Gearing change and wheelbase on a Single Speed…..

[sevenfourate]

Single Speed BMX with micro gearing: 30t front sprocket and 11t rear driver.

I want to lower the gearing / give it a bit more zip / reduce top speed.

If I remove 2 teeth on the the front sprocket OR add a 13t rear driver - along with a chain 2 links longer………will this mean in theory the rear wheel will still be in exactly the same place in the rear dropouts ?

 

[silva]

Adding teeth to front sprocket = increasing gear
Adding teeth to rear = decreasing gear
So in your case, leave the front sprocket, and 11t > 13t ?
Also, a tooth more or less differs between front and rear sprocket.
For ex, a case 3/1, rear + 2 teeth = front - 6 teeth.

 

[sevenfourate]

Adding teeth to front sprocket = increasing gear
Adding teeth to rear = decreasing gear
So in your case, leave the front sprocket, and 11t > 13t ?
Also, a tooth more or less differs between front and rear sprocket.
For ex, a case 3/1, rear + 2 teeth = front - 6 teeth.

Right. Got it…..thank you 🙏

I’d also forgotten about front / rear ratio when changing 🤦‍♂️ I’d like to make a fairly big change…..so as suggested adding 2 to the rear might be the best way to go.

So hypothetically: I’ve added 2 teeth to the rear. And fitted a chain 2 links longer:

Does that mean essentially the rear wheel will be in the same place in the dropouts / wheelbase won’t change ?

 

[Sharky]

I'm guessing that your rear dropouts are horizontal allowing the wheel to be moved fore/aft to take up any slack in the chain. You only need to add links if unable to move the wheel forward enough.

On my single speed road bike, I can cope with sprockets from 13 to 18, just by moving the wheel fore/aft and keeping the same chain length.

 

[sevenfourate]

I'm guessing that your rear dropouts are horizontal allowing the wheel to be moved fore/aft to take up any slack in the chain. You only need to add links if unable to move the wheel forward enough.

On my single speed road bike, I can cope with sprockets from 13 to 18, just by moving the wheel fore/aft and keeping the same chain length.

Thanks 🙏

I want to lower the gearing and keep exactly the same wheelbase / position of rear wheel in rear dropouts.

By definition of my requirements: I’ll have lost any adjustability via the dropouts. So all I have left to play with is gearing and chain length.

In my small brain - 2 teeth (On either front or rear) takes up 2 links of the chain. So if I add or remove 2 teeth [Rear and front respectively] and adjust the chain accordingly - nothing else ie: rear wheel position will need altering ?

 

[iandg]

You can still get 1/2 links if that helps

https://www.condorcycles.com/products/condor-1-2-x-1-8-inch-half-chain-link

 

[sevenfourate]

You can still get 1/2 links if that helps

https://www.condorcycles.com/products/condor-1-2-x-1-8-inch-half-chain-link

Cheers ! And complete 1/2 link chains of course:

https://www.customriders.com/bmx-pa...e=1&gbraid=0AAAAAD_iTgcqQtKuotYzmHkO7KpRaaLaj

…..Which is more often than not the ‘norm’ on these modern BMX’s with super short slots in the drop outs.

About to head to my LBS. I’ll inflict my question to them to get their thoughts. Maybe new gearing and a complete 1/2 link chain ? Would be as good of a chance as anything I can see or think of……

 

[silva]

Now 30t/11t=2.7
Wishes: lower gear equivalent to 2t rear, AND, same dropouts sleeves begin and end.
Requires gear change as much as can (teeth = stepwise not continuous) divided over front and rear sprocket. That is, sprocket diameters equally change, in opposite direction.
https://www.nitrochain.com/sprocket-diameters
1 tooth more/less = sprocket radius +/- 2 mm
So that the distance changes compensate eachother and thus do not alter the start/new chain position of the rear wheel.
So, 29t/12t=2.4, 28t/13t=2.1, 27t/14t=1.9

In a past I saw industrial specs half link chains designated as half strength of normal outer/inner link chains. It is possible that to compensate this, half link chain plates are made twice as thick, and thus heavier and maybe also the price tag.

 

[Ajax Bay]

"BMX with micro gearing: 30t front sprocket and 11t rear driver" I want to lower the gearing and keep exactly the same wheelbase / position of rear wheel in rear dropouts. AND ". . . front / rear ratio when changing 🤦‍♂️ I’d like to make a fairly big change"

Two teeth less on either ring means only one tooth less engaged (so half a one inch link).
Ring radius decreases by 2mm per tooth (1/2" x 1/2pi) as @silva says.

To keep the distance between the rear of the rear sprocket and the front of the front ring the same (so axis of rear wheelwrt frame/dropout remains same: proxy = "wheelbase") increase the rear sprocket to 13t (4mm to rear) and decrease the ring to 28t (4mm to rear). Gives a 42" gear (vice 54" one currently).

The 'tension' and 'return' sections of the chain therefore remain the same length.
The two sections engaging the sprocket and ring both reduce by 1/2" (1/2 of two teeth). Take a link out of the current chain.
29/12 is possible (as said but not "a fairly big change") and/but needs a half link. Avoidance adheres to the KISS principle.

This is all arithmetic: I have zero practical experience of this. And it is Monday morning (so maybe too late if LBS was open Saturday).

 

[sevenfourate]

Two teeth less on either ring means only one tooth less engaged (so half a one inch link).
Ring radius decreases by 2mm per tooth (1/2" x 1/2pi) as @silva says.

To keep the distance between the rear of the rear sprocket and the front of the front ring the same (so axis of rear wheelwrt frame/dropout remains same: proxy = "wheelbase") increase the rear sprocket to 13t (4mm to rear) and decrease the ring to 28t (4mm to rear). Gives a 42" gear (vice 54" one currently).

The 'tension' and 'return' sections of the chain therefore remain the same length.
The two sections engaging the sprocket and ring both reduce by 1/2" (1/2 of two teeth). Take a link out of the current chain.
29/12 is possible (as said but not "a fairly big change") and/but needs a half link. Avoidance adheres to the KISS principle.

This is all arithmetic: I have zero practical experience of this. And it is Monday morning (so maybe too late if LBS was open Saturday).

Awesome. Thanks muchly 🙏

In reality…….

Polished Silver sprockets were in limited options for the front. And the rear driver (Free hub - so rear sprocket just fixes onto the hub and isn’t a freewheel per-se) looks to be a specific fitment to the FIT (American made) rear hub.

So……to make a noticeable change from the current 30t front: I’ve gone for a 25t on the front. Leaving the 11t at the rear. I mocked up a front of the smaller physical size - and I should have no problem with the new chain line on the stays etc.

And I’ve bought a complete 1/2 link chain - to give me every chance of being able to be as close to where I want with the rear wheel in the dropouts.

We’ll see soon enough !!

 

[Gunk]

Expensive morning!

 

[Ajax Bay]

"I want to . . . keep exactly the same wheelbase / position of rear wheel in rear dropouts."

I’ve bought a [25t front 'sprocket' and a] complete 1/2 link chain - to give me every chance of being able to be as close to where I want with the rear wheel in the dropouts.

The 'tension' and 'return' sections of the chain will remain the same length IF the rear axle stays in the same place.
With no change to the 11t sprocket the length of the chain round it remains 2.75".
Currently the wrap round half the chainring is 7.5": with a 25t it'll reduce to 6.25" a reduction of 1.25".
A half link chain ain't going to allow you to keep the rear axle in the same place: it'll have to move 3.2mm (1/8") backwards or forwards.
This is all arithmetic: I have zero practical experience of this. And it is Monday morning (and I should be riding).

 

[sevenfourate]

"I want to . . . keep exactly the same wheelbase / position of rear wheel in rear dropouts."

The 'tension' and 'return' sections of the chain will remain the same length IF the rear axle stays in the same place.
With no change to the 11t sprocket the length of the chain round it remains 2.75".
Currently the wrap round half the chainring is 7.5": with a 25t it'll reduce to 6.25" a reduction of 1.25".
A half link chain ain't going to allow you to keep the rear axle in the same place: it'll have to move 3.2mm (1/8") backwards or forwards.
This is all arithmetic: I have zero practical experience of this. And it is Monday morning (and I should be riding).

I’ll be absolutely sure to let you know how the reality stacks up versus the raw numbers !

In the USA (Or with patience and an ‘unlimited budget’; the doing may have been more precise). The reality of what gearing was readily available ‘over here’ turned out to be more the overriding factor……🙄

 

[silva]

On the world wide web:

To calculate the chain length:
https://biketips.com/bike-chain-length-guide/

L = 2 (C) + (F/4 + R/4 + 1)

• L = Chain length in inches
• C = Chain stay length in inches
• F= Number of teeth on biggest front chainring
• R= Number of teeth on biggest back cog.

Another more detailed:
https://www.engineersedge.com/calculators/sprocket_chain_length__15580.htm

The unknown is C, something you need to measure.
This is the 25T 11T section of a pre-calculated table for bmx:
https://bmxchaincalc.com/

Sprocket	Driver	Chainstay	Exact	Chain	Halflink

25T	11T	12.50"	12.45"	68 Links	No
25T	11T	12.75"	12.70"	69 Links	Yes
25T	11T	13.00"	12.95"	70 Links	No
25T	11T	13.25"	13.20"	71 Links	Yes
25T	11T	13.50"	13.45"	72 Links	No
25T	11T	13.75"	13.70"	73 Links	Yes
25T	11T	14.00"	13.96"	74 Links	No
25T	11T	14.25"	14.21"	75 Links	Yes
25T	11T	14.50"	14.46"	76 Links	No
25T	11T	14.75"	14.71"	77 Links	Yes

Basically, the need half link or not depends on Chainstay, if it's not a multiple of .50" (=odd amount links), half link chain is required.

Note: the difference between Chainstay / Exact columns is no rounding off, the first is the next bigger step, because smaller can result in position before begin of sleeve = unable to get wheel in. That begin position is, as said before here, the most important.

Don't throw away the current (non half link) chain yet, it can help as a length (=amount links) reference / confirm measurements / table values.
Count its links, measure Chainstay length, and verify:

30T	11T	12.50"	12.53"	71 Links	Yes
30T	11T	12.75"	12.79"	72 Links	No
30T	11T	13.00"	13.04"	73 Links	Yes
30T	11T	13.25"	13.29"	74 Links	No
30T	11T	13.50"	13.54"	75 Links	Yes
30T	11T	13.75"	13.79"	76 Links	No
30T	11T	14.00"	14.04"	77 Links	Yes
30T	11T	14.25"	14.30"	78 Links	No
30T	11T	14.50"	14.55"	79 Links	Yes
30T	11T	14.75"	14.80"	80 Links	No

... it should deliver a "No" on half link chain.

 

[silva]

Expensive morning!

According to:
https://kmcchain.us/products/hl1-wide
100 links weight 384 gr = 3.84 per link.
price €26.95 = 0.2695 per link

https://kmcchain.us/products/k1_wide
112 links weight 387 gr = 3.45 per link
price €14.95 = 0.1335 per link
A half link weights 3.84-3.45=0.39=11% more.
The price - half the link = double the price = 100% more

That "wide" applies to the inner width, the 1/8" standard.
So, the rollers are same width, what remains is thicker plates = longer pins.
So the "outer" width of the chain must have increased, which makes it possible (doesn't have to, just the chance is there) the link doesn't fit in a chain splitter tool that worked before on outer/inner plates chains, and another expensive morning.
The key is whether or not half link will prove to be necessary or not. It wasn't necessary for 30/11.
The table for 30/11 shows a length range 71-80.
The table for 25/11 shows a length range 68-77.
That's 3 links less, an odd figure, which suggests half link will be needed, in which case expensive mornings will become the new normal.