20 inch versus 26 inch wheels

[jorgemartin]

Hello, I have a HP Velotechnik Grasshopper (FX). I love this bike but I lag behind my partner when we ride together. She has a conventional 26 inch trekking bike (both of us have 1.75 inch wide tyres). I was considering swapping the Grasshopper for a Street Machine. The Street Machine has a 26 inch rear wheel and a 20inch front wheel. But... is it worth the effort? Would the speed difference be noticeable?

 

[squeaker]

I was considering swapping the Grasshopper for a Street Machine. .............. Would the speed difference be noticeable?

Can't say definitely (I have an OSS Grasshopper, not tried a USS Street Machine, but would have thought not. I would try some faster tyres on the Grasshopper first (e.g. Marathon Racers), before going to the hassle of a commercial bike swap, and if the latter I'd try and find a Speed Machine

 

[ufkacbln]

The problem is the gearing.

I have a Gekko with 20" and a Catrike with 26", and the latter is faster, but I tend to spin out on the Gekko which is what limits the speed.

My Street Machine is again faster than the Gekko, but slower than the Catrike.

The speed machine for acceleration and speed is the Catrike, but in second place is the 20" wheeled Challenge Hurricane which is faster than the 26" Street Machine... mainly due to the custom 65 tooth chain ring which raises the gearing.

 

[arallsopp]

Agreed. Gearing and rubber is the only real difference. My Streetmachine takes more effort than the Furai to get to 26mph, but tops out a tad faster. They have similar input gear ratios, so the extra 2" diameter comes into play.
Put a dinner plate on the front of your grasshopper, and it'll be largely identical to its big sister.

 

[jorgemartin]

If I get a bigger chainring, I'll go substantially faster? Is this a particular kind of chainring? How do I get one? By the way, I have the SRam Dual Drive gearing system.

 

[arallsopp]

What do you have at the moment? I'm guessing an 11-34 rear cassette, 3 speed hub and 42T front. The single ring at the front is easy to replace as you don't need to worry about ramping pins, derailleur angles and capacities.
Any single speed chainwheel with the correct BCD will probably fit. You might need to lengthen the chain a little if adding a lot of teeth.
In terms of speed, yes, if you can pedal at the same speed, you will go faster.
The above gearing is the stock config for my 26". I find it way lower than needed. The dual drive gives a massive range, so I haven't bothered to change mine.

 

[jorgemartin]

Yep. My current set up is 11-34 and a 42T front chain ring. Should I go for a 48t?

 

[arallsopp]

Two questions:
- what's your target cadence- what's your target top speed
With those known, and a squizz at sheldon brown, you'll know the size you need

 

[jorgemartin]

Hmmm, not sure what target cadence is... or top speed for that matter. I'd be happy going 15% faster so I can keep up with my girlfriend's bike.

 

[arallsopp]

Well, yes, 15% more teeth will do that

 

[Riding in Circles]

What is the engine like?

 

[jorgemartin]

I went to my Local Recumbent Shop and they told me that I had two options:
- Upgrading to a 52T chainring = £78 (13% more efficient than what I have now)
- Upgrading to a 60T chainring = £178 (30% more efficient)
The second option is a lot more expensive because it involves replacing the whole crank set. My current set up is is a 46 T chainring (not 42 as I thought) a 5 bolt 110 mm BCD (the 60 T chainring is for 130 mm BCD) (and the SRAM dual drive with a 11-34 rear cassette).
These parts come from HP Velotechnik and have a safety ring or guard. The guy from the shop told me that these are mandatory.
I was thinking of sourcing a chainring from another place but I don't seem to be able to find a chain guard that is large enough to accommodate 52-60 T chainrings.
Any ideas/suggestions?
PS Is a bigger chainring going to make it more difficult to go uphill?
ie 52T chainring= 13% more difficult?

 

[Night Train]

Anything that riases the overall gear ratio will make it harder to climb hills.

Maybe what you need is to wident he gear ratios by increasing the top end without losing the bottom end.

Could you switch to a double front chainring with a deraileur? You can then have your larger chainring for speed and the smaller one for hills.

I added a triple to the front of my old Ratrike and it is is a lot more 'useful' to have 'faster' and 'slower' gears.

 

[arallsopp]

First, if your local recumbent shop said a big ring was more 'efficient', their advice can probably be discarded.
Second, I'm not aware of any legislation in the UK mandating a chainring guard, outside of HPV racing. Neither of my 'bents has a guard on, and one is a 2008 HPVelotechnik in stock configuration.
For that money, you might want to look at a capreo hub, or a TA / Stronglight chainring, which tend to come in a variety of sizes.

 

[jorgemartin]

So speed and efficiency are different things? If having a chain guard is not mandatory, then I don't see why I need to source a super-expensive chainring from HP Velotechnik... Is there anything to be aware of when I buy a new chainring? Does it have to single speed? Having a double chain wheel is a good idea, but involves a lot of work and expense... Perhaps having a 3 speed hub plus a 8-9 speed cassette plus a front derailleur is an overkill?