Why do so many ss bikes have slack chains?

[Ian H]

How do you get down the hills in Devon, drag a brake? I'm spinning out on 45x16 at 32mph-ish spinning up to 160rpm.

I have a 17t on the other side. Some years ago I clocked 42mph downhill on that. I think I worked it out to 212rpm. Couldn't get that high nowadays.

 

[silva]

It's still hard to imagine how a chainrings mount can go so offcenter that it results in a noticable chain tension variation.
Any idea about what kinda bias this is about?
Take for ex a 5 bolts mount, tolerances tend to be random, so that they all together cause a net offset in a particular direction, appears rather unlikely to me.

 

[rogerzilla]

If you loosen the bolts, you'll find the chainring can usually move quite a bit in any direction you want. Some are much tighter but it is a fitting that generally has a lot of tolerance built in. Rarely, you will find a chainring that won't fit onto a spider (Andel track cranks are rather "tight" in this respect and not all 144BCD rings actually fit), and the manufacturers want to avoid that.

 

[Deleted member 1258]

It's still hard to imagine how a chainrings mount can go so offcenter that it results in a noticable chain tension variation.
Any idea about what kinda bias this is about?
Take for ex a 5 bolts mount, tolerances tend to be random, so that they all together cause a net offset in a particular direction, appears rather unlikely to me.

With a single speed/fixed transmission it doesn't have to be much to produce slack/tight spots in the chain, a chaining can be within manufacturing tolerances but still not be exactly round, the same with the spider, it may be within manufacturing tolerances but not be exactly the shape its supposed to be, a couple of mm out here and there is all it takes. When a chainring is attatched to a spider there is usually room for a small amount of movement.

 

[silva]

I'm riding since about a decade singlespeed and later fixed gear, 3 different bicycles, had to tension my chain numerous times over that period, and never noticed any tension variation noticable to me and no I'm not blind heh.
It's only since my latest fixed gear, but as explained, it's certainly not the chainring mount that is the (by far) main reason, it's the 48/16.
Maybe a remaining question: my plan is thus to change to 47, so that there is a rotation of chain links engaging teeth at peak force moments - I wonder is there any way to quantify the wear spreading. For ex my 48/16 could be quantified as a zero spreading / worst case scenario.
Because, it's an annoyant problem, you feel it when pedaling and I'm forced to tension abit too much on the least loose point, without my chain hangs way too lose on the most loose point, risking falling off.

Adding another question about chainrings offcenter, don't they "recenter" too?
Because, if forces cause them to move abit on their mount, then forces can bring them back, at least in the case of fixed gear, when one can put force in the opposite direction.
This is also something that you may feel underway.

 

[rogerzilla]

I don't think a chainring will move once the bolts are tightened, at least not unless your chain has such a tight spot that it also wrecks your rear hub bearings.

 

[silva]

When the bolts are in place (so already before actual tightening), the position of the chainring is already fixed by reeds on the spider arms, with quite small tolerance, certainly not more than 1 mm (on my bikes its with bare sight metal on metal).
Question now is how much tolerance translates to chain tension variation. A mathematical question that is.

 

[rogerzilla]

You'd need to use catenary equations for that, but 1mm less distance between chainring and sprocket is a fair bit of sag. If you run a chain until it has the maximum recommended 1/16" of elongation per foot, it will be in danger of falling off the chainring, and that's only about 2mm over a typical chainstay length.

 

[Ajax Bay]

the chain tensioning problem is due to no derailer/tensioner (springbased), a singlespeed can have one but a fixed gear not, reason being ability to resist pedals in order to slowdown which would wreck the tensioners mount due to force direction being opposite. I wonder if there really is no solution for this.

This has been posted in the mechanical thread. But people here will be able to comment with more understanding.
Is (satisfactory for operation) chain tension a significant problem and why is there "really no solution"?

 

[Waterwheel]

A slack chain on a SS isn't especially dangerous. It can be on a fixie; the worst-case scenario is that the chain jumps off and gets caught up on the pedal spindle, which causes the bike to fold up as the rear triangle is pulled over.

I had that problem loads of time on a fixed gear bike. By the way some single speed bikes come with a chain tensioner similar to a derailleur. I think that is a great idea as then the chain is not too tight or too slack. An over tight chain will wear out too quickly and a slack chain risks coming off the chain wheel and causing an accident. Incidentally I found that single speed bike chains last far long than derailleur bike chains. A hell of a lot longer. For example I have just replace my derailleur bike chain after just two months. With a single speed bike I used to get up to 8 months wear out of it.