21” front wheel from HP2 into GSA. An enduro-inspired build thread.

[gstom89]

In an ongoing effort to truly exploit everything the bike can offer and satisfy my urge for projects, I have been searching for a 21 inch front wheel to replace the 19 inch standard spoked one on the Twin Cam GSA.

Woody’s Wheel Works in the States does a custom built one which are by all accounts fantastic, but once you factor in postage and import taxes to the UK, it quickly becomes almost the same price as an entire bike for a full set of wheels…

Some other options are available in the UK but all at quite significant cost.

My plan is to use a HP2 Enduro front rim (thanks to @Edventure on here) which I’m pretty sure has the same spoke pattern as the GSA, and lace it into a GS hub (also thanks to @ballistic on here). This should allow OEM TPMS and ABS sensors to remain as well as the ‘OEM look’ to remain.

In terms of cost, the HP2 set of wheels was £600 and the GS front wheel £100. I think it will be a couple hundred to build (awaiting quotes) and then bearings, tyres and discs. Hopefully be less than a grand all in. We shall see! Also means have a spare set of wheels for track days etc, so def a good idea right?

Anyone who has done the same and has any advice much appreciated, and equally if anyone knows a decent spoked wheel builder in the Bath/Somerset area please let me know.

Particularly interested in front mudguard solutions with the larger wheel. Any ideas?

 

[PitaNaanRoti]

Steve Buckley did it on a 1250.
He used Bart Factory wheels with the rear narrower.
Rear needs to come up or front down otherwise the geometry is a long way out and awful on road.

 

[SnowMec]

A very good idea to build the 21" wheel with parts from HP2Enduro. I have tried to get hold of such a rim but have not succeeded. I chose instead to build my wheel with an Exel rim and special spokes intended for a standard hub. I bought the parts from wheelsandparts.se and built the wheel with a used original BMW K25 hub from ebay. My bike has Öhlins EC shock absorbers and I have the lower recommended preload in the front but I have not changed the height of the rear suspension. With the larger front wheel the front trail is increased a bit and the bike also gets a slightly flatter steering angle. I see both of these changes as an improvement when riding on loose roads. The bike feels much nicer with 21” wheel, I don't miss my R100GS as much now! The rim measures 1.85" x 21" and I have a TKC80 tire, 90/90-21. Although it might be somewhat less stable at high speed on an asphalt road it doesn't matter to me. I ride dirt roads / gravel roads and trails with my GSA and own better motorcycles if I like to ride highways at high speed.

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[SnowMec]

Since the radius of the new 21" wheel is about 20mm larger, there will not be much clearance between the stock front fender and the tire. There are a few different ways to solve this and many people simply choose to ride without a front fender. I rebuild my fender by cutting a hole where it touched the lower fork bridge and making new brackets, I have moved the fender up about 15mm which seems to be enough. If I have problems with the wheel getting stuck in the fender when I ride in mud, I will probably try to ride without a fender. We will see how i works when spring arrives here in northern Sweden

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[gstom89]

Since the radius of the new 21" wheel is about 20mm larger, there will not be much clearance between the stock front fender and the tire. There are a few different ways to solve this and many people simply choose to ride without a front fender. I rebuild my fender by cutting a hole where it touched the lower fork bridge and making new brackets, I have moved the fender up about 15mm which seems to be enough. If I have problems with the wheel getting stuck in the fender when I ride in mud, I will probably try to ride without a fender. We will see how i works when spring arrives here in northern Sweden

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Hi mate,

Thanks for the comprehensive answer and detailed pictures. The Exel rim option looked a good way to go but I am glad to be able to use tubeless tyres with the HP2 21” rim, easier repairs!

Yours looks awesome and glad to hear it handles well; that is my intent and I also have ohlins front & rear so hopefully able to dial-in to suit.

Nice solution with the fender.

Any issues with speedo or warning lights or anything for abs/TC? It’ll be disabled most of the time for off road.

 

[SnowMec]

My motorcycle does not have traction control but it does have ABS and it was not affected by the change to 21" wheel. With the reservation that I have only ridden a few short rides, it seems that nothing on my bike is affected by the wheel change. No warning lights and everything works as before. Of course I always turn off ABS when I ride off-road, otherwise you have no rear brake! I was also thinking of building with a spoke wheel rim but since these are hard to get hold of so that didn't happen. An R100GS rim would have worked but they are expensive or in poor condition. I couldn't get hold of any HP2 Enduro rims so I chose Exel. The company wheelsandparts.se had everything I needed and at a reasonable price. So I ride with tubetype tire up front. My bike doesn't have TPMS so I didn't need to think about it. I will ride with the stock 17" rear rim because I think it will provide well working geometry. Now my bike has slightly less than 120mm trail and maybe a degree more steering angle (or "headrace" angle) which is then about 26 degrees. If you compare it to an HP2E it is stated to have as much as 127mm trail and 30 degrees steering angle and also a significantly longer wheelbase. So my R1200GSA is still a "more nervous bike" in terms of geometry compered to a HP2E but that may be needed when it is much heavier. So I think my conversion to 21" wheel will work well!

 

[SnowMec]

I've now ridden about 100km with my new 21" front wheel and I'm very happy with how the bike performs.
It's more stable on gravel roads, the front wheel doesn't have as much of a tendency to dive on sandy roads and on asphalt it feels about the same as before.
The slower geometry feels like an advantage in all situations and the larger and narrower front wheel seems to be exactly what I prefer.

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[Berin]

I seem to recall Wunderlich used to do a 21” conversion for the GS.

 

[gstom89]

I've now ridden about 100km with my new 21" front wheel and I'm very happy with how the bike performs.
It's more stable on gravel roads, the front wheel doesn't have as much of a tendency to dive on sandy roads and on asphalt it feels about the same as before.
The slower geometry feels like an advantage in all situations and the larger and narrower front wheel seems to be exactly what I prefer.

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Epic. Looks good. After a while day on the TET yesterday I’m looking forward to getting these wheels built!

 

[gstom89]

Pleased to report this has been successful.

Tyres are Mitas Enduro Trail XT+ Dakar

The rear tyre is 140/80/17 so is almost as tall as an 18inch rim. The HP2 rear is narrower than the GS too.

ABS still works if you insist. No faults or warnings on dash. Looks OEM if you ask me. Just need a tidy mudguard solution.

Will report back after SW TET this weekend as to how it compares….

 

[botus]

ABS ought to be an issue

the system allows for quite a bit of discrepancy but a 21" wheel should be a long way outside the 10 to 12 % leeway (I see it only just clears that lower yoke/wishbone mount) but you are around 13.5% bigger stupid thing invented its own tyre size might only be 7.2% bigger

110/80-R19 vs 110/80-R21 Tire Comparison - Tire Size Calculator | Tacoma World

110/80-R21 tires are 2 inches (50.8 mm) larger in diameter than 110/80-R19 tires and the speedometer difference is 7.1%

www.tacomaworld.com

Harley used to do chopper stuff and when the factory did 21" front option the ABS threw teddies... I guess its on the start up self check ? and I'd have thought the linked brakes have a grumble too

and where it has ASC it should be really going silly - the idea is it wants to cut power if its understanding of wheel speeds are significantly different to its internal settings - will it mean it slips more before considering that as the rear spinning up too fast ?

As in you now have std = (offroad) Sport and then S is now = (offroad) sport plus ?

 

[gstom89]

Well I spent a good five hours today on the bike. One hour on A roads/dual carriageway and the rest on the TET, a mixture of sections in Dorset and Somerset.

The only perceptible difference on roads and dual carriageway was a lighter feeling to the front of the bike. Felt less planted and therefore less confidence to push corners (this is coming from someone who has taken the GS on track days when my race bike was unserviceable). I feel that a steering damper and suspension set up would aid. The tyres are obviously more off road focused than before so not really a fair comparison.

But that was all irrelevant once off road. My god what a difference - up there with the best mod as suspension. Particularly on very rocky sections that I struggled on previously the bike seemed to be much smoother and more stable and able to roll over trickier surfaces much more easily. Obvs dropped a few times but that is the nature of the TET. No issues with the triple clamp clearance. All felt fine.

TC/ABS was disabled for off-road and all seemed to work fine on the road with some emergency stops practiced. No issues at all from what I could tell. Very pleased.

 

[botus]

that front on the road is going to feel strange its not really got any compromise for road use - and its a big lardy bike, with funny suspension

unless you lengthened the rear shock (and the shaft angles don't like that much), you'll have taken quite a bit of weight off the front ?

I get you don't want it for that usage - but are there any road based tyres at 21" that are suitable ? - Oh maybe coz the Africa twin can have one but it might be a 100 not a 110?

Thinking about Harley's issues - maybe the factory where going from 17" to chopper 21" and that exceeds the 10 to 12% limit the ABS self check is happy about

 

[botus]

I was thinking have I muddled up the impact on traction control ?

if the front is going slower as far bigger circumference - then the bike will think the back is spinning up more (not less), so more like S is now std level of assistance - so the bike with ASC on will be holding back a lot more than it already does ?

 

[gstom89]

Yes I was expecting the front to feel this way with the new weight distribution and will fettle the suspension to suit. Although it’s really not that bad but was worth mentioning.

Plenty of 21 inch road tyres available but if I want road tyres I’ll just put the 19inch front back on!

 

[SnowMec]

I am also very happy with my conversion to 21" front wheel. I have now ridden about 500km (cold spring here in northern Sweden so no long trip so far) and can safely say that it works. The ABS has exactly the same function when I use the bike for everyday riding. I have tried to provoke the ABS to intervene on all surfaces I have ridden and it works the same way as with the original wheel.
When riding active on loose surfaces I always switch off the ABS.

As I see it, the big differences are that the wheel is narrower and 21” (obviously) and that it also gives the front increased trail. I see the change in weight distribution as very small with only 20mm of front lift and I have tried raising the rear end to compensate but prefer standard.
The increased trail is an advantage and makes my bike better in my opinion.

The narrower front wheel with a coarse pattern (TKC 80) has significantly less rubber in the asphalt compered to at street tire so the maximum cornering speed on asphalt is lower and the braking distance is longer.
At high speed it is not as stable but I don't use my GSA for that.

On gravel roads it is fantastically much more fun and on terrain the difference is very big, it now feels like a offroad bike! (a heavy and big offroad bike!)
Definitely a modification I recommend to everyone who rides terrain, trails and gravel roads with their GS(A) K25.

If anyone is thinking of converting to a 21" front wheel and mounting a road tire, I wonder why they would do that?
Thanks to the narrow 1.85" front rim, I can ride with a 90/90 21" offroad tire, which makes the bike much more pleasant on the side of the road. The stock 19" wide front wheel is not great when driving over old wheel tracks in the dirt and then you have to be prepared so that the bike doesn't find the wrong track. With the narrow 21" wheel, the bike is much easier to ride, it now does what I want even when I pass old deep wheel tracks.

The larger wheel also makes the bike more pleasant in city traffic when you must drive very slow (almost zero km/h) and don't want to put your foot down. The balance is better with a large wheel and more trail.

My bike doesn't have traction control so I can't say for sure that it isn't affected. As I see it, it doesn't matter that much. If you're considering 21" front wheel, you're doing it for use at off-road/bad roads and then you turn off TC anyway.