# Factors for switching 27-inch wheels to 700c...



## ckilner (Oct 4, 2004)

I considered posting this in the wheels section, but since it mostly effects older bikes, I posted this here.

I've played around over the years with using 700c wheelsets in frames that originally came with 27-inch wheels, with varying degrees of success. I'm interested in hearing whether the experience of others tracks with my experience (mostly to figure out if it is worth the trouble of finding a 27-inch fixed gear wheelset for an old frame I might "fix").

In my experience, higher-end touring bikes (that typically have low bottom brackets and normal/long crank arms) suffer from poor handling when lowered 4mm - and have problems with clipping pedals when pedalling through turns.

Lower-end bike boom bikes targeted at casual cyclists (that typically had higher bottom brackets due to less BB drop, and often came with shorter crank arms to avoid clipping pedals in turns) seem to handle a little more like a higher end bike with 700c wheels - although this perception might just be the difference between boat anchor steel wheels and quality aluminum wheels.

While I've found older Treks, Raleighs, Bianchis, and Nishikis follow these trends, other brands like Motobecane and Schwinn vary depending on model and year.

I've also found that using taller profile tires on the 700c wheelsets (i.e. Roll-y Pol-ys or larger tires) also seems to minimize the issues, but that the overall change in BB height will usually be close to 1cm (10mm) instead of the theoretical 4mm due to the typically skinnier tires used with 700c wheels. 

And finally, I've had better results with switching from 27-inch to tubular wheelsets since it seems that tubular tires tend to have a higher profile that ends up being closer to the outer tire diameter on the original 27-inch tires.

Are these observations close to what others have found?


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## Reynolds531 (Nov 8, 2002)

When I've converted bikes form 27" to 700C wheels, I've always used better quality 32 or 35 mm tires. The bikes have always ridden bettter with the 700C wheels, which I attribute completely to the better tires. I don't think that 4 mm difference in rim radius makes any difference in handling or ride.


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## ckilner (Oct 4, 2004)

A smaller outer diameter of a wheel will reduce trail by deltaD/2*Tan(90 - head angle), so for slacker head angles, the trail is reduced more relative to the change in outer wheel circumference. Reduced trail will quicken the handling.

Also, if the bottom bracket height of the bike w/ 27-inch wheels is already in the low (10 inch range), then lowering it further will cause pedal-strike problems.

Conversely, if the bottom bracket height of the bike w/ 27-inch wheels is in the high (11 inch range), then lowering it will make it feel like more like typical bikes in the 10.5 inch range.

Perhaps my varied experience of bikes riding better and worse after switching to 700c wheels is related to whether the bikes were already close to the edge of acceptable design (i.e., trail and BB height) or not.


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## Reynolds531 (Nov 8, 2002)

ckilner said:


> A smaller outer diameter of a wheel will reduce trail by deltaD/2*Tan(90 - head angle), so for slacker head angles, the trail is reduced more relative to the change in outer wheel circumference. Reduced trail will quicken the handling.
> 
> Also, if the bottom bracket height of the bike w/ 27-inch wheels is already in the low (10 inch range), then lowering it further will cause pedal-strike problems.
> 
> ...


I think you are wrong about the trail. Trail = (wheel radius * Cos(head tube angle) - fork rake)/Sin(head tube angle)

With a 74 degree headtube and 45 mm fork rake and 340 mm radius wheel (630 rim + 25 mm tire) the trail is 50.6 mm. If you put on a smaller 336 radius wheeel the trail is only reduced to 49.5 mm.

With a 72 degree headtube and 43 mm fork rake, trail is 65.2 mm for the 340 mm radius wheel and 63.9 mm for the 336 mm wheel.

You could certainaly tell a differenc ebetween a bike with 50 mm of trail and one with 65 mm of trail, but you can't tel a difference between 50 mm and 51 mm of trail or between 64 mm and 65 mm trail.

You're also not going to be able to tell if you lower the bottom bracket by 4 mm.


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## ckilner (Oct 4, 2004)

Reynolds531 said:


> I think you are wrong about the trail. QUOTE]
> 
> My equation was for the delta (the difference) in trail, not for trail itself.
> 
> ...


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## curlybike (Jan 23, 2002)

If your brakes won't reach the rim. you had better not ride it! This I know to be correct.


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## ckilner (Oct 4, 2004)

Most of my conversions have just needed the brake pads lowered (and once, I had to file the slot to make it reach). On the few that didn't reach the rim, I used a slotted plate to lower the brakes in a manner similar to a drop bolt until I found appropriate longer reach brakes.


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## Sixty Fiver (Jul 7, 2007)

There were some very good 27 inch wheels made and not all of them were steel... I run some very nice Araya 27's on my '62 Peugeot and the bearing quality and strength of the rims and hubs is second to none.

The original steel Rigidas were very nice wheels and not inordinately heavy but being steel really fell short in the braking department.

Brake reach seemes to be the biggest issue and as stated...the outer duiameter of a 700:25 and a 27 by 1 1/4 wheel and tire is nominal... the 700:38 Schwalbes that came on my Trek 7500 had a significantly larger outer diameter than a 27 by 1 1/4 wheel / tire.


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