

1. Originally Posted by rob1
@Grunt so when moving from 26inch wheels to 24inch wheels, reducing my seat height by the difference in wheel height between 26inch and 24inch wheels (50mm), is how to keep my push mechanics the same with my method of pushing mostly on the apex of the wheel.

Is that correct?

Thanks,
Rob
No. Reducing it by half the difference maintains the same arm to hub/pushrim/top of wheel mechanics. Because the wheel is connected to the chair in the center of the wheel, only half the difference translates into changing how far the top of the wheel moves down. We push from the top of the wheel.

Edit: Connect the 2 wheels together with an axle pin. Then you can measure the exact difference between where each wheel top dead center will be. This will account for any tire size differences as well. This measurement will be very close to half the distance if you measure them with each wheel just sitting on the ground. Exactly half if the tires are the same

2. Also, changing your rear seat height will require changing the backrest angle and front height to get exactly back to the same relative seating position and pushing mechanics. It's an integrated system. Changing one thing alters the whole.

3. So, even though I'm only really pushing the top centre of the wheel, I should only reduce the seat heights by 25mm, is there not a difference because I'm only pushing the top centre of the wheel?

Yes the front seat height will be lowered by the same amount as it's for a new chair.

Why would the backrest angle need to be changed if front seat height and rear seat height were lowered by the same amount?

Thanks,
Rob

4. Right, backrest and caster angles would be same if doing both ends. It's as I said, it's because only half the difference in total diameter generates the drop in height because the wheels attach at the middle, not the bottom. Only half the wheel is above the middle, so only half the difference applies. That's a colloquial way of describing what is really a mathematical proof.

E.g. Every 1" of wheel diameter difference creates .5" of ride height change.

5. Originally Posted by rob1
------------------------------------------------------

Why would the backrest angle need to be changed if front seat height and rear seat height were lowered by the same amount? Thanks, Rob
If you changing your wheel diameter only. The backrest angle will change regardless of raising or lowering your seat height equally front & rear. Here is the extreme example that will enable you to see.
Drop your wheels off your chair and set it down. The backrest angle can be seen to have changed relative to horizontal, of course it is the same relative to the seat base but that is always true. Cause and effect.

6. A tire size related experience I am having right now - I purchased a set of tires on line that were described as 26" x 1.00" (25-559). I was fixated on the 26 x 1.00 part, so the 25-559 part didn't register until I went to put them on and they did not fit my 26" rims. Am I crazy for thinking that the description should have read 25" x 1.00"? I am trying to get the vendor to let me exchange them, but have not received a reply yet. All on me for not catching the 25-559 or should they have some responsibility for listing as 26 x 1.00?

7. Originally Posted by tw
A tire size related experience I am having right now - I purchased a set of tires on line that were described as 26" x 1.00" (25-559). I was fixated on the 26 x 1.00 part, so the 25-559 part didn't register until I went to put them on and they did not fit my 26" rims. Am I crazy for thinking that the description should have read 25" x 1.00"? I am trying to get the vendor to let me exchange them, but have not received a reply yet. All on me for not catching the 25-559 or should they have some responsibility for listing as 26 x 1.00?
Nowhere in the bicycle world is there a 25" wheel/tire. If they were selling them as "wheelchair tires" then they might have listed them as 25"x1.00". The best way to purchase wheelchair tires is by using the metric https://www.schwalbe.com/en/groessenbezeichnung.html ETRTO size. Mountain bike wheels are 559's yet labeled as 26". It's stupid but it's the way it is.

8. Backrest specs are measured to the floor, not the seat base. Raising the front and rear by the same amount doesn't change back angle to floor.

Originally Posted by slow_runner
If you changing your wheel diameter only. The backrest angle will change regardless of raising or lowering your seat height equally front & rear. Here is the extreme example that will enable you to see.
Drop your wheels off your chair and set it down. The backrest angle can be seen to have changed relative to horizontal, of course it is the same relative to the seat base but that is always true. Cause and effect.

9. Originally Posted by Oddity
Backrest specs are measured to the floor, not the seat base. Raising the front and rear by the same amount doesn't change back angle to floor.
Yes, but not if you are changing your (rear) wheel diameter only.

Originally Posted by tw
A tire size related experience I am having right now ------------------------------------ All on me for not catching the 25-559 or should they have some responsibility for listing as 26 x 1.00?
Any reasonable outlet should exchange unused/fitted tyres.Your mistake/oversight has occured before and will again.
What are the ETRTO numbers on your original tyres?
Thankfully a standard reference was introduced and is now followed by most(?) tyre manufacturers. ETRTO gives real choice.
That old sizing system veritably held consumers to the manufacturers as a captive consumer; it can be confusing and misleading.
Not so when using the ETRTO standard.

10. Cheers for all the info.

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