Search found 984 matches

by BigG
3 May 2019, 2:39pm
Forum: Does anyone know … ?
Topic: How to calculate metres travelled per crank revolution?
Replies: 22
Views: 4876

Re: How to calculate metres travelled per crank revolution?

A truly accurate measurement is difficult by the system you suggest as your wheel diameter can only be approximated. Its true rolling diameter depends not only on the size of the tyre but also on its pressure, your weight (or the proportion of this that is carried on the back wheel) and the amount the tyre is compressed when you are riding. It is probably accurate enough to add twice the tyre width to the 622 mm rim diameter and then subtract about 3 mm for local flattening. This gives a total of about 2.1 m with a 25 mm tyre and any figure using an additional decimal place is unlikely to give a better result. Using a 28 mm tyre only increases the distance per wheel revolution by about 0.75%. The difference caused by riding in a slightly wobbly line compared with a ruler-straight one are nearly as great as this.
by BigG
15 Mar 2019, 5:42pm
Forum: Bikes & Bits – Technical section
Topic: cassette gear increments
Replies: 19
Views: 1774

Re: cassette gear increments

I agree theoretically at least with MickF and try to keep to a constant power input (constant cadence and constant pedal pressure}. However, it feels more natural to me to use a greater pedal pressure when climbing together with, if necessary, a slower cadence. This has the (unintentional) advantage of allowing much wider gear gaps to be used comfortably. For me, any gap less than 10% is not worth the bother and gaps of 13-14% (two teeth) in the most used range is fine. At the bottom end of the gear range when the basic ability to keep riding begins to override any performance considerations, even larger gaps of about 20% are (again for me) quite acceptable.
by BigG
10 Feb 2019, 5:37pm
Forum: Does anyone know … ?
Topic: Changing from 1st to 12th
Replies: 50
Views: 6660

Re: Changing from 1st to 12th

The gear drop between rings (as a percentage) will be roughly equivalent to a number of rear sprocket shifts. On my bikes it is carefully arranged to be almost exactly equal to 3 rear shifts. The technique in changing down is to first drop down one ring leading to an immediate rise in cadence, and then almost immediately shift up two gears at the back. The brief spinning of the pedals in mid shift is not a problem. The more difficult shift is the other way. Then the first shift for me is down 3 at the back again leading to a large increase in cadence, followed by immediately by the shift up on the chainrings. The high cadence and thus low chain tension at the time greatly aids this sometimes awkward shift. You may notice that this second sequence leaves me in the same gear as I started. This is useful if any momentum is lost in the change. A single shift up one at the rear restores matters.
by BigG
7 Jan 2019, 9:31am
Forum: Bikes & Bits – Technical section
Topic: 9 speed cassette: 12-36. Too spaced out for front double?
Replies: 40
Views: 3452

Re: 9 speed cassette: 12-36. Too spaced out for front double?

Like CJ,I am a little sceptical about the need for closely spaced gears for leisure cyclng. I often ride behind riders with closely spaced sprockets and regularly notice that when they change gear it is more often two or three cogs at a time than just one. Of course, the roads in Devon are seldom at a constant gradient for long, but there isn't usually much sign of trimming into exactly the "right" gear.
by BigG
27 Dec 2018, 10:32am
Forum: Bikes & Bits – Technical section
Topic: 9 speed cassette: 12-36. Too spaced out for front double?
Replies: 40
Views: 3452

Re: 9 speed cassette: 12-36. Too spaced out for front double?

I use an even wider 8-speed (11-13-16-19-23-28-34-41) cassette with a RD761SGS rear mech and have no trouble at all in indexed changing despite the mixture of cogs. The 16 to 34 are coherent and came from a 7-speed but now with 9-speed spacers on an 8-speed hub. The top pair are from a 9 speed and the 41 tooth just plain and unramped off ebay. The gaps will of course be too large for many and I use an old fashioned half step (44-40) to fill them. I still find that most of my changes are rear only with the front used for trimming when needed. On hilly mid-Devon roads I am seldom in one gear long enough to get round to fine tuning. Incidentally, I have a granny ring as well now that the hills are getting steeper each year. A 26" bottom gear from the double is no longer nearly low enough.
by BigG
22 Nov 2018, 10:34pm
Forum: Bikes & Bits – Technical section
Topic: Standard versus compact gears
Replies: 25
Views: 1709

Re: Standard versus compact gears

+1 for your 22/36. I ride a comparable 22/38 on my old Freddie Grubb (a 5 x 3 triple) and use it quite often on the mid-Devon hills. The low gear does nothing to increase my speed which is controlled by my heart and lungs, but it does allow me to remain seated with a cadence of 60 or more (about 3 mph) in much greater comfort than I would have pushing a higher gear up the same hill. It is kinder to the old joints and muscles and puts less strain on at least some parts of the old bike.
by BigG
7 Nov 2018, 8:07pm
Forum: Bikes & Bits – Technical section
Topic: Alpine double
Replies: 49
Views: 3594

Re: Alpine double

Apologies to reohn2 - my mistake. However his gear gaps do not really get larger in the lower gears. The 15T/17T gap is identical to the 30T/34T one at the bottom end.
by BigG
6 Nov 2018, 9:56am
Forum: Bikes & Bits – Technical section
Topic: Alpine double
Replies: 49
Views: 3594

Re: Alpine double

Despite reohn2's comments near the top of this thread, his preferred 9-speed cassette actually has its biggest gap at the top end from 11T to 13T, a gap of 18%. The biggest gap at the bottom end is from 26T to 30T, a gap of 16%. Keeping a reasonably constant percentage gap as he does gives the most subjectively even spread of gears. I would however go further and suggest that bigger percentage gaps are acceptable, even desirable, at the lower end. This results from the difference in the retarding forces that we face. In low gears the major retarding force is likely to be gravity which for a given slope is linear with speed. In high gears wind resistance is the main opposition and this increases much more quickly than speed making smaller gear gaps seem more appropriate. Shimano went a bit too far with their Megarange cassettes, but a more modest version of this works well. For myself, I stick to a half-step and granny in which the drop onto the small ring automatically doubles the gear gap. No to everyone's taste but it suits me.
by BigG
8 Jun 2018, 9:41am
Forum: Does anyone know … ?
Topic: How many shifter types do you have?
Replies: 37
Views: 2125

Re: How many shifter types do you have?

Just one! Once I discovered Suntour Command Shifters about 30 years ago I stopped looking. I have them fitted to all 3 of my bikes controlling a 5-speed Huret Duopar, a 6-speed Shimano XT 761 SGS and an 8-speed (9-speed spacing on a 7-speed hub) XT 761 SGS. The front mechs are all old Suntour reverse action friction only triples.
by BigG
28 Mar 2018, 7:59pm
Forum: Bikes & Bits – Technical section
Topic: Cheap as chains? Or too cheap?
Replies: 27
Views: 2980

Re: Cheap as chains? Or too cheap?

I paid about £4.00 each for my TAYA chains including postage (purchased three at one time at £2.95 ea. plus postage). These and their predecessors have always served me well although I prefer (and normally use) the KMC links. Mine are 7/8 speed versions (TA 50 or 51 IIRC). Whether the Wilco ones are the same is hard to know. I have asked a couple of times on this website whether any one has had any quality problems with TAYA chains, but have never had any reported.
by BigG
21 Mar 2018, 11:24pm
Forum: Does anyone know … ?
Topic: Misunderstood terminology
Replies: 86
Views: 7781

Re: Misunderstood terminology

Power most certainly does have an element (strictly a dimension) of time despite tatanab's statement. It is work that has no time component. The work done to climb a hill slowly (to allow air resistance to be ignored) is the same whether it takes 1 hour or 10 hours. It is the power that varies being 10 times greater in the faster climb. Tatanab's 100 Watts would require 100 Watt hours of work if maintained for 1 hour. It would require 1 kW hr if maintained for 10 hours. The whole purpose of variable gears is to allow us to match an acceptable work rate to an acceptable cadence.
by BigG
18 Mar 2018, 11:07pm
Forum: Does anyone know … ?
Topic: Misunderstood terminology
Replies: 86
Views: 7781

Re: Misunderstood terminology

MickF is close, but not quite right. Work is torque (or force) multiplied by distance. Power is work divided by time. These are not definitions, just indications of the calculation process.
by BigG
9 Mar 2018, 7:17pm
Forum: Bikes & Bits – Technical section
Topic: Cheap as chains? Or too cheap?
Replies: 27
Views: 2980

Re: Cheap as chains? Or too cheap?

mjr's comment on the quality of at least some of TAYA chains underlines the reputation that this company has with some cyclists. I have used TAYA chains for many years without any problems. They may not be the most long lasting but they have always given me good value. I have asked before on this forum for anyone with specific information to the contrary to let us all know. There have been no replies so theorigin of the reputation remains obscure.
by BigG
15 Feb 2018, 10:44pm
Forum: Bikes & Bits – Technical section
Topic: Front and rear derailleurs for my retirement present (to myself)
Replies: 15
Views: 2116

Re: Front and rear derailleurs for my retirement present (to myself)

You may find the Shimano RD761 SGS better than the 771 as it has the cable adjuster built in. It is usually available on ebay. In all other respects, the two types perform equally well.
by BigG
12 Feb 2018, 8:19pm
Forum: Bikes & Bits – Technical section
Topic: Spoke tension underbraking..
Replies: 14
Views: 1169

Re: Spoke tension underbraking..

Airsporter1st wrote:What I have difficulty with is understanding why there should be any difference between discs and rim brakes.

In the fact that rim brakes effectively stop the part in contact with the road, i.e. the tyre, still puts tension on the spokes because the braking force is transmitted through the spokes to the frame. In the case of discs, they are effectively fixed to the frame and thus braking force is transmitted through the spokes to the rim/tyre.

To me its all the same, but I am not daft enough to believe that's the case. I would therefore love to see a clear and concise explanation of the differences.

With a rim brake, most of the force is transmitted from the rim to the frame directly via the brake arms and brake mounting point, not through the spokes and hub. Because the braking point is about the same distance from the hub as the tyre/road contact, both apply a similar torque to the wheel and only a small balancing torque is transmitted through the spokes. It may help to think of the situation in static terms with the wheel simplified to a beam running from the hub to the tyre edge. The moment (torque) generated about the hub by the tyre is opposed to that generated by the brake block. These moments are almost balanced because they are at almost the same radius. Only a small residual torque is needed to be supplied by the hub through the spokes. The situation with hub brakes is reversed. Here the friction point is close to the hub axis and a very much larger braking force is required to transmit the required torque to the wheel rim. This is of course transmitted through the spokes.